This paper investigates the performance of coded orthogonal frequency division multiflexing (OFDM) systems with receiving adaptive array, through which multiple users with single-element transmitting antenna are supported simultaneously by spatial division multiple access (SDMA). We characterize the performance of an OFDM/SDMA systems by effective throughput, which is essentially the average number of data bits in an OFDM symbol after considering the erroneous packet transmissions and modulation scheme by excluding the overhead from coding and pilots for channel estimation. Optimization of system operating parameters can be achieved through the maximization of effective throughput. The focus of this paper is to study the impact of pilot density and the number of users on the performance of coded OFDM/SDMA systems. Through extensive computer simulation, we how that using more pilots always improves bit error rate (BER) performance, but may reduce effective throughput. The optimal number of pilots together with the modulation scheme can he determined by maximizing the effective throughput for given operating signal-to-noise ratio (SNR). It is also shown that the system performance degrades gradually with the increase of users. For a system with a six-element adaptive array, the effective throughput with 5 users is lower than that with 4 users for a certain range of SNR. This indicates that the maximal number of users supportable by the system should consider the effective throughput. © 2003 IEEE.published_or_final_versio

Li, VOK

Pan, Z

Shen, D

Wong, KK

HKU Scholars Hub

Title Effective throughput for coded OFDM/SDMA systems with pilot-assisted channel estimationAuthor(s) Shen, D; Wong, KK; Pan, Z; Li, VOKCitation Ieee International Symposium On Personal, Indoor And MobileRadio Communications, Pimrc, 2003, v. 2, p. 1139-1143Issued Date 2003URL http://hdl.handle.net/10722/46494Rights©2003 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.The 14" IEEE 2003 International Symposium on Personal,lndoor and Mobile Radio Communicafion Proceedings Effective Throughput for Coded OFDM/SDMA Systems with Pilot-Assisted Channel Estimation Dongxu Shen, Kai-Kit Wong, Zhengang Pan, and Victor 0. K .  Li D e p a r t m e n t  of Electrical & Electronic Engineering The University of Hong Kong Pokfulam Road, Hong K o n g , C h i n a  email: {dxshen, kitwong, zgpan, v l i ) @ e e e . h k u . h k  papcr isvestiptrs the perfoi'mrnec of coded wthoEo- bioi1 hwl t ip ln ing  (O171>hl) l i \c ""r!. Ih1.ou~11 nhich multiple u\ws wilh an~ci ( i i i i  arc sapparted s in iu l r ancuur l~~  by s p  (SIl\ l&).  \e cliiimcteriLc thc pci'fhrmaoce a i  an OFDMlSDhlA systems b y  e i jmwr iliroii~lipto. which is essetitially I hc  average number of d a t ~  bits in a n  0I:l)hl symbol after eonMer isg  thc erroneous packet transmissions and niodulatioii schcmc by cx lud ing  the ovcrhvad from coding and pilots for cliilniiel ertimatius. Optimimtioii of syrtrm operating parameters fati IIC i i ch i cw t l  through the mti ini imtion of cifcctive throughput. Thc focus of (hi\ lpapcr i h  to ,ludy thc impact of pilot dcnrity and the number of users on thc ~ " f m m m c e  oicodcd OFI>MISU\lA rystcmr. Through extensive corn- lpuicr r i r i i d s f i ~ n ,  n c  how that  using more pilots always impruvca bit error m l u  (lllil<l pel-formrncc, lhut niry rcducc eifccrive throughput. The optimal I~LIIIIIWI. of pilots toxether with the niodulation scheme can he dctcrmincd irinp tlic cflective throughput fur given operating signal-to-noise <). I t  i\ also S I I O W I I  that the system performancc degrader grad- thc increasc of USPI'S. For a systcni with a sir-element adaptive effectivr throughput with 5 useis is lower than that with 4 users i n  ~-anqeolSZIK.Thisindicatesthattliemniimal numbel-uiusc~'r 'tem sheald cossirlul- Ihr effective throughput. I .  