Link-level Performance of FM-UWB in the Interfered IEEE802.15.6 Channel

Harri Vii�alaCentre for Wireless Communications,

University of OuluP.O. Box 8000

Oulu, Finland FI-90014harri.vii�ala@gmail.com

Ma�i HamalainenCentre for Wireless Communications,

University of OuluP.O. Box 8000

Oulu, Finland FI-90014ma�i.hamalainen@oulu.�

Jari Iina�iCentre for Wireless Communications,

University of OuluP.O. Box 8000

Oulu, Finland FI-90014jari.iina�i@oulu.�

ABSTRACT�is paper provides simulation results of the frequency modulatedultra wideband (FM-UWB) system with channel coding in an in-terfered IEEE 802.15.6 channel. �e paper discusses a FM-UWBreceiver structure and presents the developed simulator model. �ereceiver applies a delay-line modulator followed by an amplitudemodulation projection detection. A channel model used is the IEEE802.15.6 channel model 3 for an on-body link. An interference ismodeled as a colored Gaussian noise presenting an in-band IEEE802.15.4 interferer. �e simulation results show that the FM-UWBsystem can tolerate as low as −6 dB signal-to-interference powerratios (SIR) in the studied scenarios.

CCS CONCEPTS•Networks → Network simulations; Network performanceanalysis;

KEYWORDSFM-UWB, Interference, Simulation, WBANACM Reference format:Harri Vii�ala, Ma�i Hamalainen, and Jari Iina�i. 2017. Link-level Perfor-mance of FM-UWB in the Interfered IEEE 802.15.6 Channel . In Proceedingsof International Conference on Body Area Networks, Dalian, People’s Republicof China, September 2017 (BODYNETS’17), 4 pages.DOI: 10.475/123 4

1 INTRODUCTIONTypically, wireless body area networks (WBAN) refer to a wirelesssystem of devices carried by a human or animal. �e communica-tion architecture can be divided into three communication tiers:intra-WBAN (personal), inter-WBAN (local) and beyond-WBAN(global) based on the proximity of occurred communication arounda body. Consequentially, the intra-WBAN can be separated to in-body, on-body and o�-body communications.

Wireless communications around or vicinity of a human bodyis very challenging. A radio signal propagates around a body via

di�ractions and re�ections from extremities and surrounding envi-ronment. A direct signal propagating through a body is stronglya�enuated. In addition, movement of extremities, variation in tissuepermi�ivities and shape of a body involve. [3, 7, 13, 18]

�e IEEE 802.15.6 standard de�nes narrowband, ultra wideband(UWB) and human body communication (HBC) physical (PHY)layers to be used in the intra-WBAN communication [9]. Fromthese, UWB communications provides robustness in a dense mul-tipath environment and a low power spectral density technologyfor WBAN [11]. �e IEEE 802.15.6 standard de�nes two UWB PHYlayers, i.e., impulse radio UWB (IR-UWB) and frequency modu-lated ultra wideband (FM-UWB) [9]. FM-UWB is a very simpleUWB approach for low and medium data rates exploiting a dou-ble frequency modulation [6]. It applies a low-modulation indexfrequency shi� keying (FSK) followed by a high modulation indexfrequency modulation (FM) to achieve a constant envelope UWBsignal in the frequency domain. FM-UWB can be considered asan analog implementation of a spread-spectrum system, where amodulation index is equal to a spreading gain of a spread spectrumsystem. �e performance of FM-UWB is studied, e.g., in [4–6, 15–17]. As far as we know, the FM-UWB system is not investigatedwith Bose-Chadhuri-Hocquenghem (BCH) encoding in an inter-fered channel before. �is paper contributes simulation results ofthe FM-UWB system with BCH encoding as de�ned in the IEEE802.15.6 standard [9]. In addition, the system performance is testedagainst an in-band interference.

