2-dimensional OVSF Spreading for Chip-interleaved DS-CDMA Uplink Transmission

Transcription

1 2-dimensional OV Spreading or Chip-inerleaved DS-CDMA Uplink Transmission Le Li and Fmiyki Adahi Dep. o Elerial and Commniaion Engineering, Gradae Shool o Engineering, Tohok Universiy, 6-6-5, Aza-Aoba, Aramaki, Aoba-k Sendai , JAPA lile@mobile.eei.ohok.a.jp, adahi@eei.ohok.a.jp Absra Mli-aess inererene (MAI) limis he ransmission perormane o he DS-CDMA plink. In his paper, we propose 2-dimensional (2D)-OV spreading or hip-inerleaved DS-CDMA plink ransmission. By adoping 2D-OV spreading, he mliser hannel is ransormed ino orhogonal single-ser hannels irrespeive o sers daa raes. Ths, single-ser reqeny-domain MMSE eqalizaion an be sed o ake advanage o reqeny-seleive ading hannel or signal deeion. For a large Doppler spread, however, he orhogonaliy among he OV spreading odes may be los, hereby proding he MAI. The deision eedbak reqeny-domain MAI anellaion is applied o improve he BER perormane. The ransmission perormane in a mliser and reqeny- and ime-seleive ading environmen is evalaed by omper simlaion. Key words: MAI, 2-dimensional, OV spreading odes, plink, DS-CDMA.. ITRODUCTIO In mlimedia wireless ommniaions, a lexible sppor or he low-o-high bi rae o mlimedia servies pon a speii ser s reqes is reqired []. Using ode division mliple aess (CDMA) ehniqe [2], mli-rae servies an be oered by hanging he spreading aor (), he nmber o mliple odes, and/or modlaion orma (sh as mli-level QAM (MQAM)) [3]. There may be wo approahes in CDMA ehniqe: dire seqene (DS)-CDMA [2], [4] and mliarrier (MC)-CDMA [5], [6]. Wih he aid o reqeny-domain eqalizaion (FDE), DS-CDMA an ahieve good downlink perormane similar o MC- CDMA while he ormer has an advanage o lower signal peak-o-average power raio [7], [8]. owever, in he plink, dieren sers signals are asynhronos and go hrogh dieren hannels, resling in mlipleaess inererene (MAI). The MAI is a riial perormane limiing aor o he plink. A mally orhogonal serode-reeiver (AMOUR) sysem [9] is proposed based on he orhogonal FDMA (OFDMA) priniple [], [] or MAI-ree mliarrier plink ransmission. Generalized MC-CDMA (GMC- CDMA) [2], [3] is he mli-rae AMOUR, whih preserves all he properies o AMOUR while being able o aommodae sers o dieren daa raes. For singlearrier DS-CDMA, hip-inerleaving and ime-domain spreading is proposed o anel he MAI. Chipinerleaved DS-CDMA wih gard inerval (GI) [4], [5] an realize he MAI-ree ransmission in a qasisynhronos mlipah environmen. The se o GI (i.e., zero-padding in [4] or yli preix in [5]) no only avoids he iner-symbol inererene (ISI) b also redes he reqiremen or he synhronizaion. Freqeny-domain eqalizaion (FDE) insead o Rake ombining is applied in a severe reqeny-seleive ading hannel. The hip-inerleaved DS-CDMA was onsidered in a single-ell environmen [4], while Peng e al. exended o a mli-ell environmen by sing ellspeii srambling odes [5]. The MUI-ree ransmission is garaneed in a ime-nonseleive ading hannel, where he blok spreading mainains he ode orhogonaliy among he spreading odes i he hannel does no vary aross he onseive bloks. In his paper, we propose 2-dimensional (2D)-OV spreading or hip-inerleaved DS-CDMA plink ransmission. 2D-OV spreading an ahieve a good BER perormane, while aommodaing sers o dieren daa raes nder a qasi-synhronos mliser environmen (ransmi imings o all sers are only reqired o be kep wihin he GI). The ombinaion o 2D-OV spreading and hip-inerleaving makes i possible o ge boh he ime- and reqeny-domain diversiy gains. The average BER perormane o 2D- OV spread DS-CDMA is evalaed in a reqenyand ime-seleive Rayleigh ading hannel. Also proposed is reqeny-domain MAI-anellaion sing deision eedbak o improve he robsness agains as ading. 2. 2D-OV SPREADIG AD CIP ITERLEAVIG Uplink ransmission model o DS-CDMA sing 2D- OV spreading is illsraed in Fig., where only he h ser is shown (his sheme an also be applied o he DS-CDMA downlink ransmission, however, his is no onsidered here).