I N T I I 0 I ) l I ~ ' r I O N  C)i-thogonal frcquciicy division iiiultiplexing (OFDM) [ I ]  is a Ipromisiiig technology for high spccd data communications, and 11.1s hceri adopted i n  wireless local area networks (LAN) stan- ilar(li such as IEEE 802.1 l a  [2] and HIPERLAN II [3]. While OFDM is capable of combating intcr-symbol interference (ISI), its pcribrmance can still bc severely degraded by channel fad- ing. An cffectivc method to mitigate tlic influence offading is to cinploy adaptive array [4], [6]; through which multiple simulta- iicous users can further be suppoited by spatial division multiple lii this paper. wc study the uplink pcrfomiance of an coded OFDMiSDMA wii-clcss communication system. The base sta- tiuii (BS) is cquipped with an adaptive antenna array for sig- iiiil reception. and each inobile user ti-ansiiiits through a single- C l C l l l C l l t  aI1tC"na. The performancc of coded OFDMISDMA systems can be cli;il-;~trrizcd by cfi'ective throughput proposed in [ 5 ] ,  which cs- sciilialiy 1-eprcsciits the average number of data bits carried i n  a i  tiI:DM symbol after considering the modulation scheme and Iiacl;ct eri-or wliilc excluding the overhead from channel estinia- iioii a i d  coding. Thercfore. the opriniizatioii of system perfor- m i n c e  is cquivalent to tlic maximization ofeffective throughput. Tlirougli tlie charactcrizatioii by effective throughput, the influ- CIKC of various factors on system pcrfoi-mance can be quantified. lii this paper. the focus is on studying how the system perfoi-- niiiiicc is influenced by the pilot-aided channel estimation [8]; modulation, coding schemes, and the number of simultaneously transmitting users. Through simulation. wc show that it is tiot always beneficial to have more pilots for channel estimation. bc- cause the use of pilots reduces the throughput for data. The op- timal number of pilots should be determined together with the modulation scheme in order to maximize the cffcctive through- put given the operating signal-to-noise ratio (SNRI. Similarly. supporting more users may rcsult i n  the reduction of effectivc throughput. In particular. simulation results demonstrate for a system with a six-element adaptive array, the effective through- put with 5 users can be lower than that with 4 users for a certain range of SNR. This indicates that the maximal number of users supportable by the system is not only limited by thc frccdom of the system (number of receiving antenna elcmcnts), but is also conditioned upon effective throughput and operating SNR.  This paper is organized as follows. In Section I I .  we describe the system. The definition of effectivc throughput is prcscntcd in Section 1 1 1 .  In Section IV, the performanceofan 0I;DMiSDIVlA system is investigated. We concludc this  paper in Section V. 11. SYSTEM DESCRlf"1ION The baseband processing of a mobile user 1, .  7, = 1.. . . . L i  is depicted in Fig. 1. We assume tlie number of simultaneous users U is no greater than the number of antenna elements A at the BS. Data bits are first encoded by a convolutional encoder, and interleaved by a random interleaver. The conventional block in- tcrlcaver is inappropriate because convolutional code perfonns unsatisfactorily with bursty errors [ I O ]  due to the correlated fad- ing on adjacent subcarriers over both time and frequency. The interleaved binary bits are then mapped to a inodulation symbol using quadrature aniplitudc modulation (QAM). After serial-to- parallel conversion, i nx r sc  fast Fourier transforin (IFFT) is per- formed on the symbols StL( i ) ,  i = 1 ,'ic wlicrc i denotcs thc subcarrier index and IV, is the number of subcarriers i n  ai1 OFDM symbol. 'The outputs fi-om the IFFT transformel- cor- respond to the time samples of thc transmitting signal and arc parallel-to-serial convertcd. After cyclic prefix is added. the re- sulting signal is finally transmitted from a single-element an- tenna. At  the BS (sec Fig. 2), signal is received by A antenna cl-  cments. We assume the clianncls between a uscr and cvcry antenna cleineiit are uncorrclatcd, and signals from different users are independent, which leads to a full diversity system After sampling and removing cyclic prefix. the samples are serial-to-parallel converted, and demodulated by the fast Fourier The 14m IEEE 2003 International Symposium on Persona1,lndoor and Mobile Radio Communication Proceedings Fig. 2. Baseband processing at the basc station. We only show the antenna processing on subcamer 1. while the processing on other subcarriers is identical and i s  omitted. transfomi (FFT) processor to produce Ra(i), a = 1,. . .A ,  i = 1:. . . . iWc at antenna a. For subcarrier i ,  the signals from different antenna elements Rn(i) ,  n = 1:. . . : A  are weighted and combined to form an estimate gt,(i), U = 1 , .  . . ~U for the transmitted signal Su(i), U = I > .   . U .  The estimated signal is finally mapped back to binary bits;which are then deinterleaved and decoded. To illustrate how the antenna processing works, let S ( i )  = [ S I ( i ) .  . . . ~ S,(i)lT be the vector oftransmitted signal on sub- carrier i, where T denotes the transpose operation. Also, let R(i) = [Rl ( i ) :  . . . ~ R A ( / ) ] ~  be the receivedsignal vector, and n(i) = [n.,(i.); . . . I na(i)IT be the noise vector, where n.(i) is the noise on subcarrier i at antenna a. Since users are assumed to be perfectly synchronized in both time and frequency, the re- ceived signal for subcarrier i is cxpressed as R(1) = H ( i ) S ( i )  + n(i), i = 1, . . . :  N, (1) where H ( i )  is the channel transfer matrix on subcarrier i HI.l ( i )  " '  Hu.i(i) i ] ( 2 )  [ f f l . A ( i )  _ ' '  ". f f U . A ( i )  H(1) = with H,L.a ( i )  being the fading coefficient between user U and an- tenna element n at the BS. Denoting the antenna weight matrix as Wl,l(i)  ' . '  W U . l ( i )  W(2) = where IVu,a(i) s the antenna weight for the ith subcamer of user U at antenna a, the transmitted signal vector S ( i )  can be estimated by g ( i )  = WH(i)R(i)  (4) where H represents the Hermitian transpose. W(i)  can be computed by [9] With channel state information, the optimal weight matrix W(i)  = [H( i )H"( i )  + u ' I] - 'H( i ) ,  i = 1, . . . , N,, ( 5 )  where I is an A x A identity matrix. In this paper, the channel state information is obtained through the use ofpilots [SI. 111. EFFECTIVE THROUGHPUT We define the effective throughput in this section. This concept .is devised to  characterize the performance of an OFDM/SDMA system by considering the packet error rate (PER), overheads from pilots, modulation scheme and coding rate. Definition I-User Effective Throughput :  Let P, be the PER, N,j the number of data subcaniers in an OFDM symbol, MI the 1140 The 14" IEEE 2003 lntemalional Symposium on Persona1,lndoor and Mobile Radio Communication Proceedings Bandwidth iiiodulation indcx on data subcarriers. and rc thc coding rate. The eit'ectivc throughput for a user is defined a s  (6) 1-k iiormalizcd ctTcctivc throughput is dcfincd as the ratio bc- iwccii the user effectivc throughput and the nunibei- of subcarri- c i s  iii iiii OFDM symbol. That is n I ; ,  = (1 ~ Pc)lV<~I\I,rc. 20 MHz U+irioii ?-System Effective Throughput:  Let Tu@) be the ctt'ccrivc throughput for uscr i. When there are U users timiismit- tiiig simultaneously. the systcm effective throughput is defined , r  :1s , = I  'Slic 1101-malized system effective throughput is similarly defined IV. D I S U J S S I O N S  A N D  S I M U L A I I O N  RESULTS 111 this section. we discuss the impact of various factors on t l i c  performance of coded OFDM/SDMA through simulation, with t h e  cmphasis on the pilot density and number of users. The perfoi-mance criterion wc usc is the normalized effective throughput for a user, but sonictinics BER is used a s  a supplc- inicnt. Throughout this section, we shall use the term effective throughput interchangeably with nornialired effective through- Ipiit fnr convenience. .-1. .Similur;on Seriip We assunie all users apply the same operating parameters, and tlic channcls bctwecn cvci-y uscr and thc BS are statistically in- dependeiit and identical. Thereforc. all users would have statis- tically thc S a m  performance. 'I'hc niultipath fading channel is assumed to bc wide sense stationary with iincorrclated scattering (WSSUS). In the time domain. channel impulse response is modeled as a tapped delay Iinc at tap spacing 2:5. with each path following an exponential lpower delay profile. The amplitude on each path is Rayleigh dis- ti-ibuled. and the phase is uniformly distributed between [0: 2 ~ 1 .  