Research ArticleNonfeedback Distributed Beamforming UsingSpatial-Temporal Extraction

Pongnarin Sriploy and Monthippa Uthansakul

School of Telecommunication Engineering Suranaree University of Technology Nakhon Ratchasima 30000 Thailand

Correspondence should be addressed to Monthippa Uthansakul mtpsutacth

Received 29 October 2015 Revised 20 January 2016 Accepted 26 January 2016

Academic Editor Lei Yu

Copyright copy 2016 P Sriploy and M UthansakulThis is an open access article distributed under the Creative CommonsAttributionLicense which permits unrestricted use distribution and reproduction in anymedium provided the originalwork is properly cited

So far major phase synchronization techniques for distributed beamforming suffer from the problem related to the feedbackprocedure as a base station has to send the feedback reference signal back to the transmitting nodes This requires stability ofcommunication channel or a number of retransmissions introducing a complicated system to both transmitter and receiverTherefore this paper proposes an alternative technique so-called nonfeedback beamforming employing an operation in bothspace and time domains The proposed technique is to extract a combined signal at the base station The concept of extraction isbased on solving a simultaneous linear equation without the requirement of feedback or reference signals from base station Alsothe number of retransmissions is less compared with the ones available in literatures As a result the transmitting nodes are of lowcomplexity and also low power consumptionThe simulation and experimental results reveal that the proposed technique providesthe optimum beamforming gain Furthermore it can reduce Bit Error Rate to the systems

1 Introduction

Nowadays wireless communication networks provide a vari-ety of applications such as wireless local area networkscellular networks or wireless sensor networksThese wirelessnetworks have lots of advantages in flexibility and mobilityfor users compared with a wired communication Howeverthe transmission range of wireless communication systemsis limited due to signal attenuation [1] Therefore nodes orsensors in the systems require a higher transmitted powerto compensate the mentioned attenuation This requirementis not practical due to the limited battery lifetime of nodesTo tackle the problem array antennas may be employed atindividual devices in order to enhance beamforming gain [2ndash7] Unfortunately the installation of multiple antennas on amobile terminal is difficult due to its size and the limitationof power consumption [8ndash10] Recently a distributed beam-forming has been proposed to handle thementioned problem[11 12] which has lots of advantages such as a significantincrease in transmission range and an enhancement of bothenergy efficiency and Signal-to-Noise Ratio (SNR) [13ndash16]

The distributed beamforming concept is similar to smartantennas but the position of antennas (or nodes) is not fixed

Also it could be said that the distributed beamforming issimilar to virtual array antennas in which each node sendsthe same data at the same time to base station [17] In thedistributed beamforming networks the transmitting nodeshave to perform the synchronization in order to achievephase alignment Otherwise the phase offsets or phase errorsamong all transmitted signals may degrade the combined sig-nal at base station causing intersymbol distortion Recentlylots of phase synchronization techniques have been proposedThese techniques can be classified into two types closed-loopand open-loop synchronization techniques [18]

The closed-loop technique needs some feedback frombase station to adjust the phase offsets among transmittingnodes A one-bit feedback is one of the best techniques forclosed-loop synchronization [19 20] In thementioned workevery transmitting node in the networks has to randomlyadjust its carrier signal phases Then all nodes transmit thesame data to base station After performing an estimation ofreceived SNR at base station one bit (0 or 1) is transmittedback to all nodes The bit ldquo0rdquo means that SNR is worse thanbefore so that each node has to randomly adjust its phasesagain On the other hand bit ldquo1rdquo means that SNR is better

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2016 Article ID 7086234 16 pageshttpdxdoiorg10115520167086234

2 International Journal of Antennas and Propagation

than before so that all nodes have to update their latest phaseadjustment As we can see this technique requires a largenumber of retransmissions from nodes to base station Forexample it requires at least 5N iterations in order to achieve75 guarantee of perfect or maximum beamforming gainwhere 119873 stands for the number of transmitting nodes [21]This may be considerably impractical as the battery life ofnodes ormobile terminals is very limitedMoreover a closed-loop feedback frombase station to transmitting nodesmay beunreliable when the communication channel is weak

In order to overcome the mentioned problems twoopen-loop phase synchronization techniques master-slaveand time-slot round-trip techniques have been proposed toreduce the interaction between base station and nodes Formaster-slave technique one node in the networks is selectedas a master node while all remaining nodes are assigned tobe slave nodes The phase synchronization in this techniqueis achieved by sending the reference signals between masterand slave nodes [21] Alternatively for time-slot round-triptechnique the phase synchronization among transmittingnodes can be obtained by sending a reference data amongthemselves [22 23] The idea is based on the equivalenceof round-trip transmission delays through a multihop chainbetween transmitting nodes and base station Accordingto these procedures the open-loop techniques reduce theinteraction among nodes and base station However bothmaster-slave and round-trip techniques still require somefeedback from base station Also this interaction increasesa complexity to all transmitting nodes In addition nodesrequire a special hardware such as phase-locked loops toobtain a precise reference signal when performing phasesynchronization

Alternatively a zero-feedback distributed beamformingtechnique has been lately proposed This technique does notrequire any feedback signal from the base station and theunsynchronized carriers do notmatter [24 25] However thistechnique requires a large number of packet retransmissionsFor example in case of having 3 transmitting nodes thistechnique requires at least 50 retransmissions (iterations) toobtain the beamforming gain at 91 dB [24] Note that themaximum gain for this case is 95 dB In addition the numberof required retransmissions exponentially increases when thenumber of nodes increases

From above literatures the major disadvantage of exist-ing phase synchronization techniques can be concluded asfollows the one-bit feedback and zero-feedback require alarge number of retransmissions which extremely reducesthe battery life of transmitting nodes Also the master-slave and round-trip techniques require the reference signalamong transmitting nodes which increases a complexity totransmitting nodes

To overcome those disadvantages a nonfeedback dis-tributed beamforming technique is proposed in this paperNote that the authors use ldquononfeedbackrdquo term to avoidany confusion with the conceptual term defined for ldquozero-feedbackrdquo that appeared in [24 25] In this paper theproposed technique does not require any feedback signalfrom base station or interaction between transmitting nodesInstead the proposed technique requires a few number of

retransmissions from nodes which is only the same as thenumber of transmitting nodes N This is relatively smallcompared with the number of retransmissions for one-bitfeedback and zero-feedback techniques The proposed non-feedback beamforming performs an extraction of combinedsignal at base station which means that the transmittingnodes do not need to deal with phase synchronizationanymore hence they can save energy and also battery lifeThe concept of extraction is based on a classical equationsolving using inverse matrix This procedure requires afew retransmissions from nodes After performing a signalextraction each extracted signal is properly weighted toobtain an appropriate phase alignment at base station Finallythe base station obtains a combined signal with maximumbeamforming gain However the retransmitted signals maybe distorted when travelling through the communicationchannel

This paper is organized as follows Following an intro-duction including motivation and contribution of the pro-posed idea the basic concept and definition of distributedbeamforming techniques are presented in Section 2 Thenthe proposed nonfeedback technique and its performanceare discussed in Section 3 Afterwards experimental studiesare performed in Section 4 in order to validate the proposedconcept Finally Section 5 concludes the paper

2 Distributed BeamformingConcept and Definition

The basic concept behind a distributed beamforming issimilar to a traditional beamforming technique such assmart antennas in which the data is transmitted by arrayantennas located at one place The phases of antennas arealigned so that the transmitted signals are gainfully combinedat destination For distributed beamforming networks thetransmitting nodes representing a group of single antennaelements are randomly distributed over the networks Asthe nodesrsquo locations are unknown the phase synchroniza-tion between nodes is the key challenge over a traditionalbeamforming Figure 1 shows a configuration of distributedbeamforming networks in which each transmitting node andbase station are equipped with a single antenna elementAll nodes and base station are stationary It is assumedthat 119873 distributed nodes transmit a shared message 119909(119905)to base station over the Rayleigh flat fading channel Themathematical model of distributed beamforming networks isshown in Figure 2 which is detailed as follows The receivedpass-band complex signal 119884

119877(119905) can be written as

119884119877 (119905) = R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905) (1)

where 119909(119905) is a transmitted message ℎ119899is a fading coefficient

and 119860119899is a carrier signal amplitude at 119899th node Also 120596

119899=

120596119888+ Δ120596119899 where 120596

119888is a carrier signal frequency and Δ120596

119899

is a frequency offset In addition 120601119899= 1206010+ Δ120601119899 where

1206010is a nominal phase and Δ120601

119899is a phase offset of the

signal coming from 119899th node which depends on the relativemobility or node location between 119899th node and base station

International Journal of Antennas and Propagation 3

Base station12

3

N

4

Distributed beamforming networks (DBN)

Wireless networks

Wireless node

Figure 1 Configuration of wireless networks employing a distributed beamforming

Node 1

Node 2 Base

station

Channel

y1(t) = x(t)A1ej(1205961t+1206010)

y2(t) = x(t)A2ej(1205962t+1206010)

yN(t) = x(t)ANej(120596119873t+1206010)

h1

h2

hN

y9984001(t) = x(t)h1A1ej(1205961t+1206011)

y9984002(t) = x(t)h2A2ej(1205962t+1206012)

y998400N(t) = x(t)hNANej(120596119873t+120601119873)

AWGN W(t)

wI(t)ej120596119888t wQ(t)e

j120596119888t

Node NYR(t) = Rx(t)

N

sumn=1

hnej(120596119899t+120601119899) + W(t)

Figure 2 Mathematical model of wireless networks employing a distributed beamforming

Furthermore 119882(119905) is the Additive White Gaussian Noise(AWGN) which consists of in-phase 119908

119868(119905) and quadrature

119908119876(119905) components that is 119882(119905) = [119908

119868(119905) + 119895119908

119876(119905)]119890119895120596119905

Therefore the received signal power at base station can beexpressed as follows [24]

1003816100381610038161003816119884119877 (119905)1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120596

119888119905 minus 120596119888119905 + 120601119899minus 120601119898)

+1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120601

119899minus 120601119898) +

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2

(2)

The phase offset between each node (120601119899minus120601119898) shown in (2) is

relatively significant to the beamforming gainNote that 119899 and119898 are the index of transmitting node in the networks inwhich119899 = 119898 In the case of having a finite number of nodes119873 thedistributed beamforming gain can be defined as a normalizedreceived power at base station 119875

119877 as

119875119877= 119873

1003817100381710038171003817100381710038171003817100381710038171003817

119909 (119905)

119873

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

1003817100381710038171003817100381710038171003817100381710038171003817

2

+ 119882 (119905)2 (3)

As the phases of transmitting nodes are random thus we con-sider the normalized received power in form of an expectedvalue If we assume that the received signal amplitude from allnodes is 119860

119899= 1 the average power of the transmitted signal

is119875119879= 1 and ℎ

119899are iid randomvariables then119864[ℎ

119899] = 119864[ℎ]

4 International Journal of Antennas and Propagation

and 119864[ℎ2119899] = 1 Therefore the average power of the received

signal is adopted from [21] as follows

119864 [119875119877] =

119909 (119905)2

119873

sdot 119864[

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

119873

sum

119898=1

10038161003816100381610038161198601198981003816100381610038161003816

2119890119895(120596119888119905+120601119898)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 119864 [1003816100381610038161003816ℎ1198991003816100381610038161003816

