DS-CDMA with Frequency-domain Equalization for High Speed Downlink Packet Access

Transcription

1 wih Frequeny-domain Equalizaion for High Speed Downlink Pake Aess Deepshikha Garg and Fumiyuki Adahi Dep. of Elerial and Communiaions Engineering Tohoku Universiy, Sendai, Japan Absra The nex generaion mobile ommuniaions sysem is aniipaed o suppor daa raes up o and exeeding Mbps. In his paper, we onsider a high-speed downlink pake aess HSDPA as in G W-CDMA sysems. Adapive modulaion and oding AMC, muliode operaion and hybrid auomai repea reques HARQ will be he enabling ehnologies. Wih suh high-speed daa ransmissions, however, muliode operaion severely suffers from desruion of orhogonaliy among he spreading odes sine he wireless hannel beomes severely frequeny-seleive. In his paper, we apply frequeny-domain equalizaion FDE insead of onvenional rake ombining for reeiving he pake. I is shown ha he use of FDE for he reepion of muliode signal gives an improved hroughpu irrespeive of he hannel s frequenyseleiviy. Keywords- HSDPA,, MMSE-FDE, HARQ, AMC, muliode I. ITRODUCTIO Daa servies, as aniipaed, have beome he dominaing soure of raffi load in he G neworks based on wideband ode division muliple aess W-CDMA. High speed daa servies will be suppored by he high speed downlink pake aess HSDPA onep [], whih allows peak daa raes up o and exeeding Mbps. Adapive modulaion and oding AMC, muliode operaion MC and hybrid auomai repea reques HARQ are he enabling ehnologies used for HSDPA. The nex generaion mobile ommuniaions sysem is aniipaed o suppor even higher daa raes up o and exeeding Mbps. Wih suh high speed daa ransmissions, he wireless hannel beomes severely frequeny-seleive []. AMC, MC and HARQ will sill be ineviable. However, in a frequeny-seleive hannel, muliode operaion severely suffers from desruion of orhogonaliy among he spreading odes. Reenly, i was shown ha using minimum mean square frequeny-domain equalizaion MMSE-FDE parially resores he orhogonaliy and provides a beer bi error rae BER performane han onvenional oheren rake ombining [, 4]. In his paper, we apply MMSE-FDE for reeiving he pake daa ransmied as in HSDPA wih AMC, MC and HARQ bu over a severe frequeny-seleive hannel. The remainder of he paper is organized as follows. Seion II desribes he pake aess for wih MMSE- FDE. The MMSE weigh for pake ombining is also inrodued. The simulaion resuls are presened and disussed in Se. III. Seion IV onludes he paper. II. PACKET ACCESS FOR WITH FDE The onvenional reeivers are equipped wih a rake ombiner ha an ake advanage of he pah diversiy. When he number of propagaion pahs in he hannel inreases, he reeiver omplexiy inreases due o he inrease in he number of rake fingers. Moreover, he orhogonaliy among he spreading odes is desroyed due o severe inerpah inerferene and herefore muliode operaion anno guaranee high speed ransmissions. The orhogonaliy an be parially resored wih MMSE-FDE and is suiable for muliode operaion. In his paper, we propose he use of MMSE-FDE for he reepion of muliode pake daa. Similar o HSDPA, we onsider he muliode ransmission wih a spreading faor SF of 6 and assume ha error orreion oding is done by a rae-/ urbo ode wih he rae R varied beween ¼ and ¾ by means of punuring and repeiion. HARQ wih inremenal redundany IR eah reransmission may use a differen redundany version [5] or Chase ombing CC single redundany version [6] may be used. Wih FDE, he reeived signal is firs onvered o frequeny omponens by fas Fourier ransform FFT and MMSE equalizaion is performed for eah frequeny omponen, afer whih he signal is ransformed bak o imedomain signal wih inverse FFT IFFT. However, wih FDE, i is essenial ha he signal be periodi in he FFT window and hene guard inerval GI in he form of yli prefix needs o be insered [, 4] in he ransmi signal as for muliarrier CDMA MC-CDMA. A he ransmier, rae mahing [] an be modified so as o allow for GI inserion. The ransmier and reeiver for pake aess wih MMSE-FDE are shown in Fig.. GI inserion blok, whih is no presen in he urren HSDPA sandard, is added a he ransmier. The reeiver sruure is modified o inorporae he FDE; FFT and IFFT bloks are added and he rake ombiner is removed. The informaion sequene is firs urbo oded and punured aording o he hannel qualiy indiaor CQI. I is hen inerleaved and QPSK or 6QAM modulaed aording o CQI. The signal is hen spread and muliode muliplexing is performed. In his paper, hip-spaed disree-ime represen-

