PILOT-ASSISTED ADAPTIVE CHANNEL ESTIMATION USING MULTIPLE SETS OF TAP WEIGHTS FOR COHERENT RAKE RECEPTION OF DS-CDMA SIGNALS

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1 PILO-ASSISED ADAPIVE CHAEL ESIMAIO USIG MULIPLE SES OF AP WEIGHS FOR COHERE RAKE RECEPIO OF DS-CDMA SIGALS Shinsuke akaoka an Fuiyuki Aachi Eectrica an Counication Engineering, Grauate Schoo of Engineering, ohoku University, Japan Phone: , Fax: E-ai: Abstract For the coherent reception os-cdma signas, this paper proposes a new aaptive channe estiation schee using utipe sets of aaptive fiter tap weights, each set being use for channe estiation at ifferent sybo position. For upating each set of tap weights using the noraize east ean suare (LMS) agorith a bock of known piot sybos is tie-utipexe with the ata ouate sybos. he average bit error rate (BER) perforance in a freuency-seective Rayeigh faing channe is evauate by coputer siuation. It is confire that the propose channe estiation schee provies better BER perforance than the conventiona channe estiation schees.. Introuction Direct seuence coe ivision utipe access (DS-CDMA) is use in present ceuar obie counications systes []. For coherent rake reception os-cdma signas in a utipath faing channe [], accurate channe estiation is necessary. Aong any channe estiation schees so far propose [3]-[], piot-assiste channe estiation cae weighte uti-sot averaging (WMSA) channe estiation uses a finite ipuse response (FIR) fiter using the tie invariant tap weights [9]. However, using the tie invariant tap-weights cannot aways iniize the bit error rate perforance (BER) in a changing utipath propagation environent ue to user s oveent. In aition to faing, the presence of the freuency offset between a transitter an a receiver ay egrae the BER perforance ue to constant phase rotation in the receive signa. Aaptive channe estiation schees propose in [], [] aapt the copex-vaue tap weights accoring to a change in the faing conitions an can aso track the constant phase rotation ue to the freuency offset. he aaptive preiction channe estiation propose in [] is an extension of the conventiona WMSA channe estiation an uses the forwar preiction an backwar preiction FIR fiters. heir tap weights are upate using the noraize east ean suare (LMS) agorith [3]. In this paper, a piot-assiste aaptive channe estiation using utipe sets of tap weights is propose that siutaneousy estiates the channe gains at ifferent sybo positions. Each set of aaptive fiter tap weights is upate using the LMS agorith the reference signa for which is obtaine by reverse ouating the receive ata sybo at the sybo position of interest. he reainer of this paper is organize as foows. A transission syste oe is presente in Sect.. In Sect. 3, the operation principe of the propose aaptive channe estiation schee is escribe. Sect. 4 presents the coputer siuation resuts in a freuency-seective Rayeigh faing channe an copares the siuation resuts with those using the non-aaptive WMSA channe estiation [9] an the aaptive preiction channe estiation []. Aso iscusse is the BER perforance in the presence of freuency offset between a transitter an a receiver. Sect. 5 concues the paper.. ransission Syste Moe he transission syste oe an the sot structure are iustrate in Figs. an, respectivey. At the transitter, a binary ata to be transitte is transfore into ata-ouate sybo seuence. hen, a bock of known p piot sybos is tie-utipexe every ata sybos as shown in Fig.. p piot sybos an succeeing ata sybos ake a ata sot with a ength of sot p. Finay, the piot-inserte ata-sybo seuence is utipie by a spreaing seuence to prouce the DS-CDMA signa s(. s( can expresse in the euivaent baseban representation as s( ( p( S ( p( k k u( t / k), () c u( t / c ) where S enotes the signa power, ( represents the ata-sybo seuence wavefor p( the spreaing seuence wavefor an the ata-sybo uration. In E. (), { k } represents the ata-sybo seuence, {c } represents the spreaing seuence with a chip uration of c, an u( is the rectanguar puse with u( ( t < ) an (otherwise). he spreaing factor SF is given by SF/ c. he DS-CDMA signa s( is transitte over a utipath propagation channe. It is assue that the propagation channe is freuency-seective an has L iscrete paths having ifferent tie eays of integer utipe of c, an experiencing inepenent faing. he receiver has a tota of M spatiay separate antennas. he channe ipuse response h (t,τ) at tie t, seen on the th antenna, M-, can be expresse as [4]

