Versus Time-Hopping in Multiuser Ultra Wide and …dpopescu/papers/vtc05uwb.pdf · OFDMVersus Time-Hopping in Multiuser Ultra Wide and Communicaton Systems Dimitrie C. Popescu, Prasad

OFDM Versus Time-Hopping in MultiuserUltra Wide and Communicat on Systems

Dimitrie C. Popescu, Prasad Yaddanapudi, and Ramakoteswara KondadasuDepartment of Electrical EngineeringUniversity of Texas at San Antonio

6900 N Loop 1604 W, San Antonio, TX 78249-0669Contact e-mail: [email protected]

Abstract- In this paper we present a comparison betweenOFDM-based radios and time-hopping impulse radios for ultrawideband (UWB) communication systems operating in a mul-tiuser scenario, with additive white Gaussian noise and multipathchannels, and narrowband interference. The comparison of thetwo different schemes for generating UWB signals is based onresults obtained in numerical simulations, and is intended toassist the development of emerging UWB standards.

I. INTRODUCTION

UWB communication systems have generated increasinginterest among researchers lately because of their potentialfor providing high data rates. One of the challenges in thedesign of UWB systems comes from the large bandwidthrequired for transmission of UWB signals which is in theGigahertz range and is not available for exclusive use byUWB systems. Thus, the frequency range in which UWBsystems operate will contain slices of bandwidth in whichother existing communication systems operate, and underFCC regulations the UWB systems must appear as spuriousinterference to the already existing communication systemsthat operate in the frequency ranges below a few Gigahertz.Furthermore, UWB systems must be capable of operating inthe presence of various interfering signals coming from theexisting communication systems which may be considered asnarrowband interfering signals for the UWB signal.Two main approaches have been proposed in the litera-

ture for the design of UWB systems. One approach usesOrthogonal Frequency Division Multiplexing (OFDM), and isconsidered for the new IEEE 802.15 standard for generatingUWB signals [3]. We note that OFDM is currently used inbroadband wireless systems and has been incorporated in theIEEE 802.11 standard. The alternative approach is based onextremely short carrierless pulses (also referred to as impulsesor mono-pulses) transmitted at random or pseudo-random timeintervals by means of a time-hopping sequence, and is knownas time-hopping impulse radio (TH-IR) [7], [8].

In our paper, we consider both OFDM-based radios andTH-IR for UWB systems that operate in a multiuser scenario,and investigate their bit error rate (BER) performance in thepresence of additive white Gaussian noise, multipath channels,and narrowband interference. In the OFDM-based category weconsider the system in [4], which was dubbed interferencesuppressing OFDM (IS-OFDM), which we extend for use

in multiuser scenarios by using pseudo-random noise (PN)sequences and employing a technique similar to multicar-rier CDMA (MC-CDMA) [5]. In TH-IR multiple users cancommunicate simultaneously by using distinct time-hoppingsequences.

II. OFDM-BASED SYSTEM FOR UWB RADIOSWe consider the IS-OFDM system proposed for single-user

UWB systems in [4] and extend it for simultaneous use bymultiple users communicating with a single receiver as in thecase of the uplink of a wireless system. To enable multipleaccess and allow multiple users to transmit information si-multaneously we employ a technique similar to multicarrierCDMA (MC-CDMA) [5].The available wide bandwidth is divided into L groups withM frequency bins (or carriers) in each group, which impliesa total number of carriers N = LM. The Mf carriers in eachgroup are combined using the basic IS-OFDM and the Lbasic IS-OFDMs are combined using OFDM technique [4].The input data stream of R bits/s enters a serial-to-parallel(S/P) converter which provides L data streams each with rateR/L bits/s. Each parallel stream of rate R/L correspondingto a given group of freqency bins enters a second S/Pconverter which provides M parallel streams each with rateR/N. The M parallel streams in each group are spread usingorthogonal Hadamard sequences w so that after this spreadingoperation the signal rate becomes R/L again, followed bySIP conversion back to Ml parallel streams each with rateR/IN. These are combined in an interference suppressingscheme such that the power of each of the M symbols inthe frame carried by_a particular group of frequency bins isdistributed over all M bins in the given group while symbolsare separated by orthogonal Hadamard sequences. Differentgroups of frequency bins are made orthogonal to each otheras in an usual OFI)M system. The orthogonally modulatedsymbols of the fth group are summed to form a set of Mparallel symbols.