2 info. CRC Enoding Turbo Enoding Rae-mahing Inerleaving Buffer Modulaion Code # C- o, Code # sr GI Inserion P/S s ACK/ AK. AMC Muliode Spreading a Transmier Frequeny domain equalizaion Muliode Despreading r GI Removal S/P Conversion poin FFT w w poin IFFT P/S Conversion sr Code # C- o, Code # P/S Demodulaion Deinerleaving Rae-mahing Turbo Deoding CRC Deoding ACK/ AK. info. es. w Channel Es./Weigh Comp. AMC b Reeiver Figure Transmission sysem model wih MMSE-FDE a he reeiver -aion of signals is used. The resuling sequene is C P SF s x / SF o, mod SF sr for SF K, where P represens he ransmi power per ode, C is he number of odes muliplexed, SF is he spreading faor, { o,, C-, SF-} is he hannelizaion ode, { sr } is he ommon srambling ode and {x, C-, K-} is he daa-modulaed symbol of lengh T s. g -hip GI is insered for every blok of hips; is he number of FFT/IFFT poins a he reeiver and g is a fraion of. The resulan GI-insered muliode signal s is ransmied over he propagaion hannel. The hannel is assumed o be omposed of L disin propagaion pahs wih differen ime delays. The reeived muliode signal is sampled a he hip rae o obain { r ; SF K + g / }. Ideal sampling iming is assumed. The g -sample GI is removed and -poin FFT is applied o eah blok of hips in order o deompose he reeived signal ino he -frequeny omponens. The kh frequeny omponen for he nh blok is n+ R k r exp jπ k mod / n n for k - and n SF K /. The MMSE-FDE weigh w n k for R n k is given by [] H n k wn k, C PT + s H n k SF where represens he AWG power sperum densiy, H n k denoes he hannel gain a he nh blok s kh frequeny, whih is he FFT of he hannel impulse response, and. denoes he omplex onjugae operaion. Afer frequenydomain MMSE equalizaion, IFFT is arried ou o obain he muliode signal in he ime-domain: s wn k Rn k exp jπk mod / k s H n k + H n k k k k s τ exp jπ τ + η τ 4

3 for n n+ -, where H n k Hn k wn k and η is he noise sample a ime due o he AWG. IFFT is followed by muliode despreading o obain SF P SF s { k o, mod SF sr } H k n x + µ ICI + µ noise for C- and ik. The firs erm represens he desired daa symbol omponen and he seond and hird erms, µ ICI and µ noise, are iner-hip inerferene ICI and he noise due o AWG, respeively, given by µ µ ICI noise SF SF SF k SF { { o, H n k τ o, mod SF 5 τ s τ exp jπk. 6 mod SF sr sr } } η I an be undersood from Eq. 5 ha he equivalen hannel gain for all he symbols wihin an FFT blok is n Hn k k H and he frequeny diversiy gain is a no a funion of SF. { i; C } are parallel-o-serial P/S onvered for daa demodulaion. Then, sof deision values for urbo deoding are generaed using he log-likelihood LLR approximaion [7], given by i P / SF Hn i s i P / SF H s L b 7 σ σ for he bh bi in he ih symbol; b or for QPSK and 6QAM, respeively. Here, ŝ or ŝ is he andidae symbol, wih or in he bh bi posiion, for whih he Eulidean disane from is minimum. σ is he Gaussian approximaed ICI plus noise variane given by [8] w k k σ. 8 SF T s C PT s + H k H k SF k k The LLR values are ompued for C- and for all he bis in he symbol. Turbo deoding is performed using hese LLR values as sof inpu afer rae mahing. Then, error deeion is performed and reransmission is requesed if errors are deeed. In his paper, we inrodue MMSE weighs for frequenydomain pake ombining. When he same pake is reransmied, ime diversiy gain is obained, similar o anenna diversiy [4], and hene he weigh for frequenydomain pake ombining is modified as H n, m k w, m k M C PT, s H n m