2 A. Transmied signal The h ser s daa-modlaed symbol seqene {d (n); n( / -)} is spread sing he s OV spreading ode { ; } wih spreading aor and rher mliplied by a binary sramble sr seqene { ; } o prode a hip seqene o lengh o hips ( is he FFT/IFFT blok size or FDE). ex, he -hip seqene is spread by he 2nd OV spreading ode { ; } wih spreading aor. The s OV ode is or mli-rae onneion per ser and he 2nd OV ode is or orhogonal mliser mliplexing. As shown in Fig. 2, he overall spreading aor o 2D-OV odes is x, where an be arbirarily seleed independen o he FFT blok size, b and is hosen aording o he nmber o aive sers. I he nmber o sers is U, hen U an be sed o allow U sers aess o he base saion wiho asing he MAI. ) () sr () d (n) * [ ] () s () sˆ ˆ R Y w 2E T s () sr * [ ] Fig.. Uplink ransmission model or he h ser. Fig. 2. 2D-OV spreading. s () () s ( () s () () s ( () s () ( ) s ( ( Fig. 3. Chip-inerleaving.. ( n+ ) n Aer 2D-OV spreading, he hip inerleaving is perormed wih olmn-wise inp and row-wise op, as shown in Fig. 3. The 2D-OV spread DS- CDMA signal an be expressed sing eqivalen lowpass represenaion as dˆ s 2E T s ( mod ) ( / ). () In Eq. (), s () is given by sr s d ( ) ( mod ), (2) where x is he larges ineger smaller han or eqal o x, E is he average hip energy and T is he hip draion o he 2nd OV spreading odes. An g -hip GI is insered every -hip blok or FDE a a reeiver. B. Channel The GI-insered DS-CDMA signal is ransmied over a reqeny- and ime-seleive Rayleigh ading hannel. Assming ha he hannel has L independen propagaion pahs, he disree-ime implse response h () o he h ser is expressed as h h, lδ ( τ, l ), (3) L l where δ(x) is he dela nion and h,l is he lh pah L l 2 h, l gain wih E [ ]. Assming blok ading wih he maximm Doppler reqeny D, he pah gains remain onsan over one blok inerval TT ( + g ) b vary blok-by-blok. The T -spaed disin ime delays are assmed o be {τ,l τ +l; ll-} and τ is he h ser s ransmi iming ose; he maximm ime delay o eah ser s hannel is shorer han GI. C. Reeived signal The sm o U sers aded signals is reeived by a base saion reeiver. The reeived signal is sampled a he hip rae and he GI is removed. The GI-removed reeived signal an be wrien as U ) h s + n( ), (4) where denoes he onvolion proess and n() is he addiive whie Gassian noise (AWG) wih zero-mean and variane o 2 /T wih being he one-sided power sperm densiy. D. MAI anellaion ) is deinerleaved (see Fig. 4) and hen he s despreading is perormed sing he 2nd OV ode () as * s ˆ + m )[ ] (5) m or -. The MAI is anelled de o he orhogonal propery o OV spreading odes { ; U }. ene, he mli-ser hannel is ransormed ino orhogonal single-ser hannels and he mliser deeion problem an be onvered ino a se o eqivalen single-ser eqalizaion problems as in [5]. read ) ) ) + ) 2 Fig. 4. Chip-deinerleaving.

