Modulation Switching in OFDM for WiMAXthrough Rayleigh fading channel using GNU Radio

B. Sivakumar Reddy, Dr. Lakshmi Boppana, Abhimanyu Srivastava and Ravi Kishore KodaliDepartment of Electronics and Communications Engineering

National Institute of Technology, Warangal, INDIA

E-mail: lakshmi@nitw.ac.in

Abstract- This paper presents modulation switching in

OFDM for WiMAX specifications by considering Raleigh

fading channel using GNU Radio companion. WiMAX tech-

nology is considered as the most eminent solution able to

allow a Broadband wireless Access (BWA) in metropolitan

areas with a merer installation and lower cost. OFDM is

the fundamental block in WiMAX PHY layer. Modulation

switching is performed in WiMAX to amend the network

performance in the case of Non-LoS communication. This

work proposes a script in Python to build a block which selects

the modulation scheme to be used in OFDM. The results

show that a selector block automatically adopts the modulation

scheme to be used, while an OFDM system is in operation.

Keywords: OFDM,OFDMA, WiMAX, GNU Radio.

I. INTRODUCTION

WiMAX [1] is a wireless broadband access technology that

offers performance similar to IEEE 802.11 networks with the

coverage and QoS of mobile networks. WiMAX can offer

broadband wireless access (BWA) up to 50 km for fixed

stations (IEEE 802.16d) [1], and 5-15 km for mobile stations

(IEEE 802.16e-2005)) [1]. In contrast, the 802.11 WLAN

standard provides access up to 30-100m only and guaran-

tees good QoS. First, modulation (64-QAM/16-QAM/QPSK)

and coding schemes guarantee steady signal strength as the

distance increases. Secondly, dynamic bandwidth allocation

(DBA) mechanism that supervises the network and when

interference or other detraction occurs to signal strength, the

base station provisions more bandwidth and power for the

affected stream.

OFDM (Orthogonal Frequency Division Multiplexing) [2] is

a method of digital modulation in which a signal is broken up

into several constricted band channels at different frequencies.

The primary benefits of OFDM over single-carrier strategies

is its ability to cope with harsh channel conditions without

complex equalization filters, as OFDM uses many slowly mod-

ulated constricted band signals, rather than one rapidly mod-

ulated wideband signal. This multi-carrier modulation nature

of OFDM makes advantageous in new wireless technologies

in order to reduce multipath fading and Inter Carrier/Symbol

Interference (ICI and ISI).

Adaptive modulation [3] is applied in wireless communica-

tions referring to the similarities of the modulation, coding and

other signal and protocol parameters to the stipulations on the

radio link. Modelling of wireless propagation environments

over fading channels, adaptive modulation systems demon-

strate great enhancements in performance compared to systems

that do not exploit channel knowledge at the transmitter. In

OFDM, various coding schemes are involved, such as QPSK,

BPSK, 8PSK, QAM8, QAM16, and QAM64. The choice

of a particular scheme is made based upon the values of a

certain parameter. Such parameters can be SNR, BER, channel

attenuation factor or anything else. The limits based on the

channel estimation decides which coding scheme needs to

be used for that particular received signal. The automatic

selection of the coding schemes is based on two different

approaches A selector block automatically performs this task.

Rayleigh fading models presume that the magnitude of a

signal that has passed through such a communication channel

varies arbitrarily, according to a Rayleigh distribution.

The open source software, GNU Radio Companion [4] is

used while implementing Software defined radio (SDR) [5].

In SDR, most of the signal processing work is performed

using software. For real time applications, a peripheral called

Universal Software Radio Peripheral (USRP) is used. Using

this USRP, we can transmit and receive the real time signals.

In between the transmitter and receiver, the signal processing

can be carried out using the GNU Radio companion software.

II. OFDM AND OFDMA- MODULATION SWITCHING

A. Orthogonal Frequency Division Multiplexing

OFDM [2] is a combination of modulation and multiplexing.

Multiplexing refers to independent signals, those produced

by different sources. The multi-carrier modulation divides

the wideband high data rate R bps incoming data stream

and with a passband bandwidth B into L constricted band

subsystems, each with rate R/L and passband bandwidth B/L,

each of which is then transmitted over a different orthogonal-

frequency sub-channel.

B. OFDMA- MODULATION SWITCHING

In this subsection the Adaptive Modulation and Coding

(AMC) systems exploiting the OFDMA (Orthogonal Fre-

quency Division Multiple Access) [7] capabilities are de-

scribed considering that the modulation order and coding rate

of each complex symbol ck associated with the corresponding

k-th sub-carrier (0 ≤ k ≤ 1023) and it could be changed

according to the physical channel state and the MCS is

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2013 International Conference on Advanced Electronic Systems (ICAES)

changed accordingly for all the sub-carriers of the slot. The

selection of the modulation is based on the physical channel

state information. In this work channel state approach is

considered.

1) Channel state technique: In this subsection two algo-

rithms named Maximum Throughput [8] and Target BLER [8]

based on the channel state approach are discussed. Each adap-

tation algorithm is basically characterized by five thresholds.