2 h L ( t, τ ) ( δ ( τ τ ), () where ( an τ enote the copex path gain an tie eay of the th path, respectivey, with E[ L operation. ( t ) ] Inforation ata Antenna # exp( jπ ft ) sot sot Antenna #M- oise ouator Piot sybo MF ; E[.] is the ensebe average * MUX Mutipath Faing channe D Channe estiator Spreaing coe generator * #L- # S Rake cobiner #M- Fig. ransission syste oe. p sybos sybos piot ata Fig. Sot structure. # Recovere ata ecision Reouator he receive signa r ( on the th antenna in the presence of the freuency offset f between the transitter an the receiver can be expresse as L r ( ( exp( jπ f s( t τ ) v ( ), (3) t where v ( represents the aitive white Gaussian noise (AWG) having the singe-sie power spectru ensity of. We have assue that the receiver fiter is wie enough not to istort the receive signa at a ue to the freuency offset. he receive DS-CDMA signa is esprea an resove into L copies of transitte ata-sybo seuence by a atche fiter (MF). he MF output r (g, at the nth sybo tie epoch of the gth ata sot, associate with the th path, is represente as r g g S sot sot ( n ) τ n τ ( g, exp[ jπ f ( g sot r ( p( t τ ) t n )] v, (4) where (g, (g sot n) with sot sot, g( p ) n, an v (g, represents the noise pus inter-path interference (IPI) coponent. In the rake cobiner, a tota of M L MF outputs are coherenty sue up base on axia ratio cobining (MRC) []. Representing the channe gain estiate as, the rake cobiner output η(g,, which is the ecision variabe, can be expresse as M L η r, (5) where * enotes the copex conjugation. 3. Aaptive Channe Estiation Using Mutipe Sets of ap Weights Fig. 3 iustrates the bock iagra of aaptive FIR fiter. Q sets of the K tap weights are provie for siutaneous channe estiation, where Q (with /Q being an integer). his aaptive channe estiation fiter is necessary for each resove path. First, the instantaneous path gain is estiate by coherent aition of p receive piot sybos as in the conventiona WMSA channe estiation [9]. Without oss of generaity, the piot sybo j is assue. he instantaneous channe estiate, ˆ,, at the beginning of the gth sot is given by p r p n ˆ. (6) hen, using the K past an K future instantaneous channe gains, { ˆ, ( g k); k K K}, the channe gains at ifferent ata sybo positions in the gth sot of interest are estiate by the K-tap aaptive fiter. Q sets of the K tap weights are use. When Q, channe estiation is carrie out at each ata sybo position in a sot. On the other han, when Q, channe estiation using one set of the K tap weights is carrie out at each ata sybo position in a sot. he channe gain estiate at the nth ata sybo position of the gth sot, associate with the th path, is given by K w k K W,, X k) ˆ for n( /Q)()( /Q)-, with W X, ( g k) (7) [ w K ),, w K ) ] [ ˆ ( g K ),, ˆ ( g K ) ],,, (8) where W, (g) an X (g) are the copex tap weight vector of the th weight set (Q-) an the instantaneous channe gain estiate vector use for the gth