�is paper is organized as follows. Section 2 introduces theconcept of the FM-UWB PHY layer. �e simulator model, channelmodel and interference are discussed in Section 3. It also describesthe simulation parameters. �e simulation results are given inSection 4, and the paper is concluded in Section 5.

2 FREQUENCY MODULATED UWBFM-UWB is a simple low data rate UWB technique utilizing a dou-ble frequency modulation [6]. Basically, it constitutes an analogimplementation of a spread spectrum system [2]. In FM-UWB, adata stream is modulated with a low-modulation index digital FSKfollowed by a high-modulation index FM. �e modulation index isde�ned as β = ∆f /fm , where ∆f is the frequency deviation andfm is the frequency of a modulating signal. �e modulation indexcan be chosen freely. A small index (β < 1) yields to a narrowbandFM signal, whereas a large index (β >> 1) generates a UWB signal.A FM signal sFM(t) modulated by a signalm(t) having a frequencyfm is expressed as [6]

sFM(t) = A sin (2π fc t − β cos (2π fmt) + φ0) , (1)

where fc is the carrier frequency and φ0 is the arbitrary but time-independent constant phase. Due to wideband frequency modula-tion, the processing gain PGFM at a receiver can be expressed as theratio of the RF bandwidth BRF and subcarrier bandwidth BSUB as

PGFM = 10 log10(BRF

BSUB

). (2)

A received signal is demodulated with a delay-line FM demodu-lator followed by a FSK demodulator, as depicted in Figure 1.

LPF

LPF

90°

d

dt

d

dt

Uiω0

I = cos(φ)

Q = sin(φ)

+

–

Delay

element

(τ)

r(t)

Delay-line FM

demodulatorFSK demodulator

Figure 1: Block diagram of FM-UWB receiver

�e received signal is multiplied with the τ = Nd/(4fc ) delayedsignal, where Nd = 1, 3, 5 . . . [6] is selected so that it optimizesthe useful bandwidth of the demodulator BDEMOD [6]. �e usefulbandwidth is the maximum frequency range where a demodulatortransfer function is monotonic, de�ned as [6]

BDEMOD =2Nd

fc . (3)

�e output voltage of the FM demodulator VFM,out is given as [6]

VFM,out(t) = (−1)(Nd+1)/2A2

2 sin(Oπ

2 sin (ωmt)), (4)

where O = Nd (∆f /fc ) is the FM demodulator overdrive and ωmis the angular frequency of a modulating signal. �e overdrivede�nes the ratio between a deviation of the FM signal and a usefulbandwidth of the demodulator. From Equation (4), it can be seenthat when the overdrive is less than 1, the output voltage of thedemodulator does not exploit the full available dynamic range. �isoccurs when a deviation of the FM signal is less than one half ofthe useful bandwidth of the modulator [6].

�e output of the FM demodulator is followed by the FSK de-modulator employing an amplitude modulation (AM) projectiondetection [10]. It converts an input to zero frequency and subse-quently converts it to an AM wave with a di�erentiation stage and�nally uses the AM–projection detection.

3 SIMULATOR MODEL�e so�ware simulator implemented in Matlab was applied to studyperformance of the FM-UWB system. �e simulator consists of atransmi�er, receiver and channel. Random generated data bits areencoded by applying the BCH (63,51) code followed by FSK andFM demodulators. A transmi�ed signal s(t) propagates through amultipath channel having an impulse responseh(t). An interferencei(t) and additive white Gaussian noise (AWGN) n(t) are added tothe signal yielding to a received signal r (t)

r (t) = s(t) ∗ h(t) + i(t) + n(t), (5)where ∗ denotes the convolution operator.

�e receiver performs demodulation and decoding. Received bitsare compared to the data bits to have a bit error rate (BER) and frameerror rate (FER). Figure 2 gives a block diagram representation ofthe simulator model.