k + m SF n, 9 where M is he number of imes he same pake is reeived and H n,m k is he hannel gain a he nh blok s kh frequeny for he mh ransmission. III. SIMULATIO RESULTS The simulaion ondiions are summarized in Table. For he simulaion purpose, we assume a frequeny-seleive Rayleigh fading hannel having a hip-spaed L-pah uniform power delay profile and a normalized maximum Doppler frequeny f D T blk of., where f D is he maximum Doppler frequeny given by raveling speed/arrier wavelengh and T blk is he blok lengh wih + g hips. In he simulaion, 56 and g are assumed. The Walsh-Hadamard odes wih SF6 are used as shor spreading odes. Code muliplexing of C6 is assumed ha provides he same daa rae as he nonspread single arrier modulaion [9]. In he AMC, a rae-/ urbo ode having wo,5 reursive sysemai onvuluional RSC enoders same as in HSDPA, wih he rae R varied beween ½ and ¾, and QPSK and 6QAM are used as shown in Table. Ideal AMC is assumed, i.e., he modulaion and oding rae se MCS ha gives he highes performane a eah average reeived E s / is seleed and i is assumed ha here is no seleion error. For Case ombining CC, he same pake wih punuring marix P is ransmied unil a posiive aknowledge is reeived. For inremenal Table : Simulaion ondiions R/ and /4 Turbo oding, 5 RSC enoder Log-MAP deoding wih 8 ieraions Channel Blok Inerleaver Inerleaver Daa Modulaion ARQ Channel model Coheren QPSK and 6QAM o. of FFT poins 56 GI g Spreading faor SF6 Inremenal redundany/ Chase ombining L-pah Rayleigh fading τ l l f D T blk.

4 Table : Modulaion and oding rae se MCS Modulaion Coding rae QPSK ½ QPSK ¾ 6QAM ½ 4 6QAM ¾ Table : Punuring Paern R ½ ¾ P P redundany IR, he punuring marix P is used for he firs ransmission and P for he seond ransmission and he order repeaed for furher ransmissions. Code ombining is employed if he same pake is ransmied more han one. The punuring maries are shown in Table. Ideal hip synhronizaion and ideal hannel esimaion are assumed a he reeiver. Perfe error deeion and an error-free reverse hannel are assumed. Figure plos he hroughpu in bps/hz as a funion of he average reeived signal energy per symbol-o-he noise power speral densiy raio E s / for CC and IR wih AMC when rake ombining is employed a he reeiver. CSF6 is assumed. This is referred o as AMC. I is seen ha IR provides a slighly higher hroughpu han CC for all hannel ondiions. In IR, he seond reransmission onsiss of sending he unsen pariy bis insead of sending he same bis as in CC. From he resul i an be inferred ha he addiional oding gain due o inreased redundany is more desirable han he inreased reeived power owing o pake ombining. For L, he hroughpu gradually inreases wih he inrease in he average reeived E s /. However, even for L, he hroughpu aainable wih rake ombining is drasially redued due o he desruion of orhogonaliy among he spreading odes. Wih he inrease in L, he frequeny seleiviy of he hannel inreases and he orhogonaliy desruion is severer. Hene, he hroughpu dereases wih he inrease in L. The hroughpu wih he proposed MMSE-FDE reeiver is ploed in Figs. a and b for CC and IR, respeively when 6QAM modulaion and rae ½ urbo ode is used. The modulaion and oding rae is fixed o onenrae on he effe of he number L of pahs. For IR, he ode rae dereases afer he seond ransmission. Wih MMSE-FDE, he hroughpu is almos independen of he hannel s frequeny seleiviy for boh CC and IR. The orhogonaliy desruion is severer for sronger frequeny-seleiviy, bu MMSE-FDE is uilized ha resores orhogonaliy o a erain exen and benefis from he frequeny seleiviy of he hannel [, 4]. Inreased frequeny seleiviy resuls in more random errors whih are desirable for hannel oding bu undesirable for pake ransmissions. For lower E s / regions, he hroughpu dereases for higher frequeny seleiviy due o more pake errors bu for higher E s / regions here are few errors ha an be orreed by he Figure. Figure f DT blk. wih AMC IR CC 5 5 Average reeived E s / [db] Throughpu wih rake ombining Chase Combining 6QAM, R / SFC 6 f DT blk. L L L6 5 5 Average reeived E s / [db] a Chase Combining Inremenal Redundany 6QAM, R/ SFC 6 f D T blk. L L L4 L6 5 5 Average reeived E s / [db] b Inremenal Redundany Throughpu wih MMSE-FDE. L L L4 L6