3 sot, respectivey, an [.] enotes transposition. For the aaptation of th set of K tap weights, the LMS agorith [3] is appie. After channe estiation an ata ecision for the gth sot, the K-tap weight vector is upate. he reference signa, for LMS agorith is obtaine using reverse ouation an /Q-sybo averaging as, Q r / Q k ˆ* p / Q k), (9) where ˆ( g, is the ecision resut on the nth sybo in the gth sot. he recursive reation for upating the tap weight vector is expresse as W, e ( g ) W,,, e µ, K ˆ k K X ( g k), () where e, (g) is the estiation error an µ is the step size. Receive piot sybo ˆ, ( g K) sot p p n w,, K) w,, ) r ˆ, ( g ) ˆ ˆ, ( g K ),, sot w,, ) w,, -K ) -,, sot Aaptive Agorith /Q-sybo averaging Fro reverse ouation Fig. 3 Aaptive fiter structure. W,, ( g ) 4. Coputer Siuation We assue a freuency-seective Rayeigh faing channe with a -path unifor power eay profie. ouation an spreaing ouation are uarature phase shift keying (QPSK) an binary PSK (BPSK), respectivey. Since channe estiation accuracy epens on the vaue of p, the seection of p is an iportant issue. As p increases, the signa-to-noise ratio (SR) of the instantaneous channe estiate ˆ, of E. (6) iproves ue to coherent aition of p receive piot sybos. Accoringy, the BER perforance iproves. However, the transission of too any non-inforation bearing piot sybos egraes the BER perforance ue to the increase power oss. In the foowing coputer siuations, we set p 4 an 6 as in []. For coparison, we assue the piot-aie aaptive preiction channe estiation using inear interpoation [] (hereafter, sipy referre to as aaptive preiction channe estiatio an the WMSA channe estiation [9] using K an (the tap weight vectors for K- an K-WMSA channe estiation are {.,.} an {.6,.,.,.6}, respectivey [9]). ransitter Propagation channe oe Receiver abe. Siuation conition Spreaing factor SF8, 64 Spreaing seuence Long P seuence Mouation QPSK Spreaing BPSK Sot structure Piot p 4 sybos 6 sybos Rayeigh faing with -path unifor power eay profie M-branch antenna Diversity iversity -finger rake o. of tap K8 weights he BER perforance achievabe with the propose channe estiation schee is copare with the WMSA channe estiation an the aaptive preiction channe estiation. First, we consier the case of no freuency offset f between the transitter an receiver. Fig. 4 pots the average BER perforance as a function of the average receive E b / per antenna for sot.64. Fig. 4(a) an 4(b) are for SF8 an 64, respectivey. Q (6 sybos) sets of K tap weights are use (i.e., each sybo position in a sot is provie a ifferent set of K tap weights for channe estiatio. he WMSA channe estiation is esigne to ephasize on reucing the noise effect at the cost of sighty osing the tracking abiity against faing. In a ow (high) E b / region, K()-WMSA channe estiation provies better BER perforance than K()-WMSA channe estiation. In a high E b / region, however, the WMSA channe estiation oes not perfor we an prouces the BER foors. However, the propose aaptive channe estiation aways provies the best BER perforance. When SF64, the propose aaptive channe estiation with -antenna iversity reception reuces the reuire average E b / per antenna for BER -4 by.6(.4) B copare with K()-WMSA channe estiation an by.3 B copare with aaptive preiction channe estiation. he BER perforance using the propose aaptive channe estiation approaches that using iea channe estiation an the egraation in E b / for achieving BER -4 is as sa as.7 B (.8B of which is ue to power oss owing to piot insertio. 3

4 - - - WMSA (K) WMSA (K) Ref[] Iea estiation - -path Rayeigh (L) Δf sot, Q (6) With antenna iversity, SF64 / B -3 w/o antenna iversity -3 WMSA (K) WMSA (K) Ref[] w/ antenna iversity -path Rayeigh (L) sot.64, Δf sot SF8, Q (6) Average receive E b / per antenna (B) (a) SF f D sot Fig. 5 Ipact of the noraize axiu Dopper freuency sot when SF w/ antenna iversity -path Rayeigh (L) sot.64, Δf sot SF64, Q (6) WMSA (K) WMSA (K) Ref[] Iea estiation w/o antenna iversity path Rayeigh (L) sot.64, Q (6) Without antenna iversity, SF64 / B WMSA (K) WMSA (K) Ref[] Average receive E b / per antenna (B) (b) SF64 Fig. 4 BER perforance coparison. Fig. 5 shows how the faing rate ipacts the channe estiation accuracy when SF64 an L. he faing rate is represente by the noraize axiu Dopper freuency sot. he average BERs