2 100381610038161003816100381611986011003816100381610038161003816

2 100381610038161003816100381611986021003816100381610038161003816

22R (119890

119895(120596119888119905minus120596119888119905+1206011minus1206012))])

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 2119864 [cos (1206011minus 1206012)]) + 119864 [

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2] = 119909 (119905)

21

+ (119873 minus 1) 119864 [cos (1206011 minus 1206012)] + 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

= 119909 (119905)21 + (119873 minus 1)

sdot 119864 [cos (1206011) cos (120601

2) minus sin (120601

1) sin (120601

2)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] = 119909 (119905)

21 + (119873 minus 1) 119864 [cos (120601119894)]

2

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

(4)

where 120601119894is a phase offset among nodes which is uniformly

distributed around 0∘ between [minus120587 120587] interval Figure 3shows a normalized beamforming gain 119864[119875

119877] which is

obtained using (4) upon varying the numbers of nodes inthe networks The ldquoperfect distributed beamformingrdquo termmeans that every received signal at base station is perfectlyaligned Note that themaximumbeamforming gain is 1 as thesignal normalization is performed at base station For the caseof having no phase offset (or 120601

119894= 0∘ here defined as a perfect

distributed beamforming) the resulting beamforming gainis of maximum value Otherwise the systems experience anunstable low beamforming gain According to these resultsphase synchronization is a key success for distributed beam-forming which is the focus of this paper As mentioned in theIntroduction various phase synchronization techniques havebeen proposed with a common drawback of sending a largenumber of feedback signals and the requirement of additionalhardware Alternatively this paper proposes a phase synchro-nization technique avoiding any feedback signal from basestation and any interaction between transmitting nodes Theproposed nonfeedback phase synchronization is described inthe next section

3 Proposed Phase Synchronization

In this section we propose a nonfeedback distributed beam-forming which does not require any interaction betweennodes and feedback signals The phase synchronization isperformed at base station At base station there are twomajor parts (1) RF front end operationwhich converts the RF

0 10 20 30 40 50 60 70 80 90 1000

02

04

06

08

1

12

14

Number of nodes

Nor

mal

ized

bea

mfo

rmin

g ga

in

Perfect distributed beamformingNo phase synchronization

Figure 3 Beamforming gain in case of perfect beamforming and nophase synchronization

signals to the baseband signals and (2) proposed nonfeedbacktechnique procedure which performs signal extraction andphase synchronization Moreover the performance compar-ison between the proposed technique and some existingtechniques is presented afterwards

When base station receives signals from 119873 transmittingnodes the composite received signal 119884

119877(119905) is processed in

the RF front end receiver in order to convert a pass-bandreceived signal to digital baseband signal The RF front endreceiver consists of RF demodulator band-pass filter Analog-to-Digital Converter (ADC) and Digital Downconverter(DDC)Then the proposed nonfeedback technique is appliedto the output baseband signal 11988410158401015840(119896) which includes asignal extraction and a weighting process The mentionedprocedures are detailed as follows

31 RF Front End Operation In RF front end we assume thatthe received signal amplitude119860

119899 is equal to 1The combined

received signals at base station when they are coming from alltransmitting nodes can be written as

119884119877 (119905) = R

119873

sum

119899=1

1199101015840

119899(119905) + 119882 (119905)

= R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905)

= 119909 (119905)

119873

sum

119899=1

[ℎ119899cos (120596

119899119905 + 120601119899)]

+ 119908119868 (119905) cos (120596119888119905) minus 119908119876 (119905) sin (120596119888119905)⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

R119882119875119861(119905)

(5)

when 1199101015840119899(119905) is a signal from the 119899th node 120596

119899= 120596119888+ Δ120596119899in

which 120596119888is a carrier signal frequency and Δ120596

119899is a frequency

offset 120601119899= 1206010+ Δ120601119899 where 120601

0is a nominal phase Δ120601

119899is a

International Journal of Antennas and Propagation 5

Base station

123N

1

2

N minus 1

N

Phase shift of each retransmissionSymbol period

middot middot middot

middot middot middot

middot middot middot

middot middot middot

middot middot middot

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej180∘

ej180∘

ej180∘

Y998400998400N(k)

Figure 4 Proposed phase shifting pattern

phase offset and119882(119905) is AWGN which consists of in-phase119908119868(119905) and quadrature 119908

119876(119905) components Then the received

signal that appeared in (5) is modulated and downconvertedusing RF modulator and DDC in order to obtain the digitalbaseband signal as follows

11988410158401015840(119896) = 119909 (119896)

119873

sum

119899=1

ℎ119899119890minus119895(ΔΩ

119899119896+Φ119899)+119882119861119861 (119896) (6)

where 119896 is a sampling time variableΔΩ119899is a frequency offset

and Φ119899is a phase offset at 119896th sampling time In addition

119882119861119861(119896) is a baseband AWGN and119882

119861119861(119896) = 119908

119868(119896) + 119895119908

119876(119896)

Equation (6) presents the baseband signal which can bedegraded by the phase offset Φ

119899 Thus the baseband signal

of each transmitting node requires a phase synchronization inorder to obtain the maximum beamforming gain Therefore11988410158401015840(119896) is passed to the procedure of proposed nonfeedback

technique which is presented in the following section

32 Proposed Nonfeedback Technique In this paper a non-feedback technique is proposed employing a signal extractionat base station to achieve a phase synchronization for dis-tributed beamforming networks The received signal 11988410158401015840(119896)mentioned earlier is processed in two steps Spatial-TemporalExtraction and OptimumWeighting

Step 1 (Spatial-Temporal Extraction) The combined signalthat appeared in (6) needs to be extracted before performinga phase synchronization The steps of signal extraction are asfollows

All transmitting nodes in the networks transmit thesimilar message to the base station at the same time Thistransmission repeats only 119873 times where 119873 stands for thenumber of transmitting nodes in the networks For eachretransmission the transmitting nodes adjust their phasesaccording to a fixed phase adjustment pattern matrix A

119873119873

which can be obtained by the following algorithm Note thatthis retransmission does not require any interaction amongtransmitting nodes

(1) At 1st symbol period all transmitting nodes send thesignal without any phase adjustment

(2) At the 119899th symbol period where 119899 = [2 3 119873]only (119899 minus 1)th transmitting node shifts its phase by180∘ For example at the 2nd symbol period the 1stnode shifts its phase by 180∘ and at the 3rd symbolperiod the 2nd node shifts its phase by 180∘ as seenin Figure 4 which shows the summary of proposedphase adjustment pattern

(3) Repeat phase shifting pattern until119873 symbol period

According to the proposed phase adjustment pattern aspresented above we obtain the fixed coefficient matrix A

119873119873

by retransmitting signal for119873 times as shown in the followingequations

A119873119873

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]

(7)

After having completed 119873 retransmissions all receivedsignals are simultaneously arranged to form vector Y10158401015840

119873as

follows

Y10158401015840119873(119896) = A

119873119873y119873 (119896) +W

119861119861119873 (119896) (8)

or

[[[[[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

119873(119896)

]]]]]]]]]]

]

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A119873119873

[[[[[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

119910119873 (119896)

]]]]]]]]]

]

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Submit your manuscripts athttpwwwhindawicom

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

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DistributedSensor Networks

International Journal of

2 International Journal of Antennas and Propagation

than before so that all nodes have to update their latest phaseadjustment As we can see this technique requires a largenumber of retransmissions from nodes to base station Forexample it requires at least 5N iterations in order to achieve75 guarantee of perfect or maximum beamforming gainwhere 119873 stands for the number of transmitting nodes [21]This may be considerably impractical as the battery life ofnodes ormobile terminals is very limitedMoreover a closed-loop feedback frombase station to transmitting nodesmay beunreliable when the communication channel is weak

In order to overcome the mentioned problems twoopen-loop phase synchronization techniques master-slaveand time-slot round-trip techniques have been proposed toreduce the interaction between base station and nodes Formaster-slave technique one node in the networks is selectedas a master node while all remaining nodes are assigned tobe slave nodes The phase synchronization in this techniqueis achieved by sending the reference signals between masterand slave nodes [21] Alternatively for time-slot round-triptechnique the phase synchronization among transmittingnodes can be obtained by sending a reference data amongthemselves [22 23] The idea is based on the equivalenceof round-trip transmission delays through a multihop chainbetween transmitting nodes and base station Accordingto these procedures the open-loop techniques reduce theinteraction among nodes and base station However bothmaster-slave and round-trip techniques still require somefeedback from base station Also this interaction increasesa complexity to all transmitting nodes In addition nodesrequire a special hardware such as phase-locked loops toobtain a precise reference signal when performing phasesynchronization

Alternatively a zero-feedback distributed beamformingtechnique has been lately proposed This technique does notrequire any feedback signal from the base station and theunsynchronized carriers do notmatter [24 25] However thistechnique requires a large number of packet retransmissionsFor example in case of having 3 transmitting nodes thistechnique requires at least 50 retransmissions (iterations) toobtain the beamforming gain at 91 dB [24] Note that themaximum gain for this case is 95 dB In addition the numberof required retransmissions exponentially increases when thenumber of nodes increases

From above literatures the major disadvantage of exist-ing phase synchronization techniques can be concluded asfollows the one-bit feedback and zero-feedback require alarge number of retransmissions which extremely reducesthe battery life of transmitting nodes Also the master-slave and round-trip techniques require the reference signalamong transmitting nodes which increases a complexity totransmitting nodes

To overcome those disadvantages a nonfeedback dis-tributed beamforming technique is proposed in this paperNote that the authors use ldquononfeedbackrdquo term to avoidany confusion with the conceptual term defined for ldquozero-feedbackrdquo that appeared in [24 25] In this paper theproposed technique does not require any feedback signalfrom base station or interaction between transmitting nodesInstead the proposed technique requires a few number of

retransmissions from nodes which is only the same as thenumber of transmitting nodes N This is relatively smallcompared with the number of retransmissions for one-bitfeedback and zero-feedback techniques The proposed non-feedback beamforming performs an extraction of combinedsignal at base station which means that the transmittingnodes do not need to deal with phase synchronizationanymore hence they can save energy and also battery lifeThe concept of extraction is based on a classical equationsolving using inverse matrix This procedure requires afew retransmissions from nodes After performing a signalextraction each extracted signal is properly weighted toobtain an appropriate phase alignment at base station Finallythe base station obtains a combined signal with maximumbeamforming gain However the retransmitted signals maybe distorted when travelling through the communicationchannel

This paper is organized as follows Following an intro-duction including motivation and contribution of the pro-posed idea the basic concept and definition of distributedbeamforming techniques are presented in Section 2 Thenthe proposed nonfeedback technique and its performanceare discussed in Section 3 Afterwards experimental studiesare performed in Section 4 in order to validate the proposedconcept Finally Section 5 concludes the paper