3 Aer despreading, -poin as Forier ransorm (FFT) is applied o deompose he despread hip seqene { sˆ ; ( } ino reqeny omponens { R ˆ ; k ( }. R an be expressed as ˆ R ˆ s exp( j2πk ), (6) 2E S + Π T where S is he kh reqeny omponen o s (), is he kh reqeny hannel gain o he h ser wih 2 E[ ] and Π is he noise omponen de o he AWG wih zero-mean and variane o 2 /T. Then, one-ap MMSE-FDE is arried o on eah reqeny omponen as Y ˆ w R, (7) where w is he MMSE-FDE weigh given by [7], [8] w. (8) 2 E + ex, -poin inverse FFT (IFFT) is applied o {Y ; k -} o obain he ime-domain hip seqene: k y Y exp( j2π ). (9) k Finally, he 2nd despreading sing he s OV spreading ode () is perormed o ge he deision variable dˆ ( n) or he deeion o d (n) as ( n+ ) n sr [ ] ˆ d ( n) y, () based on whih daa symbol deision is arried o. 3. FREQUECY-DOMAI MAI CACELLATIO Time-domain MAI anellaion is based on he orhogonal propery o he 2nd OV odes { () ; (U-), ( -)}. owever, or hannels wih large Doppler spreads, he orhogonal propery may be los and he MAI is proded. To rede he MAI or large Doppler spread, reqeny-domain MAI anellaion is proposed below. The kh reqeny omponen Rˆ in Eq. (6) an be rewrien as Rˆ 2E T S + M + Π () or k( -), where M is he MAI omponen, given by U 2E M S [, m ],(2) T m where,m is he kh sbarrier hannel gain assoiaed wih he mh reeived hip blok or he h ser. The deision eedbak is sed o generae he replia o S as ( ) S k s exp( j2π k ), (3) where s ( ) is he replia o s (), whih is he spread and srambled hip seqene represened by Eq. (2). The MAI omponen esimae M ( k ) is obained by replaing S in Eq. (2) wih S ( k ). As shown in Fig. 5, reqeny-domain MAI-anellaion is perormed as R ˆ R M. (4) Alhogh he above reqeny-domain MAI anellaion reqires oher sers hannel sae inormaion and spreading odes o generae he MAI esimaed M ( k ), i is mh simpler han MUD ha reqires he marix inversion. ˆ s ˆ R R ( k ) Y w ( k ) M ( k ) S sr () () () s ( ) ( n+ ) n sr * ) ( )] [ ( Fig. 5. Freqeny-domain MAI-anellaion. 4. COMPUTER SIMULATIO dˆ d ( n ) We assme QPSK daa-modlaion, 256 and g 32. An L6-pah reqeny-seleive blok Rayleigh ading hannel having he niorm power delay proile is assmed. The ransmi iming oses {τ } are niormly disribed over [- /2, /2] wih <( g -L) and he maximm ime delay dierene is less han GI. Ideal hannel esimaion is assmed. Fig. 6 plos he BER perormane o plink DS- CDMA as a nion o he average reeived bi energyo-he AWG power sperm densiy raio /, whih is given by /.5(E / )( )(+ g / ), or U, 8 and 6 when D T -6, where T x xt is he daa symbol draion. The overall o 2D-OV spreading is x 6 and we sed U and 6/U. For omparison, he BER perormane o he onvenional DS-CDMA sing D-OV spreading is ploed in Fig. 6 or single-ser deeion (SUD) [6] and MUD [7] based on MMSE rierion. SUD anno work or U 8. When he sysem is moderaely loaded, he MUD exhibis beer perormane; however, when

4 DS-CDMA wih x 6 L6, Uniorm proile, D T D-OV odes (6) SUD MUD U U8 U6 2D-OV odes ( U, 6/U ) U U8 U / -3-4 DS-CDMA L6, Uniorm proile D T -6, U / Fig. 6. BER perormane omparison. Fig. 7. BER perormane o mli-rae ll-loaded CI-DS-CDMA - -2 DS-CDMA wih MMSE-FDE L6, Uniorm proile U DS-CDMA wih MMSE-FDE L6, Uniorm proile / 8 db x 6 U, /U -3-4 x 6 U, /U D T..5. U8 U -4-5 Freqeny-domain MAI anellaion w/o wih U6 U / D T Fig. 8. Impa o D T. Fig. 9. Ee o reqeny-domain MAI anellaion. he sysem is heavily loaded (i.e., U ), he BER perormane degrades severely and or proposed 2D- OV spread DS-CDMA operorms he MUD. MUD reqires he knowledge o he spreading odes and mlipah hannels o all sers and he ompaional omplexiy o MUD grows exponenially wih he nmber o sers; while he reeiver omplexiy o 2D- OV spreading is linear. ene, 2D-OV spreading is onsidered o be promising or he plink ransmission. Fig. 7 shows he BER perormane o ll loaded 2D-OV spread DS-CDMA wih U6. The overall spreading aor is 6x. The imedomain diversiy is ahieved by he despreading sing he 2nd OV spreading ode () and he MAI is anelled almos ompleely when D T -6. The daa symbol is spread over all reqenies sing he s OV spreading ode (), yielding large reqeny diversiy ee irrespeive o. The reason or degrading perormane wih dereasing is de only o inreasing sel-hip inererene (ICI) among hips o s OV spreading ode [7]. Fig. 8 shows he impa o Doppler spread on he 2D-OV spread DS-CDMA or x 6, where