The method in [ I  I ]  is used for the siniulation of  channel fading. The simulation parameters are summarized in Table I .  In oui- simulation. pilots arc inserted for channel estima- tion. To confine the overheads from pilots. only onc of  eveiy !lIf OFDM symbols is inserted with pilots on subcarriers that arc !If, subcarriers apart, and the arrangement is illustrated in Fig. 3:  When a subcarricr is uscd as a pilot subcarrier for a uscr. othci- users place nu l l  symbols on the subcarrier to avoid inrcrference. i.e.. no signal (neither pilot nor data) is carried by the subcarrier. After estimating the channel on pilot sub- am-iers in a pilot-bearing OFDM symbol, the channels on all Nuniber of  antennas I A = (i U = 3 4 o r  5 Number of users I Nuniber of subcarricrs I lV, = 128 Carrier swacinz I A f  = 20/128 = 162.5 kHz Guard interval Z'C. = 0.8ps Svtnbol interval I " , , .  1 ( W 3 .  561). (5571 663. 771). 1 (in octals) (765. 6 i 1 ,  51% 4i3) lnterleaver Random intcrlcaver Channel delav soread I 0.8 lis , I  I Donnler freauencv I F,r = 13119 H z  , ~ r  ~ ~ ~ ,~ , , ~~ Pilot density !U, = 4 or 8, !If, = 5 Pilot SNR 2 0  dB other subcai-riers ofthat OFDM symbol are cstimated by a niax- imal likclihood (ML) estimator [12]. Then linear interpola- tion is performed based on the estimation from two consecutive pilot-bearing OFDM symbols for estimating the channels of the Aft - 1 OFDM symbols in bctwccn [ 131. To ensure the quality of channel estimation, w e  rcquii-c tlic SNR on the pilot subcar- riers to be always maintained at 20 dB regardless of the SNR on data subcarriers. r:1 D,lo,bt,hcnrr8er -". *$ ,1111, ellllirrriui drlr rllbrrrliil -. . _I 1 1 ,"bC.,,,,<, hl I B. Pilof Density We first compare thc performance under different numbcr of pilots. In the simulation. Adt is fixed at 5 .  AJ, is 4 or 8 and U = 3. The BER for the uncoded QPSK, 16-QAM, and 64:QAM with M ,  = 4 or E, i l l t  = 5_ U = 3 is plotted in Fig. 4. From Fig. 4, we notice that by doubling thc pilot dcnsity in the h-e- quency domain (from A{, = 8 to !lf, = 4). there is consistently about 2 dB gain in SNR for all modulation schemes undcr inves- tigation. At the same AJf and AJL, there is 6 to 8 dB diffcrence 1141 The 14Ih IEEE 2003 International Symposium on Persona1,lndoor and Mobile Radio Communication Proceedings i i i  SNR between QI'SK and 16-QAM for the same BER, and 4 to h d B  ditrcrcncc between 16-QAM and 64-QAM. I:,. 5 i:tkCtive ~ I , ~ ~ , ~ ~ & ~ u I  lor UW-: ~ d e d  QPSK. I ~ - Q A M  anti 64 QAM U~l l l l  .Ai, =.I 0 1  8. \ I ,  = 5 .  U = :<. Thc cffcctivc thi-oughput for a rate-; coded system with . I / \  = 4 or 3. df, = 5. U = :1 is plotted in Fig. 5 .  We first i,b.;cr\jc li-niii Fig. 5 that tlic curve ofeffective throughput can be chili-actcl-ized by three segments dcpendiiig on the SNR. When S N R  is lowel- than a ccitain threshold. effective throughput is zero. When S N R  is higher than a ccitain cutoff value, effec- tlve throughput bccoincs saturated. Between the two values, ef- fective thi-oughput curve is almost linear. Fuither, the effective till-ouglipiit with 11f.f = 4 is highci-than that with A l f  = 8 before i t  bccoiiics saturated. and is lower than that with A t ,  = 8 in the saturation scgment. The iioi-malized effective thi-oughput is expressed in (7) as Tu = i!,,./.Vc, where N,, is the averazc number of data subcari-icrs i n  an OFDM symbol. From our ai-rangement of pi- Tlic above phenomena can bc explained as follows. lots, .V,i can be calculated as IV, = NC - lv,/ll.r,/nit. (10) Thus when SNR is low. f": = I ,  so that T,, = 0:  when S N R  is high. P ,  i 0, so that I=',, is closc to the upper liiiiitcd !\7d1\.fi I .JIV~. ( 1  1 )  Obviously thc limit with ~lf, = 8 is higher than that with AI, = 4. Although more pilots always improves the BER perfoi-mancc. it increases overheads and thus reduces effective throughput. as shown in Fig. 5 .  To maximize effective throughput. the optimal number of pi- lots can be selected togethei- with the modulation scheme from the effective throughput curves in Fig. 5 when given the oper- ating SNR. For example, when S N R  is 10 dB. 16-QAM with 114, = 4 is used; when SNR is 20 dB_ 64-QAM with Ai, = 8 is chosen. C. Nuinher. uf Use,s Fig. 6 .  