2 Distributed BeamformingConcept and Definition

The basic concept behind a distributed beamforming issimilar to a traditional beamforming technique such assmart antennas in which the data is transmitted by arrayantennas located at one place The phases of antennas arealigned so that the transmitted signals are gainfully combinedat destination For distributed beamforming networks thetransmitting nodes representing a group of single antennaelements are randomly distributed over the networks Asthe nodesrsquo locations are unknown the phase synchroniza-tion between nodes is the key challenge over a traditionalbeamforming Figure 1 shows a configuration of distributedbeamforming networks in which each transmitting node andbase station are equipped with a single antenna elementAll nodes and base station are stationary It is assumedthat 119873 distributed nodes transmit a shared message 119909(119905)to base station over the Rayleigh flat fading channel Themathematical model of distributed beamforming networks isshown in Figure 2 which is detailed as follows The receivedpass-band complex signal 119884

119877(119905) can be written as

119884119877 (119905) = R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905) (1)

where 119909(119905) is a transmitted message ℎ119899is a fading coefficient

and 119860119899is a carrier signal amplitude at 119899th node Also 120596

119899=

120596119888+ Δ120596119899 where 120596

119888is a carrier signal frequency and Δ120596

119899

is a frequency offset In addition 120601119899= 1206010+ Δ120601119899 where

1206010is a nominal phase and Δ120601

119899is a phase offset of the

signal coming from 119899th node which depends on the relativemobility or node location between 119899th node and base station

International Journal of Antennas and Propagation 3

Base station12

3

N

4

Distributed beamforming networks (DBN)

Wireless networks

Wireless node

Figure 1 Configuration of wireless networks employing a distributed beamforming

Node 1

Node 2 Base

station

Channel

y1(t) = x(t)A1ej(1205961t+1206010)

y2(t) = x(t)A2ej(1205962t+1206010)

yN(t) = x(t)ANej(120596119873t+1206010)

h1

h2

hN

y9984001(t) = x(t)h1A1ej(1205961t+1206011)

y9984002(t) = x(t)h2A2ej(1205962t+1206012)

y998400N(t) = x(t)hNANej(120596119873t+120601119873)

AWGN W(t)

wI(t)ej120596119888t wQ(t)e

j120596119888t

Node NYR(t) = Rx(t)

N

sumn=1

hnej(120596119899t+120601119899) + W(t)

Figure 2 Mathematical model of wireless networks employing a distributed beamforming

Furthermore 119882(119905) is the Additive White Gaussian Noise(AWGN) which consists of in-phase 119908

119868(119905) and quadrature

119908119876(119905) components that is 119882(119905) = [119908

119868(119905) + 119895119908

119876(119905)]119890119895120596119905

Therefore the received signal power at base station can beexpressed as follows [24]

1003816100381610038161003816119884119877 (119905)1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120596

119888119905 minus 120596119888119905 + 120601119899minus 120601119898)

+1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120601

119899minus 120601119898) +

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2

(2)

The phase offset between each node (120601119899minus120601119898) shown in (2) is

relatively significant to the beamforming gainNote that 119899 and119898 are the index of transmitting node in the networks inwhich119899 = 119898 In the case of having a finite number of nodes119873 thedistributed beamforming gain can be defined as a normalizedreceived power at base station 119875

119877 as

119875119877= 119873

1003817100381710038171003817100381710038171003817100381710038171003817

119909 (119905)

119873

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

1003817100381710038171003817100381710038171003817100381710038171003817

2

+ 119882 (119905)2 (3)

As the phases of transmitting nodes are random thus we con-sider the normalized received power in form of an expectedvalue If we assume that the received signal amplitude from allnodes is 119860

119899= 1 the average power of the transmitted signal

is119875119879= 1 and ℎ

119899are iid randomvariables then119864[ℎ

119899] = 119864[ℎ]

4 International Journal of Antennas and Propagation

and 119864[ℎ2119899] = 1 Therefore the average power of the received

signal is adopted from [21] as follows

119864 [119875119877] =

119909 (119905)2

119873

sdot 119864[

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

119873

sum

119898=1

10038161003816100381610038161198601198981003816100381610038161003816

2119890119895(120596119888119905+120601119898)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 119864 [1003816100381610038161003816ℎ1198991003816100381610038161003816

2 100381610038161003816100381611986011003816100381610038161003816

2 100381610038161003816100381611986021003816100381610038161003816

22R (119890

119895(120596119888119905minus120596119888119905+1206011minus1206012))])

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 2119864 [cos (1206011minus 1206012)]) + 119864 [

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2] = 119909 (119905)

21

+ (119873 minus 1) 119864 [cos (1206011 minus 1206012)] + 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

= 119909 (119905)21 + (119873 minus 1)

sdot 119864 [cos (1206011) cos (120601

2) minus sin (120601

1) sin (120601

2)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] = 119909 (119905)

21 + (119873 minus 1) 119864 [cos (120601119894)]

2

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

(4)

where 120601119894is a phase offset among nodes which is uniformly

distributed around 0∘ between [minus120587 120587] interval Figure 3shows a normalized beamforming gain 119864[119875

119877] which is

obtained using (4) upon varying the numbers of nodes inthe networks The ldquoperfect distributed beamformingrdquo termmeans that every received signal at base station is perfectlyaligned Note that themaximumbeamforming gain is 1 as thesignal normalization is performed at base station For the caseof having no phase offset (or 120601

119894= 0∘ here defined as a perfect

distributed beamforming) the resulting beamforming gainis of maximum value Otherwise the systems experience anunstable low beamforming gain According to these resultsphase synchronization is a key success for distributed beam-forming which is the focus of this paper As mentioned in theIntroduction various phase synchronization techniques havebeen proposed with a common drawback of sending a largenumber of feedback signals and the requirement of additionalhardware Alternatively this paper proposes a phase synchro-nization technique avoiding any feedback signal from basestation and any interaction between transmitting nodes Theproposed nonfeedback phase synchronization is described inthe next section

3 Proposed Phase Synchronization

In this section we propose a nonfeedback distributed beam-forming which does not require any interaction betweennodes and feedback signals The phase synchronization isperformed at base station At base station there are twomajor parts (1) RF front end operationwhich converts the RF

0 10 20 30 40 50 60 70 80 90 1000

02

04

06

08

1

12

14

Number of nodes

Nor

mal

ized

bea

mfo

rmin

g ga

in

Perfect distributed beamformingNo phase synchronization

Figure 3 Beamforming gain in case of perfect beamforming and nophase synchronization

signals to the baseband signals and (2) proposed nonfeedbacktechnique procedure which performs signal extraction andphase synchronization Moreover the performance compar-ison between the proposed technique and some existingtechniques is presented afterwards

When base station receives signals from 119873 transmittingnodes the composite received signal 119884

119877(119905) is processed in

the RF front end receiver in order to convert a pass-bandreceived signal to digital baseband signal The RF front endreceiver consists of RF demodulator band-pass filter Analog-to-Digital Converter (ADC) and Digital Downconverter(DDC)Then the proposed nonfeedback technique is appliedto the output baseband signal 11988410158401015840(119896) which includes asignal extraction and a weighting process The mentionedprocedures are detailed as follows

31 RF Front End Operation In RF front end we assume thatthe received signal amplitude119860

119899 is equal to 1The combined

received signals at base station when they are coming from alltransmitting nodes can be written as

119884119877 (119905) = R

119873

sum

119899=1

1199101015840

119899(119905) + 119882 (119905)

= R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905)

= 119909 (119905)

119873

sum

119899=1

[ℎ119899cos (120596

119899119905 + 120601119899)]

+ 119908119868 (119905) cos (120596119888119905) minus 119908119876 (119905) sin (120596119888119905)⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

R119882119875119861(119905)

(5)

when 1199101015840119899(119905) is a signal from the 119899th node 120596

119899= 120596119888+ Δ120596119899in

which 120596119888is a carrier signal frequency and Δ120596

119899is a frequency

offset 120601119899= 1206010+ Δ120601119899 where 120601

0is a nominal phase Δ120601

119899is a

International Journal of Antennas and Propagation 5

Base station

123N

1

2

N minus 1

N

Phase shift of each retransmissionSymbol period

middot middot middot

middot middot middot

middot middot middot

middot middot middot

middot middot middot

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej180∘

ej180∘

ej180∘

Y998400998400N(k)

Figure 4 Proposed phase shifting pattern

phase offset and119882(119905) is AWGN which consists of in-phase119908119868(119905) and quadrature 119908

119876(119905) components Then the received

signal that appeared in (5) is modulated and downconvertedusing RF modulator and DDC in order to obtain the digitalbaseband signal as follows

11988410158401015840(119896) = 119909 (119896)

119873

sum

119899=1

ℎ119899119890minus119895(ΔΩ

119899119896+Φ119899)+119882119861119861 (119896) (6)

where 119896 is a sampling time variableΔΩ119899is a frequency offset

and Φ119899is a phase offset at 119896th sampling time In addition

119882119861119861(119896) is a baseband AWGN and119882

119861119861(119896) = 119908

119868(119896) + 119895119908

119876(119896)

Equation (6) presents the baseband signal which can bedegraded by the phase offset Φ

119899 Thus the baseband signal

of each transmitting node requires a phase synchronization inorder to obtain the maximum beamforming gain Therefore11988410158401015840(119896) is passed to the procedure of proposed nonfeedback

technique which is presented in the following section

32 Proposed Nonfeedback Technique In this paper a non-feedback technique is proposed employing a signal extractionat base station to achieve a phase synchronization for dis-tributed beamforming networks The received signal 11988410158401015840(119896)mentioned earlier is processed in two steps Spatial-TemporalExtraction and OptimumWeighting

Step 1 (Spatial-Temporal Extraction) The combined signalthat appeared in (6) needs to be extracted before performinga phase synchronization The steps of signal extraction are asfollows

All transmitting nodes in the networks transmit thesimilar message to the base station at the same time Thistransmission repeats only 119873 times where 119873 stands for thenumber of transmitting nodes in the networks For eachretransmission the transmitting nodes adjust their phasesaccording to a fixed phase adjustment pattern matrix A

119873119873

which can be obtained by the following algorithm Note thatthis retransmission does not require any interaction amongtransmitting nodes

(1) At 1st symbol period all transmitting nodes send thesignal without any phase adjustment

(2) At the 119899th symbol period where 119899 = [2 3 119873]only (119899 minus 1)th transmitting node shifts its phase by180∘ For example at the 2nd symbol period the 1stnode shifts its phase by 180∘ and at the 3rd symbolperiod the 2nd node shifts its phase by 180∘ as seenin Figure 4 which shows the summary of proposedphase adjustment pattern

(3) Repeat phase shifting pattern until119873 symbol period

According to the proposed phase adjustment pattern aspresented above we obtain the fixed coefficient matrix A

119873119873

by retransmitting signal for119873 times as shown in the followingequations

A119873119873

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]

(7)

After having completed 119873 retransmissions all receivedsignals are simultaneously arranged to form vector Y10158401015840

119873as

follows

Y10158401015840119873(119896) = A

119873119873y119873 (119896) +W

119861119861119873 (119896) (8)

or

[[[[[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

119873(119896)

]]]]]]]]]]

]

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A119873119873

[[[[[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

119910119873 (119896)

]]]]]]]]]

]

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 3

Base station12

3

N

4

Distributed beamforming networks (DBN)