The modulation and coding scheme (MCS) remains the same

until the measured parameter, such as SNR, BER, Throughput

or attenuation parameter overcomes a threshold. This work

considers SNR as channel state parameter. The main difference

between the two channel state algorithms is how the thresholds

are derived: their values strongly influence the behaviour of

the adaptation algorithm in the performance measurement. The

Maximum Throughput (MT) algorithm aims to maximize the

system throughput without constraints on target Block error

probability. The Target BLER (TBLER) algorithm, aims to

respect a certain target BLER (Block Error Rate) due to

specific QoS requirements. The target value of the BLER can

be the same for each SNR, or may vary with it.

2) MT algorithm: The MT algorithm aspires to maximize

the overall link throughput by choosing for each SNR value

the MCS that allows the maximum throughput. Hence, the

transmission efficiency is enhanced but the performance in

terms of error rate could be worse, since no constraint on

the maximum acceptable error probability is introduced. The

objective is to choose, for each frame and SNR value, the

modulation order that maximize the total throughput. This

algorithm is more devoted to such services that request the

maximum achievable data rate, with a lower sensitivity to the

error probability, such as video streaming or VoIP services.

This algorithm has no degree of freedom and thresholds can

be found by comparing the throughput of the considered

transmission schemes [9]. Towards this end, the classical

throughput formulation provided in [10] has been modified

here to match the slot structure of the system: the throughput

(η) can be expressed as a function of the channel attenuation

factor α and used MCS for a fixed SNR, [8] given as:

η =4.(1− Pblock(α, SNR,M,Rc))Nc.Nsdlog2M.Rc)

4.Tf,

(1)

where M= 4, 16, 64 represents the modulation order, Pblock

stands for the block error probability related to each (mod-

ulation, coding rate) tuple, Nc is the number of sub-carriers

per user,Nsd is the number of OFDM symbols forming the

Downlink OFDMA subframe, Rcis the adopted coding rate

and Tf is the duration of the OFDMA frame. The symbol

error probability psym in a Rayleigh channel is given by [8]:

Psym(SNR, α,M) =4(√M − 1)√M

.Q(

√3× SNR× α2

M − 1)

− [2√M − 1√M

Q(

√3× SNR× α2

M − 1)]

3) TBLER Technique: The main feature of the TBLER

technique is that of keeping the error rate below a target limit,

hence guaranteeing a fixed level of QoS in terms of P-block.

Therefore this algorithm is more suitable to services requesting

a strict respect of a certain maximum error rate value as best

effort traffics. For each value of SNR we can derive BLER as

a function of α.

III. SIMULATION RESULTS

OFDM can be implemented by using a open source software

GNU Radio Companion as shown in 1. OFDM parameters are

configured and executed. In this paper a selector block has

been created by using Python scripting for the automatic se-

lection of modulation scheme in OFDM block. The parameter

value (k) can be decided by the channel, which is used. The

waveforms are matched when modulation scheme is selected

by selector block with the manual selection of scheme in

OFDM block.

As for the MT technique, whenever a dissimilar propagation

environment is conceived, it is straightforward to note that due

to different performance in terms of BLER, the thresholds have

different values. The specific modulation scheme selection has

been made, based on the two approaches.

Fig. 2. 64QAM modulation scheme with manual selection in OFDM Block.

Fig. 3. 64QAM modulation scheme accessed by Selector Block.

For the TBLER the system holds working with the lowest

MCS, QPSK modulation with coding rate 1/2, until the SNR

grants to encounter the error rate constraint, then the system

changes to higher transmission schemes since a better spectral

32013 International Conference on Advanced Electronic Systems

Fig. 1. GNU Radio schematic for 64QAM implementation

efficiency is achieved without loosing the target performance.

For the MT scheme the BLER performance are worse com-

pared to the TBLER technique due to the lack of control on

the error rate for the MT method.Based on the results, it can be stated that the selection of

the best adaptive algorithm depends on the specific system

requirements. While the MT technique represents a well

suited method for applications requesting a strict respect of

throughput, the TBLER technique is well suited for generic

data applications where a target error rate needs to be fulfilled

based on the requested QoS level. As a matter of fact, the

TBLER technique has been designed to offer the possibility

of switching the target value whenever the QoS requirement

of a specific application needs it.

IV. CONCLUSIONS

This paper has discussed the use of adaptive modulation

schemes in an OFDMA based system. Channel state approach

has been used by taking into account the channel behaviour in

terms of SNR. This technique has been implemented using two

different algorithms: i)TBLER, aiming to keep the error rate

under a specified value and to employ the MCS which guaran-

tees the highest efficiency while respecting the error target, and

ii) MT that aims to maximize the system throughput without

controlling the error rate performance. The paper shows the

creation of a selector block. This selector block automatically

selects a modulation scheme based on the value of a parameter

which can be different for various users as per the availability

of the estimation of a parameter. It can be either SNR, BER or

even channel attenuation factor. After estimating the value of

this parameter, the block selects automatically the modulation

scheme through a conditional flow of statements. Whichever,

condition gets satisfied, the corresponding modulation scheme

gets selected and the OFDM system is executed.

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