Data bitsFSK

Modulation

FM

Spreading Multipath

Channel

h(t)

FM

Demodulator

AWGN

n(t)

s(t)

r(t)FSK

DemodulatorReceived

bits

Interference

i(t)

BCH

Encoder

BCH

Decoder

Figure 2: Block diagram of the FM-UWB simulator

3.1 IEEE 802.15.6 Channel Model�e simulator model uses the IEEE 802.15.6 channel model for abody-to-body link [18]. �e model is based on the measurementscarried out in a hospital room where the applied frequency bandwas from 3.0 GHz to 11.0 GHz [1]. A subject was laying down in abed in a room of 5.0 m by 7.0 m, a receive antenna was placed on themiddle of a torso and a location of a transmit antenna varies froma head to an ankle. �e distance between the antennas was from176 mm to 984 mm. �e S21–parameter was measured 10 times foreach location.

A single cluster model where a path amplitude al is modeledby an exponential decay Γ with a Rician factor γ0 was extracted as[18]

10 log10 |al |2 ={0, l = 0γ0 + 10 log10

(exp

(− tlΓ

))+ S, l , 0,

(6)

where S is the normal distribution with zero-mean and a standarddeviation of σS . A path arrival time tl is modeled by the Poissondistribution [1]

p (tl |tl−1) = λ exp (−λ(tl − tl−1)) , (7)

where λ is the path arrival rate. �e number of arrival paths L aremodeled by the Poisson distribution, given as[1]

p(L) = LL exp(L)L! , (8)

where L is the average number of L. ϕl is modeled by a uniformdistribution over [0, 2π ). �e parameter values for the channelmodel are tabulated in Table 1.

Table 1: Parameters for the IEEE 802.15.6 CM3

Γ [ns] γ0 [dB] σS λ [1/ns] L

59.7 −4.60 5.02 0.541 38.1

3.2 Interference�e simulator applies the colored Gaussian noise (CGN) noise tomodel an in-band interference, i.e., thewhite Gaussian noise process�ltered by a bandpass �lter. Using the band-limited characteristic

of CGN, it is easy to implement speci�c interferences to the de-�ned frequencies with the desired bandwidths. �e interferencegeneration procedure is presented in [14].

�e bandpass �lter uses the raised cosine waveform to have abaseband impulse response hrc(t) having spectrum H rc(f ) as [12]

Hrc(f ) =

T 0 ≤ | f | < 1−α

2TT2

(1 + cos

(πTα

(| f | − 1−α

2T

)))1−α2T ≤ | f | ≤

1+α2T

0, | f | > 1+α2T ,

(9)where α is the roll-o� factor and T is the symbol time related tothe desired bandwidth BW. �e �lter is shi�ed to the desired centerfrequency fc as

hcgn(f ) = hrc(t) cos (2π fc t + φ) (10)

An in-band interference i(t) is generated by �ltering the whiteGaussian noise n(t) with a �lter hcgn having a frequency responseHcgn(f ) as illustrated in Figure 3. An interference power is adjustedaccording to a given signal-to-interference power ratio (SIR).

Hcgn(f)

fc, BW, α

i(t)n(t)

𝑆𝐼𝑅

Figure 3: Generation of the interference

3.3 Simulation Parameters�e simulation parameters are given in Table 2. �e �rst frequencyband from the high band group was selected as de�ned in theIEEE 802.15.6 standard, i.e., the channel number 3 at the 6489.6MHz center frequency and the bandwidth is 499.2 MHz [9]. �eIEEE 802.15.6 standard de�nes the uncoded bit rate of 250 kbpsfor FM-UWB as a mandatory rate [9]. When using the BCH code(63,51), the bit rate is 202.5 kbps. A modulating-carrier signal inFSK follows a triangular waveform. �e selected parameters givesNd = 27 and O = 1.038, which means that the FM demodulator isworking almost an optimal range [6].