Wireless networks

Wireless node

Figure 1 Configuration of wireless networks employing a distributed beamforming

Node 1

Node 2 Base

station

Channel

y1(t) = x(t)A1ej(1205961t+1206010)

y2(t) = x(t)A2ej(1205962t+1206010)

yN(t) = x(t)ANej(120596119873t+1206010)

h1

h2

hN

y9984001(t) = x(t)h1A1ej(1205961t+1206011)

y9984002(t) = x(t)h2A2ej(1205962t+1206012)

y998400N(t) = x(t)hNANej(120596119873t+120601119873)

AWGN W(t)

wI(t)ej120596119888t wQ(t)e

j120596119888t

Node NYR(t) = Rx(t)

N

sumn=1

hnej(120596119899t+120601119899) + W(t)

Figure 2 Mathematical model of wireless networks employing a distributed beamforming

Furthermore 119882(119905) is the Additive White Gaussian Noise(AWGN) which consists of in-phase 119908

119868(119905) and quadrature

119908119876(119905) components that is 119882(119905) = [119908

119868(119905) + 119895119908

119876(119905)]119890119895120596119905

Therefore the received signal power at base station can beexpressed as follows [24]

1003816100381610038161003816119884119877 (119905)1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120596

119888119905 minus 120596119888119905 + 120601119899minus 120601119898)

+1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2= 119909 (119905)

2

119873

sum

119899=1

ℎ2

1198991198602

119899

+ 2sum

119899 =119898

[ℎ119899ℎ119898] [119860119899119860119898] cos (120601

119899minus 120601119898) +

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2

(2)

The phase offset between each node (120601119899minus120601119898) shown in (2) is

relatively significant to the beamforming gainNote that 119899 and119898 are the index of transmitting node in the networks inwhich119899 = 119898 In the case of having a finite number of nodes119873 thedistributed beamforming gain can be defined as a normalizedreceived power at base station 119875

119877 as

119875119877= 119873

1003817100381710038171003817100381710038171003817100381710038171003817

119909 (119905)

119873

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

1003817100381710038171003817100381710038171003817100381710038171003817

2

+ 119882 (119905)2 (3)

As the phases of transmitting nodes are random thus we con-sider the normalized received power in form of an expectedvalue If we assume that the received signal amplitude from allnodes is 119860

119899= 1 the average power of the transmitted signal

is119875119879= 1 and ℎ

119899are iid randomvariables then119864[ℎ

119899] = 119864[ℎ]

4 International Journal of Antennas and Propagation

and 119864[ℎ2119899] = 1 Therefore the average power of the received

signal is adopted from [21] as follows

119864 [119875119877] =

119909 (119905)2

119873

sdot 119864[

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

119873

sum

119898=1

10038161003816100381610038161198601198981003816100381610038161003816

2119890119895(120596119888119905+120601119898)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 119864 [1003816100381610038161003816ℎ1198991003816100381610038161003816

2 100381610038161003816100381611986011003816100381610038161003816

2 100381610038161003816100381611986021003816100381610038161003816

22R (119890

119895(120596119888119905minus120596119888119905+1206011minus1206012))])

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 2119864 [cos (1206011minus 1206012)]) + 119864 [

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2] = 119909 (119905)

21

+ (119873 minus 1) 119864 [cos (1206011 minus 1206012)] + 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

= 119909 (119905)21 + (119873 minus 1)

sdot 119864 [cos (1206011) cos (120601

2) minus sin (120601

1) sin (120601

2)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] = 119909 (119905)

21 + (119873 minus 1) 119864 [cos (120601119894)]

2

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

(4)

where 120601119894is a phase offset among nodes which is uniformly

distributed around 0∘ between [minus120587 120587] interval Figure 3shows a normalized beamforming gain 119864[119875

119877] which is

obtained using (4) upon varying the numbers of nodes inthe networks The ldquoperfect distributed beamformingrdquo termmeans that every received signal at base station is perfectlyaligned Note that themaximumbeamforming gain is 1 as thesignal normalization is performed at base station For the caseof having no phase offset (or 120601

119894= 0∘ here defined as a perfect

distributed beamforming) the resulting beamforming gainis of maximum value Otherwise the systems experience anunstable low beamforming gain According to these resultsphase synchronization is a key success for distributed beam-forming which is the focus of this paper As mentioned in theIntroduction various phase synchronization techniques havebeen proposed with a common drawback of sending a largenumber of feedback signals and the requirement of additionalhardware Alternatively this paper proposes a phase synchro-nization technique avoiding any feedback signal from basestation and any interaction between transmitting nodes Theproposed nonfeedback phase synchronization is described inthe next section

3 Proposed Phase Synchronization

In this section we propose a nonfeedback distributed beam-forming which does not require any interaction betweennodes and feedback signals The phase synchronization isperformed at base station At base station there are twomajor parts (1) RF front end operationwhich converts the RF

0 10 20 30 40 50 60 70 80 90 1000

02

04

06

08

1

12

14

Number of nodes

Nor

mal

ized

bea

mfo

rmin

g ga

in

Perfect distributed beamformingNo phase synchronization

Figure 3 Beamforming gain in case of perfect beamforming and nophase synchronization

signals to the baseband signals and (2) proposed nonfeedbacktechnique procedure which performs signal extraction andphase synchronization Moreover the performance compar-ison between the proposed technique and some existingtechniques is presented afterwards

When base station receives signals from 119873 transmittingnodes the composite received signal 119884

119877(119905) is processed in

the RF front end receiver in order to convert a pass-bandreceived signal to digital baseband signal The RF front endreceiver consists of RF demodulator band-pass filter Analog-to-Digital Converter (ADC) and Digital Downconverter(DDC)Then the proposed nonfeedback technique is appliedto the output baseband signal 11988410158401015840(119896) which includes asignal extraction and a weighting process The mentionedprocedures are detailed as follows

31 RF Front End Operation In RF front end we assume thatthe received signal amplitude119860

119899 is equal to 1The combined

received signals at base station when they are coming from alltransmitting nodes can be written as

119884119877 (119905) = R

119873

sum

119899=1

1199101015840

119899(119905) + 119882 (119905)

= R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905)

= 119909 (119905)

119873

sum

119899=1

[ℎ119899cos (120596

119899119905 + 120601119899)]

+ 119908119868 (119905) cos (120596119888119905) minus 119908119876 (119905) sin (120596119888119905)⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

R119882119875119861(119905)

(5)

when 1199101015840119899(119905) is a signal from the 119899th node 120596

119899= 120596119888+ Δ120596119899in

which 120596119888is a carrier signal frequency and Δ120596

119899is a frequency

offset 120601119899= 1206010+ Δ120601119899 where 120601

0is a nominal phase Δ120601

119899is a

International Journal of Antennas and Propagation 5

Base station

123N

1

2

N minus 1

N

Phase shift of each retransmissionSymbol period

middot middot middot

middot middot middot

middot middot middot

middot middot middot

middot middot middot

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej180∘

ej180∘

ej180∘

Y998400998400N(k)

Figure 4 Proposed phase shifting pattern

phase offset and119882(119905) is AWGN which consists of in-phase119908119868(119905) and quadrature 119908

119876(119905) components Then the received

signal that appeared in (5) is modulated and downconvertedusing RF modulator and DDC in order to obtain the digitalbaseband signal as follows

11988410158401015840(119896) = 119909 (119896)

119873

sum

119899=1

ℎ119899119890minus119895(ΔΩ

119899119896+Φ119899)+119882119861119861 (119896) (6)

where 119896 is a sampling time variableΔΩ119899is a frequency offset

and Φ119899is a phase offset at 119896th sampling time In addition

119882119861119861(119896) is a baseband AWGN and119882

119861119861(119896) = 119908

119868(119896) + 119895119908

119876(119896)

Equation (6) presents the baseband signal which can bedegraded by the phase offset Φ

119899 Thus the baseband signal

of each transmitting node requires a phase synchronization inorder to obtain the maximum beamforming gain Therefore11988410158401015840(119896) is passed to the procedure of proposed nonfeedback

technique which is presented in the following section

32 Proposed Nonfeedback Technique In this paper a non-feedback technique is proposed employing a signal extractionat base station to achieve a phase synchronization for dis-tributed beamforming networks The received signal 11988410158401015840(119896)mentioned earlier is processed in two steps Spatial-TemporalExtraction and OptimumWeighting

Step 1 (Spatial-Temporal Extraction) The combined signalthat appeared in (6) needs to be extracted before performinga phase synchronization The steps of signal extraction are asfollows

All transmitting nodes in the networks transmit thesimilar message to the base station at the same time Thistransmission repeats only 119873 times where 119873 stands for thenumber of transmitting nodes in the networks For eachretransmission the transmitting nodes adjust their phasesaccording to a fixed phase adjustment pattern matrix A

119873119873

which can be obtained by the following algorithm Note thatthis retransmission does not require any interaction amongtransmitting nodes

(1) At 1st symbol period all transmitting nodes send thesignal without any phase adjustment

(2) At the 119899th symbol period where 119899 = [2 3 119873]only (119899 minus 1)th transmitting node shifts its phase by180∘ For example at the 2nd symbol period the 1stnode shifts its phase by 180∘ and at the 3rd symbolperiod the 2nd node shifts its phase by 180∘ as seenin Figure 4 which shows the summary of proposedphase adjustment pattern

(3) Repeat phase shifting pattern until119873 symbol period

According to the proposed phase adjustment pattern aspresented above we obtain the fixed coefficient matrix A

119873119873

by retransmitting signal for119873 times as shown in the followingequations

A119873119873

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]

(7)

After having completed 119873 retransmissions all receivedsignals are simultaneously arranged to form vector Y10158401015840

119873as

follows

Y10158401015840119873(119896) = A

119873119873y119873 (119896) +W

119861119861119873 (119896) (8)

or

[[[[[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

119873(119896)

]]]]]]]]]]

]

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A119873119873

[[[[[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

119910119873 (119896)

]]]]]]]]]

]

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Chemical EngineeringInternational Journal of Antennas and

Propagation

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International Journal of

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DistributedSensor Networks

International Journal of

4 International Journal of Antennas and Propagation

and 119864[ℎ2119899] = 1 Therefore the average power of the received

signal is adopted from [21] as follows

119864 [119875119877] =

119909 (119905)2

119873

sdot 119864[

119873

sum

119899=1

1003816100381610038161003816ℎ1198991003816100381610038161003816

2 10038161003816100381610038161198601198991003816100381610038161003816

2119890119895(120596119888119905+120601119899)

119873

sum

119898=1

10038161003816100381610038161198601198981003816100381610038161003816

2119890119895(120596119888119905+120601119898)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 119864 [1003816100381610038161003816ℎ1198991003816100381610038161003816

2 100381610038161003816100381611986011003816100381610038161003816

2 100381610038161003816100381611986021003816100381610038161003816

22R (119890

119895(120596119888119905minus120596119888119905+1206011minus1206012))])

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] =

119909 (119905)2

119873(119873 +

119873 (119873 minus 1)

2

sdot 2119864 [cos (1206011minus 1206012)]) + 119864 [

1003816100381610038161003816119882119875119861 (119905)1003816100381610038161003816

2] = 119909 (119905)

21

+ (119873 minus 1) 119864 [cos (1206011 minus 1206012)] + 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