For a given user u these symbols are then spread usinga spreading sequence c(u) of length N, Thus, each groupcontains NC parallel data sets. The corresponding parallel datasets of all L grouys are combined to form a parallel sequenceof length N = ML. The N, parallel sequences are appliedto an FFTl block, and then fed to a parallel-to-serial (P/S)

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converter to form the UWB IS-OFDM symbol. In a multiuserscenario distinct users transmitting simultaneously to the samereceiver are assigned different spreading sequences as in [5].The transmitter diagram is depicted schematically in Figure 1.

Mathematically, the operation of the transmitter is describedas follows. The complex data points of the qth parallel streamof group e of user u, xqu) = ,qu) +j,) are spread bythe orthogonal codes wq = [wq,o,Wq,, ...,wq,] yielding

Wke'u) = EXq,',U)Wq,k for k=O,...,M 1 (1)q=O

These are then spread by the user spreading sequence c(u) toyield

M-1 k= O , ...

.= x(eU)wq,kc$iL) for e = 1,.. L

k qOq=O m N, -..,-1(2)

(u)2

which are combined to form the parallel sequence ai{ ~~M-1| e{b( fum)} E5 (e,u,m)WO for i= 0

q=Ob(e,u,m) for i= kL+±-1a(u,m) _ k and V(k, t) z# (0, 1)ai - ~~~M-13m{b( } = E do for i = N

q=O-(E U)}* for i = 2ML- (kL + e -1)LkJ and V(k, e) 7# (O, 1)

(3)where N = 2N and m = O,..., N, - 1. Thus we have Nparallel data sets that are fed to the IFFT block whose ouputis given by

N-1S(u,m) = 1 Ea ,m)ej27r(in/N)

The Nc parallel outputs of the IFFT block are then fed to aparallel to serial converter whose output sequence of lengthNN, forms the basic UWB IS-OFDM symbol.

At the receiver corresponding to the uth user, the receivedsignal is serial to parallel converted and fed to an FFT blockto obtain N parallel data points z(,m). These are then fed to adecoder where the data points are demapped, after which eachof the N parallel sequences of length Nc are despread usingthe uth user spreading sequence. The output of the despreaderis given by

Nu Nc-1

Z(Iu) =b- ) + EPE ccubu +w (5)Zk k kkmmp=1,pOu m=O

for k = 0, . . .,M-1 and e = 1, . . ., L, where the second termrepresents the multi-access interference and uk is the additivenoise contribution. These points are then demapped, parallelto serial converted and despread by N Hadamard sequences inparallel to obtain the original data symbols [4]. The receiverdiagram is depicted schematically in Figure 2.

WM-1

c(u)

Fig. 1. The Transmitter for the Multiuser UWB IS-OFDM system.

Fig. 2. The Receiver for the Multiuser UWB IS-OFDM system.

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III. TIME-HOPPING IMPULSE RADIOS

The TH-IR is based on transmission of ultra short pulses[7], [8], and the transmitted signal by a given user u is

OC

S(u)(t) - E E p(t -jTf-c(k)T0-dfPNI6) (6)i= -00'

where p(t) is the pulse (or monocycle) used, Ep is the energyper pulse, Tf is the pulse repetition interval, and Np is thenumber of pulses that are modulated by a given binary symbol.The information symbols d(u) change only at multiples of

Np. The time-hopping sequence )provides an additionaltime shift of c(k)T seconds to the jth monocycle in thepulse train, necessary to avoid collisions due to multipleaccess. Assuming that transmitted signals suffer only constantattenuation Au and delay Tu, V, the received signal for amultiuser system with K active users is

K

R(t) - 3 A,S(u)(t - Tu) + N(t)U=1

(7)

where the uth user received signal is given by00

S(u)(t-Tu) S/ Eprec(t_jTfC_(k)Tc- d(ju) 6)(8)

with prec(t) being the second derivative of the monocycle usedfor transmission [8].A single-user receiver using pulse correlation as described

in [8] is employed to decode users. This forms the templatesignal v(t) = Prec(t) -prec(t - 5), and calculates the decisionstatistic by adding up the Np correlations of v(t) with thereceived signal S(u)(t -ru) at various instances of time. In amultipath environment a selective Rake receiver [3, Sec. 8.2.4]with Ls fingers is used to select the Ls best components fromall available paths.

Performance of this system for a single active user in thepresence of narrowband interference has been analyzed in [9],where criteria like SIR, processing gain, and jam resistanceare defined. An analysis of the multiaccess performance ofthis system with random time-hopping sequences can be foundin [7]. In our paper we look at the BER performance of thesystem in a multiuser scenario when PN sequences are used.

IV. SIMULATION SETUP

We have performed simulations to investigate the BERperformance of UWB systems using OFDM-based radios aswell as TH-IR for a system with available bandwidth of528 MHz.