5 U and 6/U. ere, eah ser s signal is assmed o pass hrogh independen ading hannel b wih he same D T. I is observed ha he BER perormane wih U remains almos he same i D T is less han.. owever, he BER perormane o he heavy-loaded ase (i.e., U8 and U6) degrades severely or large D T sine he orhogonaliy among he 2nd OV spreading odes anno be mainained and hene he MAI appears. Thereore, he MAI anellaion ehniqe is neessary o minimize he perormane degradaion ased by he orhogonaliy desrion o he s OV spreading odes. Fig. 9 plos he BER perormane as a nion o D T or U8 and 6 when / 8dB. Or proposed reqeny-domain MAI anellaion is very eeive o improve he BER perormane. The MAI anellaion ee is more signiian as he nmber U o aessing sers inreases. For he ll-loaded ase (U6), he BER perormane an be almos he same nil D T approahes 8x -3 (his orresponding o a mobile moving speed o 6 km/h a 5 Gz arrier reqeny or Mbps ransmission). 5. COCLUSIOS In his paper, 2D-OV spreading was proposed o allow mli-rae ransmission while miigaing he MAI or DS-CDMA plink ransmission. The se o 2D- OV spreading and hip inerleaving an onver a mliser deeion (MUD) problem ino a se o singleser eqalizaion problems. 2D-OV spreading an no only inrease he plink apaiy, b also has an a lexibiliy in providing mli-rae servies. Comper simlaion resls have shown ha he proposed 2D- OV spreading and deision eedbak reqenydomain MAI-anellaion provides good BER perormane in a mliser reqeny- and imeseleive ading environmen wiho sing sophisiaed MUD ehniqe. REFERECES [] F. Adahi, Reverse link apaiy o orhogonal mli-ode DS-CDMA wih mliple onneions, IEICE Trans. Commn., vol. E85-B, no., pp , ov. 22. [2] A. J. Vierbi, CDMA: Priniples o spread sperm ommniaions, Addison Wesely, 995. [3] T. Oosson and A. Svensson, On shemes or mlirae sppor in DS/CDMA, J. Wireless Personal Commn., vol. 6, no. 3, pp , Mar [4] F. Adahi, M. Sawahashi, and. Sda, Wideband DS- CDMA or nex generaion mobile ommniaions sysems, IEEE Commn. Mag., vol. 36, no. 9, pp , Sep [5] S. ara and R. Prasad, Overview o miarrier CDMA, IEEE Commn. Mag., vol. 35, no. 2, pp , De [6] M. elard, R. Le Goable, J. F. elard, and J. Y. Badais, Mliarrier CDMA ehniqes or re wideband wireless neworks, Ann. Teleommn., vol. 56, pp , 2. [7] F. Adahi, D. Garg, S. Takaoka, and K. Takeda, Broadband CDMA ehniqes, IEEE Wireless Commn. Mag., vol. 2, no. 2, pp. 2-3, Apr. 25. [8] D. Faloner, S. L. Ariyavisiakl, A. Benyamin- Seeyar, and B. Eidson, Freqeny domain eqalizaion or singlearrier broadband wireless sysems, IEEE Commn. Mag., vol. 4, no. 4, pp , April 22. [9] G. B. Giannakis, Z. D. Wang, A. Saglione and S. Barbarossa, AMOUR-generalized mliarrier ranseivers or blind CDMA regardless o mlipah, IEEE Trans. Commn., vol. 5, no. 2, pp , Feb. 22. []. Sari and G. Karam, Orhogonal reqeny-division mliple aess and is appliaions o CATV nework, Er. Trans. Teleommn., pp , ov./de []A. Samolis, Z. Q. Li, and G. B. Giannakis, Spae-ime blok-oded OFDMA wih linear preoding or mlirae servies, IEEE Trans. Signal Proessing, vol. 5, no., pp. 9-29, Jan. 22. [2]Z. D. Wang and G. B. Giannakis, Wireless mliarrier ommniaions: Where Forier mees Shannon, IEEE Signal Proessing Mag., vol. 7, no. 3, pp , May 2. [3]Z. Wang and G. B. Giannakis, Blok preoding or MUI/ISI-resilien generalized mliarrier CDMA wih mlirae apabiliies, IEEE Trans. Commn., vol. 49, no., pp , ov. 2. [4]S. Zho, G. B. Giannakis and C. L. Marre, Chipinerleaved blok-spread ode division mliple aess, IEEE Trans. Commn., vol. 5, no. 2, pp , Feb. 22. [5]X. Peng, F. Chin, T. T. Tjhng and A. S. Madhkmar, A simpliied ranseiver srre or yli exended CDMA sysem wih reqeny domain eqalizaion, IEEE Pro. VTC 5 Spring, Sweden, pp , May. 25. [6]K. Ishihara, K. Takeda, and F. Adahi, DS-CDMA reqeny-domain mli-aess inererene aneller, IEICE Teh. Repor, RCS24-36, pp.55-6, Jan. 25. [7]S. Tsmra, S. ara, and Y. ara, Perormane omparison o MC-CDMA and ylially preixed DS- CDMA in an plink hannel, IEEE Pro. VTC 4 Fall, Los Angeles, USA, pp , Sep. 24.