= 119909 (119905)21 + (119873 minus 1)

sdot 119864 [cos (1206011) cos (120601

2) minus sin (120601

1) sin (120601

2)]

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2] = 119909 (119905)

21 + (119873 minus 1) 119864 [cos (120601119894)]

2

+ 119864 [1003816100381610038161003816119882119875119861 (119905)

1003816100381610038161003816

2]

(4)

where 120601119894is a phase offset among nodes which is uniformly

distributed around 0∘ between [minus120587 120587] interval Figure 3shows a normalized beamforming gain 119864[119875

119877] which is

obtained using (4) upon varying the numbers of nodes inthe networks The ldquoperfect distributed beamformingrdquo termmeans that every received signal at base station is perfectlyaligned Note that themaximumbeamforming gain is 1 as thesignal normalization is performed at base station For the caseof having no phase offset (or 120601

119894= 0∘ here defined as a perfect

distributed beamforming) the resulting beamforming gainis of maximum value Otherwise the systems experience anunstable low beamforming gain According to these resultsphase synchronization is a key success for distributed beam-forming which is the focus of this paper As mentioned in theIntroduction various phase synchronization techniques havebeen proposed with a common drawback of sending a largenumber of feedback signals and the requirement of additionalhardware Alternatively this paper proposes a phase synchro-nization technique avoiding any feedback signal from basestation and any interaction between transmitting nodes Theproposed nonfeedback phase synchronization is described inthe next section

3 Proposed Phase Synchronization

In this section we propose a nonfeedback distributed beam-forming which does not require any interaction betweennodes and feedback signals The phase synchronization isperformed at base station At base station there are twomajor parts (1) RF front end operationwhich converts the RF

0 10 20 30 40 50 60 70 80 90 1000

02

04

06

08

1

12

14

Number of nodes

Nor

mal

ized

bea

mfo

rmin

g ga

in

Perfect distributed beamformingNo phase synchronization

Figure 3 Beamforming gain in case of perfect beamforming and nophase synchronization

signals to the baseband signals and (2) proposed nonfeedbacktechnique procedure which performs signal extraction andphase synchronization Moreover the performance compar-ison between the proposed technique and some existingtechniques is presented afterwards

When base station receives signals from 119873 transmittingnodes the composite received signal 119884

119877(119905) is processed in

the RF front end receiver in order to convert a pass-bandreceived signal to digital baseband signal The RF front endreceiver consists of RF demodulator band-pass filter Analog-to-Digital Converter (ADC) and Digital Downconverter(DDC)Then the proposed nonfeedback technique is appliedto the output baseband signal 11988410158401015840(119896) which includes asignal extraction and a weighting process The mentionedprocedures are detailed as follows

31 RF Front End Operation In RF front end we assume thatthe received signal amplitude119860

119899 is equal to 1The combined

received signals at base station when they are coming from alltransmitting nodes can be written as

119884119877 (119905) = R

119873

sum

119899=1

1199101015840

119899(119905) + 119882 (119905)

= R119909 (119905)

119873

sum

119899=1

ℎ119899119890119895(120596119899119905+120601119899)+119882(119905)

= 119909 (119905)

119873

sum

119899=1

[ℎ119899cos (120596

119899119905 + 120601119899)]

+ 119908119868 (119905) cos (120596119888119905) minus 119908119876 (119905) sin (120596119888119905)⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

R119882119875119861(119905)

(5)

when 1199101015840119899(119905) is a signal from the 119899th node 120596

119899= 120596119888+ Δ120596119899in

which 120596119888is a carrier signal frequency and Δ120596

119899is a frequency

offset 120601119899= 1206010+ Δ120601119899 where 120601

0is a nominal phase Δ120601

119899is a

International Journal of Antennas and Propagation 5

Base station

123N

1

2

N minus 1

N

Phase shift of each retransmissionSymbol period

middot middot middot

middot middot middot

middot middot middot

middot middot middot

middot middot middot

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej180∘

ej180∘

ej180∘

Y998400998400N(k)

Figure 4 Proposed phase shifting pattern

phase offset and119882(119905) is AWGN which consists of in-phase119908119868(119905) and quadrature 119908

119876(119905) components Then the received

signal that appeared in (5) is modulated and downconvertedusing RF modulator and DDC in order to obtain the digitalbaseband signal as follows

11988410158401015840(119896) = 119909 (119896)

119873

sum

119899=1

ℎ119899119890minus119895(ΔΩ

119899119896+Φ119899)+119882119861119861 (119896) (6)

where 119896 is a sampling time variableΔΩ119899is a frequency offset

and Φ119899is a phase offset at 119896th sampling time In addition

119882119861119861(119896) is a baseband AWGN and119882

119861119861(119896) = 119908

119868(119896) + 119895119908

119876(119896)

Equation (6) presents the baseband signal which can bedegraded by the phase offset Φ

119899 Thus the baseband signal

of each transmitting node requires a phase synchronization inorder to obtain the maximum beamforming gain Therefore11988410158401015840(119896) is passed to the procedure of proposed nonfeedback

technique which is presented in the following section

32 Proposed Nonfeedback Technique In this paper a non-feedback technique is proposed employing a signal extractionat base station to achieve a phase synchronization for dis-tributed beamforming networks The received signal 11988410158401015840(119896)mentioned earlier is processed in two steps Spatial-TemporalExtraction and OptimumWeighting

Step 1 (Spatial-Temporal Extraction) The combined signalthat appeared in (6) needs to be extracted before performinga phase synchronization The steps of signal extraction are asfollows

All transmitting nodes in the networks transmit thesimilar message to the base station at the same time Thistransmission repeats only 119873 times where 119873 stands for thenumber of transmitting nodes in the networks For eachretransmission the transmitting nodes adjust their phasesaccording to a fixed phase adjustment pattern matrix A

119873119873

which can be obtained by the following algorithm Note thatthis retransmission does not require any interaction amongtransmitting nodes

(1) At 1st symbol period all transmitting nodes send thesignal without any phase adjustment

(2) At the 119899th symbol period where 119899 = [2 3 119873]only (119899 minus 1)th transmitting node shifts its phase by180∘ For example at the 2nd symbol period the 1stnode shifts its phase by 180∘ and at the 3rd symbolperiod the 2nd node shifts its phase by 180∘ as seenin Figure 4 which shows the summary of proposedphase adjustment pattern

(3) Repeat phase shifting pattern until119873 symbol period

According to the proposed phase adjustment pattern aspresented above we obtain the fixed coefficient matrix A

119873119873

by retransmitting signal for119873 times as shown in the followingequations

A119873119873

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]

(7)

After having completed 119873 retransmissions all receivedsignals are simultaneously arranged to form vector Y10158401015840

119873as

follows

Y10158401015840119873(119896) = A

119873119873y119873 (119896) +W

119861119861119873 (119896) (8)

or

[[[[[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

119873(119896)

]]]]]]]]]]

]

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A119873119873

[[[[[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

119910119873 (119896)

]]]]]]]]]

]

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Chemical EngineeringInternational Journal of Antennas and

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International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 5

Base station

123N

1

2

N minus 1

N

Phase shift of each retransmissionSymbol period

middot middot middot

middot middot middot

middot middot middot

middot middot middot

middot middot middot

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej0∘

ej180∘

ej180∘

ej180∘

Y998400998400N(k)

Figure 4 Proposed phase shifting pattern

phase offset and119882(119905) is AWGN which consists of in-phase119908119868(119905) and quadrature 119908

119876(119905) components Then the received

signal that appeared in (5) is modulated and downconvertedusing RF modulator and DDC in order to obtain the digitalbaseband signal as follows

11988410158401015840(119896) = 119909 (119896)

119873

sum

119899=1

ℎ119899119890minus119895(ΔΩ

119899119896+Φ119899)+119882119861119861 (119896) (6)

where 119896 is a sampling time variableΔΩ119899is a frequency offset

and Φ119899is a phase offset at 119896th sampling time In addition

119882119861119861(119896) is a baseband AWGN and119882

119861119861(119896) = 119908

119868(119896) + 119895119908

119876(119896)

Equation (6) presents the baseband signal which can bedegraded by the phase offset Φ

119899 Thus the baseband signal

of each transmitting node requires a phase synchronization inorder to obtain the maximum beamforming gain Therefore11988410158401015840(119896) is passed to the procedure of proposed nonfeedback

technique which is presented in the following section

32 Proposed Nonfeedback Technique In this paper a non-feedback technique is proposed employing a signal extractionat base station to achieve a phase synchronization for dis-tributed beamforming networks The received signal 11988410158401015840(119896)mentioned earlier is processed in two steps Spatial-TemporalExtraction and OptimumWeighting

Step 1 (Spatial-Temporal Extraction) The combined signalthat appeared in (6) needs to be extracted before performinga phase synchronization The steps of signal extraction are asfollows

All transmitting nodes in the networks transmit thesimilar message to the base station at the same time Thistransmission repeats only 119873 times where 119873 stands for thenumber of transmitting nodes in the networks For eachretransmission the transmitting nodes adjust their phasesaccording to a fixed phase adjustment pattern matrix A

119873119873

which can be obtained by the following algorithm Note thatthis retransmission does not require any interaction amongtransmitting nodes

(1) At 1st symbol period all transmitting nodes send thesignal without any phase adjustment

(2) At the 119899th symbol period where 119899 = [2 3 119873]only (119899 minus 1)th transmitting node shifts its phase by180∘ For example at the 2nd symbol period the 1stnode shifts its phase by 180∘ and at the 3rd symbolperiod the 2nd node shifts its phase by 180∘ as seenin Figure 4 which shows the summary of proposedphase adjustment pattern

(3) Repeat phase shifting pattern until119873 symbol period

According to the proposed phase adjustment pattern aspresented above we obtain the fixed coefficient matrix A

119873119873

by retransmitting signal for119873 times as shown in the followingequations

A119873119873

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]

(7)

After having completed 119873 retransmissions all receivedsignals are simultaneously arranged to form vector Y10158401015840

119873as

follows

Y10158401015840119873(119896) = A

119873119873y119873 (119896) +W

119861119861119873 (119896) (8)

or

[[[[[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

119873(119896)

]]]]]]]]]]

]

=

[[[[[[[[[[

[

1 1 sdot sdot sdot 1 1

119890119895180∘

1 sdot sdot sdot 1 1

1 119890119895180∘

sdot sdot sdot 1 1

d

1 1 sdot sdot sdot 119890119895180∘

1

]]]]]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A119873119873

[[[[[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

119910119873 (119896)

]]]]]]]]]

]

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

6 International Journal of Antennas and Propagation

+

[[[[[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

119882119861119861119873 (119896)

]]]]]]]]]

]

(9)

where Y10158401015840119873(119896) is the vector of combined received signal

obtained by retransmitting signal for 119873 times y119873(119896) is

the vector of transmitted message from 119899th node 119899 =

[1 2 119873] andW119861119861119873

(119896) is the vector of baseband AWGNEquation (9) confirms that the retransmitting signal ofY10158401015840

119873(119896)

provides the coefficient matrix A119873119873

From the combined signal vector that appeared in (8)Y10158401015840119873(119896) can be extracted by applying an inverse matrix Aminus1