For the OFDM-based system this implies a total of N-= 256parallel channels, split into L = 16 IS-OFDM groups, eachgroup using M = 16 carriers. The L = 16 IS-OFDM groupsare combined using ordinary OFDM as described in Section II.Single user matched filter detection was employed to decodeusers for this system.

For the TH-IR system we considered a pulse repetition timeof 23.80 ns and a pulse duration of 2.1023 ns. One pulse pertransmitted information symbol was used, Np = 1, and thetime hopping interval was taken to be 3.0 ns. A correlationreceiver was used to decode users for this system. In the caseof multipath channels a selective Rake receiver with Lp 5fingers was used.The two systems were simulated in an additive white Gaus-

sian noise (AWGN) channel and in a multipath channel, withand without a narrowband interferer present. The multipathchannel model used in simulations is similar to the one in [1],and is based on the Saleh-Valenzuela (S-V) model [6]. Themodel, which was selected by the IEEE 802.15.3a standardand was also used in other recent work dealing with UWBcommunication systems [2] assumes that the multipaths arrivein clusters with arrival rate A given by a Poisson process, andthe rays within a cluster also arrive as a Poisson process witharrival rate A. The impulse response of this multipath modelis mathematically described by

L K

hi(t) = Xi ic 6(t- - T-kie)4=O k=O

(9)

where* {ja} represents the multipath gain for the kth ray

within the fth cluster.* {Te} is the delay of the fth cluster* {'r e} is the delay of the kth multipath relative to the eth

cluster arrival time.* {Xi} is the log-normal shadowing.

The cluster and ray arrival time follow an exponential distri-bution given by

p(TejTe-i) - Aexp[-A(Te-Tt_e)]p(Tk,ee Tk-1,e) = Aexp[-A(Tk,,e-Tk-1,e)]

and we assumed that o,e = 0. The channel gains are definedas follows

a, - Pk,,eGe3k,e (10)

where* Pk,e= tl with equal probability, and accounts for signal

inversion due to reflections.* (e represents the fading associated with the fth cluster.* k,e represents the fading associated with the kth ray of

the £th cluster.The small scale fading coefficients (e and /3k,e are modeled

as random variables with a log-normal distribution as givenbelow

20 log,0 (de/3k,e) X KAf(4k,e, O + F2 )The power delay profile is given as

E[j '(7e3k,e 12] = Qoee

(11)

(12)The mean Jk,l is given by

lOln(o) 'T0lTelOrke (or2 + f2)/I1k,01nQo) rl' 1±2 In(10) (13)I'k,l= l~~n(10) 20

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The large-scale fading coefficient is modeled also as a log-normal random variable and is given as

20 log 10(Xi) ocxK(°, o) (14)

We used the parameters in Table II in [1] to obtain the channelmodel used in our numerical simulations.

The narrowband interference signal used insimulations was generated similar to [4] by usinga linear bandpass FIR filter with 60 taps, passbandequal to 10 MHz, and stopband attenuation of-35 dB, driven by white Gaussian noise with unit varianceat the input.

V. NUMERICAL RESULTSWe have first looked at the performance of the IS-OFDM

UWB system in AWGN and multipath environment with oneand two active users. Results of this experiment are presentedin Figure 3. We note that, while in AWGN channel theBER curve has the typical "waterfall" shape, in the presenceof multipath the BER performance of the IS-OFDM UWBsystem degrades, and the BER curve flattens. For the systemunder consideration increasing the ratio Eb/No beyond 10 dBimplies almost no improvement in the BER. We also notethat adding one active user in the system does not affectperformance drastically.We have also compared performance of the IS-OFI)M UWB

system with that of the TH-IR system with one active user inthe system for both AWGN and multipath channels. Resultsof this experiment are presented in Figure 4. We note thatthe TH-IR displays similar performance degradation in thepresence of multipath, and that its corresponding BER curveflattens as well. We also note that the IS-OFDM UWB systemoutperforms the TH-IR system in both cases. For the systemunder consideration, in AWGN the same BER requires a ratioEb/No about 3-4 dB lower for the IS-OFDM UWB system,while for multipath the BER flattens at a lower BER value.

This experiment was followed by a similar one in whichperformance of IS-OFDM UWB and TH-IR systems was com-pared for multipath channels in the presence of a narrowbandjamming signal with different values of the Jammer-to-SignalRatio (JSR). Results of this experiment are presented in Fig-ure 5. Similar BER curves for a comparable IS-OFDM UWBsystem can be found in [4]. We note again the flattening ofthe BER curve for both IS-OFDM UWB and TH-IR systems.We also note that IS-OFDM UWB system outperforms againthe TH-IR system.