5 urbo deoder and he hroughpu inreases. Figure 4 ompares he hroughpu in bps/hz wih MMSE- FDE and rake ombining as a funion of he average reeived E s / wih AMC when MMSE-FDE is applied for CC and IR. Even wih AMC, here is almos no differene in hroughpu wih he inrease in L. I an also be noed ha here is no advanage of IR over CC employing frequeny-domain pake ombining. For L, he hroughpu is slighly lower wih MMSE-FDE ompared o rake ombining, due o he GI inserion. However, for L6, he hroughpu is muh beer han ha of rake ombining. Rake ombining is no suiable for hannels wih many srong pahs when MC operaion is applied. Some form of orhogonaliy resoraion is needed. The MMSE- FDE provides a good rade-off beween orhogonaliy resoraion and noise enhanemen [4] and is suiable for hannels wih high frequeny seleiviy. The IR hroughpu wih MMSE-FDE is ompared wih ha of non-spread single arrier ransmission [9] in Figure 5 when L6 and AMC is uilized. The non-spread single arrier ransmission is equivalen o wih SF. To mainain he daa rae fixed, C6 is assumed. The hroughpu of wih rake ombining is also ploed for referene. The hroughpu of wih MMSE-FDE is he same as ha of non-spread single arrier ransmission. As was disussed in Se. II, he frequeny diversiy effe in DS- CDMA wih MMSE-FDE does no depend on SF. The inerferene depends on he C/SF raio as an be seen from Eq. 8. Hene, when CSF, he performane is he same irrespeive of SF. IV. COCLUSIO In his paper, we showed ha he use of MMSE-FDE for he reepion of muliode pake signal ransmied as in HSDPA gives an improved hroughpu. When rake ombining is used, he hroughpu degrades drasially wih he inrease in he frequeny-seleiviy of he hannel due o orhogonaliy desruion among he spreading odes. However, MMSE-FDE an give improved hroughpu irrespeive of he hannel ondiions. In addiion, MMSE weigh for frequeny-domain pake ombining was inrodued. I was found ha wih adapive modulaion and oding, here was no advanage of inremenal redundany over Chase ombining employing frequeny-domain pake ombining. REFERECES [] GPP TR5.858, High speed downlink pake aess: physial layer aspes, version 5... [] W. C., Jakes Jr., Ed., Mirowave mobile ommuniaions, Wiley, ew York, 974. [] F. Adahi, T. Sao, and T. Iagaki, Performane of muliode DS- CDMA using frequeny domain equalizaion in a frequeny seleive fading hannel, Eleronis Leers, vol. 9, no., pp. 9-4, Jan.. [4] T. Iagaki and F. Adahi, Join frequeny domain equalizaion and anenna diversiy ombining for orhogonal muliode signal ransmissions in a frequeny seleive fading hannel, Pro. of IEEE WPMC, vol., pp , O., Yokosuka, Japan. [5] J. Hagenauer, Rae-ompaible punured onvoluional odes RCPC odes and heir appliaion, IEEE Trans. Commun., vol. 6, no. 4, pp.89-4, April [6] D. Chase, Code Combining--A Maximum-Likelihood Deoding Approah for Combining an Arbirary umber of oisy Pakes, IEEE Trans. on Commun., vol., no. 5, pp. 85-9May 985. [7] A. Sefanov and T. Duman, Turbo oded modulaion for wireless ommuniaions wih anenna diversiy, Pro. of IEEE VTC99-Fall, pp , eherlands, Sep [8] F. Adahi and K. Takeda, Bi error rae analysis of wih join frequeny-domain equalizaion and anenna diversiy ombining, o appear in IEICE Trans. on Commun. [9] D. Faloner, S. L. Ariyavisiakul, A. Benyamin-Seeyar, B. Eidson, Frequeny domain equalizaion for single-arrier broadband wireless sysems, IEEE Commun. Mag., vol. 4, no. 4, pp , April. Figure 4. Figure f DT blk. SFC6 wih AMC Average reeived E s / [db] IR CC L L6 rake MMSE-FDE Throughpu omparison wih rake ombining Inremenal Redundany SFC 6 wih AMC L6, f DT blk. non-spread single arrier 5 5 Average reeived E s / [db] wih MMSE-FDE wih rake Throughpu omparison wih non-spread single arrier sysem.