119873119873

as seen in (10) Thus we can extract the combined signal byutilizing the inverse matrix as follows

Aminus1119873119873

Y10158401015840119873(119896) = Aminus1

119873119873A119873119873

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) (10)

Then the expression of y119873(119896) that appeared in (8) becomes

y119873 (119896) + Aminus1

119873119873W119861119861119873 (119896) = Aminus1

119873119873Y10158401015840119873(119896) (11)

Equation (11) represents the extracted signals from each nodey119873(119896)+Aminus1

119873119873W119861119861119873

(119896)which can be extracted by applying theproposed Aminus1

119873119873 Equation (11) also reveals that the extracted

signals are affected by baseband AWGN Aminus1119873119873

W119861119861119873

(119896)This proposed technique will be demonstrated through

an example of a beamforming network that is composed offour transmitting nodes119873 = 4 Note that each transmittingnode and base station are equipped with a single antennaelement Also all nodes are stationary and the operatingfrequency is 245GHzThe received signals at base station areassumed to be equal to 1 having SNR of 20 dB referring tothe minimum SNR of commercial Wi-Fi networks [26] Thisconfirms the feasibility of proposed concept when operatedin real circumstances having rich noise signal The phaseoffset is distributed over minus120587 to 120587 The utilized frequencyoffset is referred to as a typical frequency offset of clockcrystals which is 1ndash20 parts per million (ppm) [24] As theoperating frequency is 245GHz the maximum frequencyoffset is 245GHz times 20 times 10minus6 = 49 kHz and the minimumfrequency offset is 245GHz times 1 times 10minus6 = 245 kHz Thus thefrequency offset is distributed over minus49 kHz to 49 kHz Asthe systems are stationary the effect of fading channel is nowneglectedThus the received signal amplitude from all nodesis assumed to be 1 As the focus of this paper is only phasesynchronization the perfect timing synchronization acrossall nodes is assumed in this system

In case of 119873 = 4 expressions (8) can be rewritten asfollows

Y101584010158404(119896) = A

44y4 (119896) +W

1198611198614 (119896) (12)

or

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

=

[[[[[[

[

1 1 1 1

119890119895180∘

1 1 1

1 119890119895180∘

1 1

1 1 119890119895180∘

1

]]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

A44

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

(13)

where 119884101584010158401(119896) 119884101584010158402(119896) 119884101584010158403(119896) and 11988410158401015840

4(119896) are the received signal

from retransmissions for the first second third and fourthtime respectively Also 119910

1(k) 119910

2(k) 119910

3(k) and 119910

4(k) are

the transmitting signal where 1198821198611198611

(k) 1198821198611198612

(k) 1198821198611198613

(k)and119882

1198611198614(k) are baseband AWGN Then we can extract the

original signal by utilizing the inverse matrix Aminus144

as follows

y4 (119896) + Aminus1

44W1198611198614 (119896) = Aminus1

44Y101584010158404(119896) (14)

or

[[[[[

[

1199101 (119896)

1199102 (119896)

1199103 (119896)

1199104 (119896)

]]]]]

]

+

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[

[

1198821198611198611 (119896)

1198821198611198612 (119896)

1198821198611198613 (119896)

1198821198611198614 (119896)

]]]]]

]

=

[[[[[

[

05 minus05 0 0

05 0 minus05 0

05 0 0 minus05

minus05 05 05 0

]]]]]

]⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

Aminus144

[[[[[[

[

11988410158401015840

1(119896)

11988410158401015840

2(119896)

11988410158401015840

3(119896)

11988410158401015840

4(119896)

]]]]]]

]

(15)

Figure 5(a) shows the original signals of 4 transmittingnodes which are separately sent to base station 119910

1(k) 1199102(k)

1199103(k) and 119910

4(k) As we can see in the figure the initial phase

of each signal is 45∘ 1081∘ minus1059∘ and minus255∘ Note thatthe phase of transmitting signals is random using uniformdistribution After all original signals are transmitted theyare destructively combined at base station Figure 5(b) showsthe 4 retransmissions of combined signals 11988410158401015840

1(k) 11988410158401015840

2(k)

11988410158401015840

3(k) and11988410158401015840

4(k) for the first second third and fourth time

respectively As we can see the maximum combination ofreceived signal cannot be achieved as their phases are notsuitably aligned Thus we propose an extraction of receivedsignal at base station by applying the inverse matrix asshown in (14) After having done the proposed extractionFigure 5(c) shows the 4 extracted signals y

4(119896)+Aminus1

44W1198611198614

(k)obtained from (14) or (15) As we can see in the comparisonbetween Figures 5(a) and 5(c) phases of extracted signalsare similar to original signals at 48∘ 1107∘ minus1043∘ and

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

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Shock and Vibration

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Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

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Electrical and Computer Engineering

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Advances inOptoElectronics

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Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Navigation and Observation

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DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 7

0 05 1 15 2minus15

minus1

minus05

0

05

1

15A

mpl

itude

(vol

tage

v)

Times (k)

Signal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9

(a) Four original transmitted signals

Am

plitu

de (v

olta

ge v

)

minus4

minus3

minus2

minus1

0

1

2

3

4

Sending 1st timeSending 2nd timeSending 3rd timeSending 4th time

0 05 1 15 2Times (k) times10minus9

(b) Combined retransmission signals

minus15

minus1

minus05

0

05

1

15

Am

plitu

de (v

olta

ge v

)

Extracted signal of node 1Extracted signal of node 2Extracted signal of node 3Extracted signal of node 4

0 05 1 15 2Times (k) times10minus9

(c) Four extracted signals

Figure 5 Simulation results from Spatial-Temporal Extraction

minus263∘ At this point phase offsets among those 4 signals stillremain in which a suitable phase synchronization techniqueis required next

Step 2 (Optimum Weighting) After we obtain the correctextracted signals as pointed out in previous step the signalsare sent to the weighting procedure In this process thephases of extracted signals will be synchronized by thefollowing simple algorithm The transmitted signal from the1st node is given to be a reference signal Then signals fromremaining nodes 119910

119899(k) will be weighted by shifting their

phases from 0∘ to 360∘ in order to find the best weightingcoefficients which provide the maximum combined signalstrength between the reference node 119910

1(k) and the remaining

nodes 119910119899(k) where 119899 = [2 3 119873] As a result the

synchronized signals have equal phases referring to thechosen reference signal Figure 6 presents the flow chart ofthe proposed phase synchronization concept In the processof finding the best weighting coefficients we can choose aweighting step size larger than 1∘ in order to reduce theprocessing time Figure 7 shows a normalized beamforminggain upon employing several weight steps varied from 1∘to 100∘ In this simulation we assume that the receivedsignal amplitude from all nodes is equal to 1 having SNR =20 dB the number of nodes is 20 and the phase offset isuniformly distributed over 0∘ to 360∘ As we can see in thisfigure a largeweighting step provides the lower beamforminggain This is because a large weighting step may skip theOptimum Weighting value However the weighting step ofsim10∘ provides a similar beamforming gain as employing a

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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DistributedSensor Networks

International Journal of

8 International Journal of Antennas and Propagation

Start

Endall weighted signals are combined

No

Yes

n = n + 1

n = N

Initial weight 120595 = 0∘ n = [2 N]

ywn(k) = y1(k) + ynj120595 + WBB(k)(k) middot e

and find the best weighting coefficient that provides the highest combined signal

Loop for weight 120595 = 0∘ 3600

Figure 6 Summary flow chart of proposed phase synchronization

0 10 20 30 40 50 60 70 80 90 100086

088

09

092

094

096

098

1

Nor

mal

ized

bea

mfo

rmin

g ga

in

Weight step (degrees (∘))

Figure 7 Normalized beamforming gain versus weighting step

smaller weighting from 1∘ to 10∘ Thus we can utilize theweighting step of 10∘ to reduce the processing time

After having done signal extraction and phase syn-chronization we obtain the gainfully combined signal in acontinuous time domain 119884opt(t) as shown in Figure 8 Wefinally obtain the maximum 4 times of transmitted signalamplitudes using the proposed techniques The phase ofcombined signal equals the phase of the reference node 119910

1(k)

Without a phase synchronization a lower beamforming gainis achieved due to the phase offset

The simulation results in this section also reveal thatthe proposed nonfeedback technique has an efficiency overthe one-bit feedback and zero-feedback techniques as theproposed technique requires a lower number of retransmis-sions comparing to the one-bit feedback and zero-feedbacktechniques The proposed nonfeedback technique provides

0 02 04 06 08 1 12 14 16 18 2minus5

minus4

minus3

minus2

minus1

0

1

2

3

4

5

Am

plitu

de (v

olta

ge v

)

Proposed techniqueWithout proposed techniqueSignal from node 1Signal from node 2Signal from node 3Signal from node 4

times10minus9Times (k)

Figure 8 Received signal at base station from 4 transmitting nodes

1198732119873 = 119873 beamforming gain per transmission while the

work presented in [18] has stated that the one-bit feedbackoffers 11987325119873 = 1198735 beamforming gain per transmissionwith the requirement of at least 5119873 retransmissions in orderto achieve 75 guarantee of maximum beamforming gainFor example considering employing 3 nodes in the networksthe proposed one and one-bit feedback offer gain per trans-mission of 323 = 3 and 35 = 06 respectively In additionfrom [24] in case of having 3 transmitting nodes the zero-feedback technique provides 3250 = 018 beamforming gainper transmission as it requires at least 50 retransmissions inorder to achieve asymp95 guarantee of maximum beamforminggain As we can see the proposed concept offers higherbeamforming gain per one transmission comparing to othertechniques

Thenext section presents the beamforming gain compari-son between the proposed technique and some existing phasesynchronization techniques

33 Performance Comparison In this section the averagebeamforming gains of some existing techniques and the pro-posed one are compared where the number of transmittingnodes is varied from 2 to 10 nodes Note that the numberof iterations for an average value is 100 In the simulationwe assume that the received signals at base station have unitamplitudes 119860

119899= 1 and SNR of 20 dB The random initial

phase of each node is uniformly distributed over minus120587 to 120587The effects of fading channel and Doppler are neglected Thenumber of retransmitting signals (retransmissions) is limitedto 50 Also this paper focuses on only phase synchronizationand the perfect timing and frequency synchronization acrossall transmitting nodes are assumed in this system For the

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Submit your manuscripts athttpwwwhindawicom

VLSI Design

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 9

2 3 4 5 6 7 8 9 1056789

1011121314151617181920

Number of nodes

Aver

age b

eam

form

ing

gain

(dB)

Perfect phase synchronizationNonfeedback (proposed)1-bit feedback

Time-slot round-tripMaster-slaveZero-feedback

Figure 9 Beamforming gain of proposed nonfeedback techniqueversus other techniques

proposed nonfeedback beamforming we utilize the weight-ing step of 10∘ Note that the reason of choosing the step sizehas been mentioned in the previous section