Table 2: Simulation Parameters

Parameter ValueOperating frequency band 3Channel center frequency [MHz] 6489.6Bandwidth [MHz] 499.2Uncoded bit rate [kbps] 250BCH coding (63,51)Packet size, MPDU [octets] 50FSK waveform triangularModulation index, FSK 1

0 5 10 15 20 25 30 35

Eb/N

0 [dB]

10-5

10-4

10-3

10-2

10-1

100

Bit e

rro

r ra

te

no intf.

SIR=0

SIR=-3

SIR=-4

SIR=-5

SIR=-6

SIR=-9

Figure 4: BER of uncoded FM-UWB

�e applied channel model is the IEEE 802.15.6 channel model 3as introduced in Section 3.1. An in-band interference is consideredto be an IEEE 802.15.4 UWB system with the same bandwidth andcenter frequency as a victim IEEE 802.15.6 FM-UWB system, i.e.,6489.6 MHz and 499.2 MHz, respectively [8]. �e roll-o� factor forthe raised cosine �lter is 0.5.

4 SIMULATION RESULTS�is section gives the simulation results of the uncoded and codedFM-UWB system. �e performance of the system was simulated inthe IEEE 802.15.6 channel model 3 without or with an IEEE 802.15.4in-band interference. �e channel model 3 models a body-to-bodycommunication link.

Figure 4 and Figure 5 represent the uncoded BER and FER per-formances, respectively. �e do�ed line without markers is thereference performance without interference. A SIR value is reducedfrom 0 dB to -9 dB, i.e., the interference power is increased. As itcan be seen from the results, the performance saturates when SIRis equal or less than -5dB. Otherwise, a reliable communication linkcan be established in the IEEE 802.15.6 channel model 3 even whenan interferer is present.

�e BCH code improves the system performance, and hence, ittolerates higher SIR level as showed in Figures 6 and 7. �e codingimproves the system performance approximately 3 dB at the BERlevel of 10−5 without interference. �e higher interference powercan be tolerated with encoding, i.e., SIR of -6 dB. FM-UWB takesadvantage of a large receiver processing gain to mitigate an in-bandinterference. A�er the FM demodulator, the part of the demodu-lated interference signal falls inside the subcarrier bandwidth. �isexplains a good performance with an in-band interference.

5 CONCLUSIONFM-UWB is a UWB PHY layer included in the IEEE 802.15.6 stan-dard [9]. It is a simple technique where a double frequency modu-lation is applied to form an UWB waveform.

�is paper presented the simulation results of the FM-UWBsystem with and without the BCH encoding in the interfered IEEE

0 5 10 15 20 25 30 35

Eb/N

0 [dB]

10-3

10-2

10-1

100

Fra

me

err

or

rate

no intf.

SIR=0

SIR=-3

SIR=-4

SIR=-5

SIR=-6

SIR=-9

Figure 5: FER of uncoded FM-UWB

0 5 10 15 20 25 30 35

Eb/N

0 [dB]

10-5

10-4

10-3

10-2

10-1

100

Bit e

rror

rate

no intf.

SIR=0

SIR=-3

SIR=-6

SIR=-7

SIR=-8

SIR=-9

Figure 6: BER of coded FM-UWB

0 5 10 15 20 25 30 35

Eb/N

0 [dB]

10-3

10-2

10-1

100

Fra

me e

rror

rate

no intf.

SIR=0

SIR=-3

SIR=-7

SIR=-8

SIR=-6

SIR=-9

Figure 7: FER of coded FM-UWB

802.15.6 channel model 3. �e interference was modeled as CGNand it was intended to simulate an in-band IEEE 802.15.4 system. Asthe results showed, the FM-UWB system performs well in moderateinterference environments, i.e., SIR is less than -6 dB.

Future work could include modeling of the IEEE 802.15.4 inter-ference system in detail.

6 ACKNOWLEDGEMENTResponsible author would like to thank Ulla Tuominen and SeppoSaynajakangas Foundations for �nancing the work with personalgrants.

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