RER for wooded QPSK. 16-QAM and 64-QAM will> 3. 4 and 5 users. AI1 = 8. Mt = 5 .  We investigate thc performancc impact by the number of users. According to [O], an  A-clcnient antenna w a y  has A - 1 degrees offreedoiii. To cancel the co-channel interference (CCI) from a uscr, one degree of antcnna freedom is needed. 4 s  a rc- wlt ,  with U users ( U  5 A) ,  U degrees of freedom arc nccdcd for CCI canccllation. so that the dcgree offreedom tor diversity is il - U. Thus the degree of diversity diminishes with iiiorc users. We plot the BER performancc of  uncoded systcnis in Fig. 6. and the corresponding uncoded effective throughput i n  Fig. 7. It is clear that both the BER and effective throughput pel-foi-mance degrades witli more users. An interesting observation from Fig. 7 is that the effective throughput for uncoded 16-QAM with 4 users is always higher than that of 5 users using either QPSK or 16-QAM. Wc can also calculate from the definition in (9) that the system effective throughput with 5 users is Iowcr than that with 4 uscrs when 1142 The 14" IEEE 2003 International Symposium on Persona1,lndoor and Mobile Radio Communication Proceedings 4 V. CONCI.LlSION ___-- In this paper, we study the performance of coded OFDMiSDMA systems through effective throughput. Specifi- cally, the focus is on investigating the influence ofpilot density and number of users. We show that the BER performance im- proves consistcntly with inore pilots, but the effective through- put can be reduced due to the overhead from pilots. The optimal nuniber of pilots can be determined together with the modula- tion scheme by maximizing effective throughput at the opcrating SNR. Simulation results also show that system pciformancc dc- with niorc uscrs. It is possiblc for the effective throiiph- put for a user and the systcni be reduccd at the same timc by ihc n of a user. Thus thc niaxinial number of iisci-s that can poited by an OFDMiSDMA system should bc cai-cfully , ~ + uncoded QPSK. 3 users I ? 2 5 ,  g 7  il 2 ,  r:, . m  I.chosen by considering effectivc throughput. AC:KNOWI.EDGMEN'I I ' ig 7. Ii1Yecl~~'c lllrnoghpur Wr iincodrd QPSK. 16-QAM and 64-QAM wit11 3. 4 ;md 5 users. A i i  = X. A i ,  = 5 .  This research is supported in part by the Research Grants Council of Hong Kong under grant no. H K U  7047100E. R E  FEI IENC ES SNR is lower than :is dB. For exaniple, whcn S N R  is 32 dB, the effectl,,e tllrougllpll~ for a i s  about : ~ , ~ j  for [I] R. YBII Nee. R. Prasad. OFDMIUr IWirrim ,Midiimedio CLini,iiirni~niioi~~. Aifrcli House. 2000. I (>-QAM whcn thcre are 4 users, and is about 2.7 bitsisubcal-ricr [2] W~reless I.AN Medium A C C ~ S S  Con~rol (MAC) itnd i'hysical lhycl- ( I w Y )  spccificalios: High-specd Physical Layer ill the 5 Gllz Rand. l K t E  itd to r  I&Q.AM whcn there are 5 users. The corresponding sys- . . XUL.1 Itl-I'YYY. 1.ocal Arca Networks (HIPLRLAN) Tvvpe 2 .  Svstem Ouervirw. l i l S l  .I'll tcin cffcctive throughput is 3 . 6  x 4 = 14.4 bitsisubcarrier and 131 Bioadhand Access tliell 2 . i  Y .5 = 1.1.5 bitsisubcarrier respectively. This reveals that . .  the addition of a user can degrade the effective throughput per- Ibrniancc f o r  a user and the system at the same timc, which is 101 683.v0.1.2. 1999 ar the mobiie and basc statiolis in an O I D M I l D M A  s~sfem." I E I X  %uri r .  141 K. K~ w~ng.  R. CIIW~. K. 13. i . m e i :  I<. ri. Murcii. w ~ ~ x ~ ~ ~  R ~ , I C ~ I ~ I ' , S  Tlic coded performance for effective throughput is plotted in IFiy. X.  'The effective throughput with 5 users employing QPSK is better than  that of 4 usei-s employing 16-QAM when SNR is lower t h a n  16 dB, which is much improved as compared with tlic iiiicoded case. However, when SNR is above 16 dB, the effective throughput for 5 user with either QPSK or 16-QAM is still Iowcr than that o f 4  users with 16-QAM. 1143 

Effective throughput for coded OFDM/SDMA systems with pilot-assisted channel estimation

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Effective throughput for coded OFDM/SDMA systems with pilot-assisted channel estimation

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