As seen in Figure 9 the obtained results show thatthe average beamforming gains for the cases of proposednonfeedback and time-slot round-trip are equal to the caseof perfect phase synchronization Although nonfeedback andtime-slot round-trip techniques are comparable in terms ofbeamforming gain the proposed nonfeedback technique ispreferable as it does not require any feedback from basestation while the time-slot round-trip technique requires areference signal transmitted back from base station More-over the proposed nonfeedback technique does not requireany interaction between nodes while time-slot round-triptechnique does Although the master-slave technique alsodoes not require any feedback from the base station it requiresan interaction between transmitting nodes This introducesa complexity to the systems Moreover the beamforminggain of the master-slave technique may be distorted by anuncompensated VCO phase drift This phase drift occurredby the internal oscillator noise and over time of phasecompensation in the open-loop mode while the slave nodesare transmittingThus the slaversquos carrier signals can be driftedout of phase [21]

According to the number of retransmissions which islimited to 50 the one-bit feedback and zero-feedback tech-niques provide lower beamforming gain compared with theproposed one This is because the one-bit feedback and zero-feedback technique require a large number of retransmissionsto achieve the maximum beamforming gain The one-bitfeedback technique requires the number of retransmissionsto be at least 5N retransmissions to achieve 75 guarantee ofmaximumbeamforming gain [18]Thus the one-bit feedbackrequires the number of retransmissions to be larger than 50to achieve the maximum beamforming gain upon having10 transmitting nodes 119873 = 10 Figure 9 shows that the 10

transmitting nodes for one-bit feedback technique provide(147 dB20 dB) times 100 = 735 of the maximum beamforminggain which is close to the numerical results of [18] Figure 9also presents that the one-bit feedback technique cannotprovide the maximum beamforming gain when 119873 gt 3 Thereason is that only 50 retransmissions may be not enoughto achieve the maximum beamforming gain while the zero-feedback requires at least 50 and 250 retransmissions toachieve asymp95 of the maximum beamforming gain uponhaving 3 and 4 transmitting nodes respectively [24] Thatmeans only 50 retransmissions are not enough to achieve themaximum beamforming gain when119873 ge 3

In summary the proposed nonfeedback technique hasthe following advantages over other phase synchronizationtechniques It offers higher effective gain with lower numberof retransmissions Also it avoids interactions between trans-mitting nodes and also does not require any feedback signalfrom the base station

4 An Experimental Study of theProposed Nonfeedback DistributedBeamforming Technique

In a real circumstance the proposed nonfeedback distributedbeamforming can be affected by characteristic of communi-cation channel such as phase variation or fading Thereforethe experimental study of proposed techniques is consideredin order to validate the proposed technique A testbedconsisting of two transmitting nodes and one base stationwas developed under SDR technology We utilize a UniversalSoftware Radio Peripheral (USRP) as it provides high speedADCs DACs FPGA andUSB interface support [27 28]Theexperiments are separated into two parts (A) an experimentfor received signal power and (B) an experiment for BERThe first one is to prove if the proposed concept provides againfully combined signal at base station while the latter is toconfirm the enhancement of system performance in terms ofBER

41 An Experiment on Received Signal Power Figure 10shows the configuration of experiment setup to measure thereceived signal power A cosine wave is transmitted usingUSRP1 which includes two transmitting nodes nodes Aand B Note that XCVR2450 is employed as a daughterboard for both cases Then base station receives the trans-mitted signal and conveys it to laptop for data recordingWe use a single laptop in order to avoid the problem oftiming synchronization among transmitting nodes As thetransmitted cosine wave is very sensitive to frequency offsettwo daughter boards (or RF boards) on a single USPR1 arechosenTheUSRP1 is connected to a laptopwhich is operatedby Ubuntu 1004 Figure 11(a) presents the configuration oftransmitting nodes (A and B) which are placed at thesidewall Figure 11(b) presents the placement of base stationwhich is situated 7 meters away from the two transmittingnodes According to a distance between the base station andlaptop which is limited to about 2 meters (a maximum rangeof USB cable) we utilize a transmission line to extend a

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

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Active and Passive Electronic Components

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RotatingMachinery

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Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

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Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

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Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

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Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

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Navigation and Observation

International Journal of

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DistributedSensor Networks

International Journal of

10 International Journal of Antennas and Propagation

Daughter board A A

B BDaughter board

USRP1

Tx node A

Tx node BDaughter board

Daughter board

Base station

Figure 10 Configuration of measurement for received signal power

Transmitting

Transmittingnode A

node B

(a) Two transmitting nodes

Base stationTransmission

Transmitting nodes

range = 7m

(b) Base station

Figure 11 The configuration of the experiment on received signal power

communication range the transmission lines are connectedbetween USRP1 and antenna as shown in Figure 10 A loss ofused transmission line is minus162 dB Therefore the 7 meters isthe longest distance to ensure that the received signal is nottoo weak We cannot use a farther distance as the USRP isvery sensitive for the signal strength The antennas havinggain of 3 dBi are employed at both transmitting nodes andbase station

For the programming we utilize GNURadio Companion(GRC) version 374 which can build GNU Radio flow graphsusing a graphical user interface Figure 12(a) presents a blockdiagram of the transmitting nodes Note that the phase differ-ences between the two transmitting nodes are random by thenode locations as shown in Figure 11(a) The ldquoSignal Sourcerdquogenerates a cosine wave which has the setup parameters asfollows signal amplitude is 1 volt signal frequency is 1 kHzcarrier frequency is 245GHz and sampling rate is 250 kHzThen the signal is weighted by a ldquoPhase Shifterrdquo considered asa weighting coefficientThis weighting coefficient depends onthe proposed phase adjustment patterns shown in Figure 4

Finally the signals are transmitted by ldquoUHD USRP Sinkrdquowhere UHD is the USRP Hardware Driver compatible for allUSRPs Note that the transmitting gain of USRP parameterin GRC is 29 dB Note that the testbed has two transmittingnodes (N = 2)Thus the required number of transmission is 2times as the proposed nonfeedback beamforming techniquerequires onlyN transmissions to performbeamformingwhenN is the number of transmitting nodes Figure 12(b) presentsa block diagram of base station The received signal isobtained by ldquoUHD USRP Sourcerdquo Then the received signalis locked to the center frequency and downconverted tobaseband signal by a ldquoCostas Looprdquo The loop bandwidthof Costas Loop is 00065 radians per sample Finally thesignal is saved at ldquoFile Sinkrdquo The saved files are used foran offline processing to be performed for the proposedtechnique as shown in Figure 12(c)The saved files are loadedby ldquoFile SourcerdquoThe ldquoFile Source 1rdquo and ldquoFile Source 2rdquo areobtained at the 1st- and 2nd-time transmission respectivelyThen the ldquoExtractionrdquo proposed in Section 32 provides thetwo extracted signals which related to the signal transmitted

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 11

Table 1 Mean and standard deviation of measured average magnitude

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 0012 0026 0030 0030 0038 0026Standard deviation (v) 00022 00037 00042 00046 00051 00078

Signal Source Phase Shifter

Signal Source Phase Shifter

UHDUSRPSink

Transmitting node A

Transmitting node B

(a) Block diagram of transmitting nodes

UHDUSRPSource

File SinkCostas Loop

(b) Block diagram of base station

FileSource 1

FileSource 2

Extraction Weighting andcombining

Extracted signal A

Extracted signal B Output

signal

(c) Block diagram of the proposed technique

Figure 12 The programming block diagram for experiment on received signal power

from nodes A and B Finally the extracted signals areweighted and combined according to proposed weightingalgorithm discussed in Section 32

The measured results are presented in a histogram ofaverage combined magnitude at base station Note thatthis magnitude is average from 100-time data recordingFigure 13(a) shows the results in the case of only a singlenode (node A) that transmits a cosine wave to base sta-tion while Figure 13(b) is for the case when only node Btransmits a cosine wave to base station Figures 13(c) and13(d) show the average of combined magnitude when bothnodes A and B transmit a signal for the 1st time and 2ndtime respectively Then Figure 13(e) shows the output ofcombined signal when the proposed beamforming schemehas been performed But when the proposed scheme is off(without phase synchronization) the combined signal turnsto be lower as shown in Figure 13(f) As 100-time data ofexperiments is recorded Table 1 shows a mean value andstandard deviation of all cases The results present that theproposed technique provides an optimum gain as 0038 voltswith respect to the optimumbeamforming gain Note that theoptimumgain can be calculated by summation of the receivedsignal power from nodes A and B (0012 + 0026 = 0038)Thus the gain of proposed technique is significantly betterthan that without phase synchronization which provides asignal gain as only 0026 voltsMoreover a standard deviation

in case of proposed technique (120590 = 00051) is lower than thatin the casewhen the proposed scheme is off (120590 = 00078)Thisimplies that the proposed technique provides higher stabilityin terms of received signal power

The experimental results in this section validate that theproposed technique provides a gainfully combined signal atbase station However the power of received signal cannottotally guarantee the quality of the received data This isbecause the received signal can be affected by transmissionchannel such as fading noise interference and bit synchro-nization between the two transmitting nodes Therefore wefurther investigate the BER in the next section

42 An Experiment on Bit Error Rate A testbed for BERmeasurement is shown in Figure 14 In this experiment wetransmit the randombinary bits to a base stationThenumberof transmitting bits is 1 million which has a carrier frequencyat 245GHz The USRP is employed at base station and twoUSRPB100s are employed as the transmitting nodes nodesA and B SBX-120 is used as the daughter boards forUSRPB100 All USRPs are connected to a laptop for datarecording The two transmitting nodes are placed at thesidewall as shown in Figure 15 The configuration of the basestation is the same as the one for previous experiment shownin Figure 11(b) Also all losses in transmission line have beencalibrated before performing the measurement

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

12 International Journal of Antennas and Propagation

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 002 004 006 008 010

5

10

15

20

25

30

Average magnitude (V)

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 13 A histogram of average magnitude

Figure 16(a) shows a block diagram of the transmittingnodes The ldquoSignal Sourcerdquo generates the random binary bitswhich has sample rate of 250 kHzThen the signal is encodedby ldquoPacket Encoderrdquo In this block the signal is wrappedinto a packet which provides a payload length with a headeraccess code and preambleThe setup parameters of this blockare samplessymbol of 2 and bitssymbol of 1 Afterwardsthe encoded signals are demodulated by the ldquoDifferential

Binary Phase Shift Keying (DBPSK)rdquo modulation The setupparameter of this block is that an excess bandwidth (orroll-off factor) is 035 and Gray code is enabled Then themodulated signal is weighted by a ldquoPhase Shifterrdquo consideredas weighting coefficient Note that the mentioned weightingscheme has been proposed in Figure 4 Finally the signals aretransmitted at ldquoUHD USRP Sinkrdquo The transmitted gain inGRC is 31 dBwhich related to the optimum transmitting gain

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 13

Daughter board B

Daughter board A

Base station

Daughter board

USRP B100

Daughter board

USRP B100

Transmitting node B

Transmitting node A

Figure 14 Configuration of measurement for BER

Transmittingnode B

Transmittingnode A

Figure 15 Two transmitting nodes employing USRP B100

Figure 16(b) shows a block diagram of base station includingldquoUHDUSRP Sinkrdquo and ldquoFile Sinkrdquo In the offline processingthe saved files are extracted weighted and combined asshown in Figure 16(c) After that the combined signalsare demodulated using ldquoDBPSK Demodulationrdquo The setupparameters of this block are as follows an excess bandwidth(or roll-off factor) is 035 frequency lock loop bandwidth is00628 radians per sample phase recovery loop bandwidthis 00628 timing recovery loop bandwidth is 00628 radiansper sample and Gray code is enabled Finally a demodulatedsignal is decoded by ldquoPacket Decoderrdquo