Finally, we have also looked at the performance of the twosystems in a multiuser/multipath scenario. The BER curvesthat correspond to 2, respectively 5, active users in the systemare presented in Figure 6. We note again the fact that theIS-OFDM UW*B system displays better performance. We alsonote that the addition of active users into the system seems tohave less impact in the IS-OFD)M UWB system than in theTH-IR system. For the considered scenarios, the addition

of 3 users in the IS-OFDM UWB system implies a smallerdecrease in the BER than in the TH-IR system.We conclude presentation of numerical results by noting that

the flattening of the BER curves in the multiuser case suggeststhat there are upper bounds on the number of users that can besupported in a multiuser UWB system with specified qualityof service (BER) using single-user detectors. This observation,which has been mentioned for multiuser TH-IR UWB systemusing random time-hopping sequences in [7], seems to be validalso for TH-IR using PN time-hopping sequences, as well asfor IS-OFDM UWB systems.

VI. CONCLUSIONSIn this paper we provided a performance comparison be-

tween OFDM-based radios and TH-IR for UWB systems. Inthe OFDM-based category we extended the IS-OFDM UWBsystem in [4] for use in multiuser scenarios. We extendedalso previous performance analysis for OFDM-based radiosand TH-IR for UWB systems [4], [7], [9] to a multiuserscenario, with AWGN, multipath channels, and narrowbandinterference. Our numerical analysis showed that the IS-OFDM UWB system outperformed the TH-IR system in allcases.

Future work should include an analytical investigation ofthe BER for the two considered schemes for generating UWBsignals to provide a theoretical basis for the experimentalresults obtained through simulations.

ACKNOWLEDGEMENTSThis work was supported in part by the Texas Higher

Education Coordinating board (THECB) under AdvancedTechnology Program (ATP) grant 000512-0261-2003.

REFERENCES[1] A. Batra, J. Balakrishnan, G. R. Aiello, J. R. Foerester, and A. Dabak.

Design of a Multiband OFDM system for Realistic UWB ChannelEnvironments. IEEE Transactions on Microwave Theory and Techniques,52(9):2123-2138, September 2004.

[2] D. Cassioli, M. Z. Win, and A. F. Molisch. The Ultra-Wide BandwidthIndoor Channel - From Statistical Model to Simulations. IEEE Journalon Selected Areas in Communications, 20(6):1247-1257, August 2002.

[3] M.-G. Di Benedetto and G. Giancola. Understanding Ultra Wide BandRadio Fundamentals. Prentice Hall, Upper Saddle River, NJ, 2004.

[4] D. Gerakoulis and P. Salmi. An Interference Suppressing OFDM Systemfor Ultra Wide Bandwidth Radio Channels. In Proceedings 2002 IEEEConference on Ultra Wideband Systems and Technologies, pages 259-264, Baltimore, MD, May 2002.

[5] S. Hara and R. Prasad. Overview of Multicarrier CDMA. IEEECommunications Magazine, 35(12):126-133, December 1997.

[6] A. Saleh and R. Valenzuela. A Statistical Model for Indoor MultipathPropagation. IEEE Joumal on Selected Areas in Communications, SAC-5(2):128-137, February 1987.

[7] M. Z. Win and R. A Scholz. Impulse Radio: How It Works. IEEECommunications Letters, 2(2):36-38, February 1998.

[8] M. Z. Win and R. A Scholz. Ultra-Wide Bandwidth Time-HoppingSpread-Sprectrum Impulse Radio for Wireless Multiple-Access Commu-nications. IEEE Tran. on Communications, 48(4):679-689, April 2000.

[91 L. Zhao and A. M. liaimovich. Performance of Ultra-Wideband Com-munications in the Presence of Interference. IEEE Joumal of SelectedAreas in Communications, 20(9):1684-1691, December 2002.

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-5 0 5 10 15Eb/No [dB]

20 25 30

100

10

lo'

ul

102

-1 C -5 0 5 10 15Eb/No

20 25 30

Fig. 3. BER vs. Eb/No for an IS-OFDM UWB system with AWGN andmultipath channels, and different number of active users.

Fig. 5. BER vs. Eb/No for single user IS-OFDM UWB and TH-IR systemswith multipath channel and narrowband interferer with different powers.

100

10~1

aum

10i

1A3L15 20 25 30 -10

........... I! -

-5 0 5 10Eb/NO [dB]

15 20 25 30

Fig. 4. BER vs. Eb/No for single user IS-OFDM UWB and TH-IR systemswith AWGN and multipath channels

Fig. 6. BER vs. Eb/No for single user IS-OFDM UWB and TH-IR systemswith multipath channels and different number of active users.

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