The measurement results are presented in a histogramof BER where 100-time recorded data has been averagedNote that the measured BER employing USRP is relativelysensitive with noise Thus the major portion of measuredBER is the optimum case as 00 or the worst case as 05Note that BER = 00 means that there is no bit error at allwhile BER = 05 means that bit error turns out to be a halfof transmitted bits for example bit error is 500000 upontransmitting 1 million bits Figure 17(a) presents the BER

in the case of transmitting data from only node A whileFigure 17(b) presents the case when only node B transmitsthe data The results show that transmitting data from only asingle node provides a low performance in terms of BER theportion of BER = 00 which is only 11100 in the case of onlynode A and the portion of BER = 00 which is only 22100in the case of only node B Then the proposed techniqueis applied in order to enhance the BER Figures 17(c) and17(d) show the BER value at the base station when the twonodes transmit data at the 1st and 2nd time respectivelyFigure 17(e) shows the BER of combined signal at base stationwhen the proposed technique has been applied Figure 17(f)shows the BER for the case without the proposed techniqueThe results present that the proposed technique provides alower BER than the case of transmitting signal from a singlenode and when the proposed technique is not applied Theportion of BER = 00 in case of using the proposed techniqueis 45100 The portion of BER = 00 is only 25100 17100and 14100 when the two nodes transmit data at the 1st and2nd time and without phase synchronization respectivelyTable 2 shows a mean and standard deviation BER of allcases as the experiments have been recorded for 100 timesThe results in this table confirm that the proposed techniquemakes the system BER lower comparing to other cases Themean BER of the proposed technique is only 027 whilethe mean BER values in case of transmitting signal froma single node A and a single node B are 043 and 039respectively The mean BER is 035 039 and 040 when thetwo nodes transmit data at the 1st and 2nd times and withoutphase synchronization respectively The BER performanceobtained in the experiment is considered high as the receivedsignals are too weak as shown in Table 1 However the BERperformance of proposed technique is significantly lowerthan the BER performance of using a single node (1 times 1 Aand 1 times 1 B) and without phase synchronization (2 times 1 off)Therefore the experimental results in this section validate

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

14 International Journal of Antennas and Propagation

Table 2 Mean and standard deviation of measured BER

1 times 1 A 1 times 1 B 2 times 1 1st 2 times 1 2nd 2 times 1 on 2 times 1 offMean (v) 043 039 035 039 027 040Standard deviation (v) 016 021 022 020 025 019

Signal Source Packet Encoder DBPSK modulation Phase Shifter

Transmitting node A

Transmitting node B

UHD USRP Sink

Signal Source Packet Encoder DBPSK modulation Phase Shifter UHD USRP Sink

(a) A block diagram of the transmitting nodes

UHDUSRPSource

File Sink

(b) A block diagram of the basestation

File Source 2

File Source 1

Extraction Weighting andcombining

Packet Decoder

DBPSK Demodulation

Extracted signal A

Extracted signal B Output

signal

(c) A block diagram of the proposed technique

Figure 16 The programming block diagram experimental BER

the proposed technique that it can be utilized to realize adistributed beamforming network with an optimum gain andlower BER

5 Conclusion

This paper has proposed an alternative phase synchronizationtechnique so-called nonfeedback distributed beamformingtechniqueThe proposed nonfeedback requires a lower num-ber of retransmissions comparing to the one-bit feedbackand zero-feedback techniques Also the proposed nonfeed-back does not require any feedback signal or interactionbetween transmitting nodes Using this technique phasesynchronization can be accomplished at base station insteadof mobile terminals From simulation results the proposed

nonfeedback technique provides a high beamforming gaincompared with some existing phase synchronization tech-niques Also the proposed nonfeedback technique has beenanalyzed under real indoor environment The measuredresults have revealed that the proposed technique providesthe optimum beamforming gain Also it can enhance thesystem performance by lowering a Bit Error Rate (BER)comparing to the case when the phase synchronization is notapplied

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 15

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(a) Transmitting signal from node A (1 times 1 A)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(b) Transmitting signal from node B (1 times 1 B)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(c) Transmitting signal from two nodes at the 1st transmission (2 times 11st)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(d) Transmitting signal from two nodes at the 2nd transmission (2 times1 2nd)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(e) Transmitting signal with proposed technique (2 times 1 on)

0 02 04 06 08 10

10

20

30

40

50

60

70

80

BER

Freq

uenc

y

(f) Transmitting signal without phase synchronization (2 times 1 off)

Figure 17 A histogram of Bit Error Rate

Acknowledgments

This work was supported in part by The Royal GoldenJubilee PhD (RGJ-PhD) Program no 1QTS52B1BXXand Suranaree University of Technology Thailand

References

[1] P Sriploy P Uthansakul andMUthansakul ldquoEffect of path losson the distributed beamforming forWireless Sensor Networksrdquo

in Proceedings of the 10th International Conference on Elec-trical EngineeringElectronics Computer Telecommunicationsand Information Technology (ECTI-CON rsquo13) pp 1ndash4 KrabiThailand May 2013

[2] C A Balanis AntennaTheory Analysis and Design JohnWileyamp Sons New York NY USA 2nd edition 1997

[3] P Meerasri P Uthansakul and M Uthansakul ldquoSelf-interference cancellation-based mutual-coupling model forfull-duplex single-channel MIMO systemsrdquo International

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

16 International Journal of Antennas and Propagation

Journal of Antennas and Propagation vol 2014 Article ID405487 10 pages 2014

[4] A Innok P Uthansakul and M Uthansakul ldquoAngular beam-forming technique for MIMO beamforming systemrdquo Interna-tional Journal of Antennas and Propagation vol 2012 ArticleID 638150 10 pages 2012

[5] M Uthansakul and M E Bialkowski ldquoInvestigations into awideband spatial beamformer employing a rectangular array ofplanar monopolesrdquo IEEE Antennas and Propagation Magazinevol 47 no 5 pp 91ndash99 2005

[6] C Bunsanit P Uthansakul and M Uthansakul ldquoRefinementmethod for weighting scheme of fully spatial beamformerrdquoInternational Journal of Antennas and Propagation vol 2012Article ID 359847 13 pages 2012

[7] P Uthansakul A Innok and M Uthansakul ldquoOpen-loopbeamforming technique for MIMO system and its practicalrealizationrdquo International Journal of Antennas and Propagationvol 2011 Article ID 723719 13 pages 2011

[8] F Rayal ldquoWhy have smart antennas not yet gained tractionwithwireless network operatorsrdquo IEEE Antennas and PropagationMagazine vol 47 no 6 pp 124ndash126 2005

[9] T Kaiser ldquoWhen will smart antennas be ready for the marketPart Irdquo IEEE Signal Processing Magazine vol 22 no 2 pp 87ndash92 2005

[10] M Kosanovic and M Stojcev ldquoSensor node lifetime prolong-ingrdquo in Proceedings of the 20th Telecommunications Forum(TELFOR rsquo12) pp 178ndash181 Belgrade Serbia November 2012

[11] Y T Lo ldquoA mathematical theory of antenna arrays withrandomly spaced elementsrdquo IEEETransactions onAntennas andPropagation vol 12 no 3 pp 257ndash268 1964

[12] H Ochiai P Mitran H V Poor and V Tarokh ldquoCollaborativebeamforming for distributed wireless ad hoc sensor networksrdquoIEEE Transactions on Signal Processing vol 53 no 11 pp 4110ndash4124 2005

[13] P Sriploy P Uthansakul and M Uthansakul ldquoThe optimumnumber of nodes and radius for distributed beamforming net-worksrdquo ECTI Transactions on Electrical Engineering Electronicsand Communications (EEC) vol 13 no 2 pp 35ndash47 2014

[14] S Amini D-J Na and K Choi ldquoPerformance comparisonbetween distributed beamforming and clustered beamformingrdquoin Proceedings of the 11th International Conference on Informa-tion Technology NewGenerations (ITNG rsquo14) pp 174ndash179 IEEELas Vegas Nev USA April 2014

[15] L Yang K A Qaraqe E Serpedin and M-S Alouini ldquoPer-formance analysis of distributed beamforming in a spectrumsharing systemrdquo IEEETransactions onVehicular Technology vol62 no 4 pp 1655ndash1666 2013

[16] M Dohler and Y Li Cooperative Communications HardwareChannel and PHY JohnWileyamp SonsHobokenNJ USA 2010

[17] K Yao R E Hudson C W Reed D Chen and F LorenzellildquoBlind beamforming on a randomly distributed sensor arraysystemrdquo IEEE Journal on Selected Areas in Communications vol16 no 8 pp 1555ndash1567 1998

[18] R Mudumbai D R Brown III U Madhow and H V PoorldquoDistributed transmit beamforming challenges and recentprogressrdquo IEEE Communications Magazine vol 47 no 2 pp102ndash110 2009

[19] R Mudumbai J Hespanha U Madhow and G Barriac ldquoScal-able feedback control for distributed beamforming in sensor

networksrdquo in Proceedings of the IEEE International Symposiumon Information Theory (ISIT rsquo05) pp 137ndash141 Adelaide Aus-tralia September 2005

[20] R Mudumbai J Hespanha U Madhow and G BarriacldquoDistributed transmit beamforming using feedback controlrdquoIEEE Transactions on InformationTheory vol 56 no 1 pp 411ndash426 2010

[21] R Mudumbai G Barriac and U Madhow ldquoOn the feasibilityof distributed beamforming in wireless networksrdquo IEEE Trans-actions onWireless Communications vol 6 no 5 pp 1754ndash17632007

[22] D R Brown and H V Poor ldquoTime-slotted round-trip carriersynchronization for distributed beamformingrdquo IEEE Transac-tions on Signal Processing vol 56 no 11 pp 5630ndash5643 2008

[23] D R Brown III B Zhang B Svirchuk and M Ni ldquoAnexperimental study of acoustic distributed beamforming usinground-trip carrier synchronizationrdquo in Proceedings of the 4thIEEE International Symposium on Phased Array Systems andTechnology (Array rsquo10) pp 316ndash323 Waltham Mass USAOctober 2010

[24] A Bletsas A Lippman and JN Sahalos ldquoSimple zero-feedbackdistributed beamforming with unsynchronized carriersrdquo IEEEJournal on Selected Areas in Communications vol 28 no 7 pp1046ndash1054 2010

[25] G Sklivanitis andA Bletsas ldquoTesting zero-feedback distributedbeamforming with a low-cost SDR testbedrdquo in Proceedings ofthe 45th Asilomar Conference on Signals Systems and Comput-ers (ASILOMAR rsquo11) pp 104ndash108 Pacific Grove Calif USANovember 2011

[26] Wi-Fi Define Minimum SNR Values for Signal CoverageMay 2015 httpwwwenterprisenetworkingplanetcomnetsparticlephp3747656

[27] C Clark Software Defined Radio With GNU Radio and USRPMcGraw-Hill New York NY USA 2008

[28] Ettus Research Company April 2015 httpwwwettuscom

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of