with iversity reception at average receive E b / B are potte in the figure. With the propose aaptive channe estiation schee, aost the sae BER can be achieve if sot <.. However, with the other channe estiation schees, as sot increases, the tracking abiity of channe estiation against faing tens to be ost an hence, the achievabe BER increases f sot Fig. 6 Ipact of the noraize freuency offset f sot when sot.64. Faing prouces rano phase rotations in the receive DS-CDMA signa. If the freuency offset is present between the transitter an receiver, the receive signa experiences the constant phase rotation in aition to the rano phase rotation. Beow, we wi iscuss how the freuency offset affects the channe estiation accuracy an then, the achievabe BER perforance. Fig. 6 pots the average BER at average receive E b / B as a function of the noraize freuency offset f sot for the case of no iversity an SF64. Q6 sets of K tap weights are use. For very sa vaues of f sot, the preoinant cause of 4

5 ecision errors is the AWG an hence, the BER is aost constant. As f sot increases, the BER starts to increase ue to the presence of freuency offset. However, the propose aaptive channe estiation schee is very robust against the constant phase rotation ue to freuency offset as we as rano phase rotation ue to faing, as seen in Fig. 5. So far, we have assue that each sybo position is provie the ifferent set of K tap weights for channe estiation (Q 6). he sae weight set is share for channe estiation at /Q consecutive sybo positions. However, reucing the nuber of tap weight sets ay egrae the achievabe BER perforance. his is because ifferent sybo positions ay experience ifferent channe gains in a fast faing environent. Fig. 7 pots the average BER perforance with iversity as a function of Q for various vaues of sot when SF64. Athough the achievabe BER increases as Q becoes saer, as sa as Q6 sets of tap weights can be use with ony a sight perforance egraation path Rayeigh (L) Δf sot, SF64 With antenna iversity / B sot.64 f.9 D sot f.3 D sot uber Q of aveiabe weight sets Fig. 7 Ipact of the nuber Q of tap weight sets. 5. Concusion In this paper, for the coherent rake reception of DS-CDMA signas, a piot-assiste aaptive channe estiation using utipe sets of tap weights was propose that siutaneousy estiates the channe gains at ifferent sybo positions. In the propose aaptive channe estiation schee, each set of tap weights is upate using the LMS agorith. he reference signa for LMS agorith is obtaine by reverse ouating the receive ata sybo. he achievabe BER perforance in a freuency-seective Rayeigh faing channe was evauate by coputer siuation an was copare with those of the non-aaptive WMSA channe estiation [9] an the aaptive preiction channe estiation using inear interpoation []. It was confire that the propose aaptive channe estiation aways provies the best BER perforance. he propose channe estiation was foun to be aost insensitive to the faing rate; aost the sae BER can be achieve unti sot approaches.. he E b / egraation at BER -4 fro iea channe estiation is as sa as.7 B when sot.64. It was aso confire that the propose aaptive channe estiation is very robust against the freuency offset an that the nuber Q of tap weight sets can be reuce to as sa as Q6 with ony a sight perforance egraation. Acknowegent his work is supporte by Grant-in-Ai fro the Japan Society for the Prootion of Young Scientists. References [] F. Aachi, M. Sawahashi an H. Sua, Wieban DS-CDMA for next generation obie counication syste IEEE Coun. Mag., vo. 36, pp , Sept [] W. C., Jakes Jr., E., Microwave Mobie Counications, Wiey, ew York, 974. [3] J. K. Cavers, An anaysis of piot sybo assiste ouation for Rayeigh faing channes, IEEE rans. Veh. echno., Vo. 4, o. 4, pp , ov. 99. [4] S. Sapei an. Sunaga, Rayeigh faing copensation for QAM in an obie raio counications, IEEE rans. Veh. echno., Vo. 4, pp , May 993. [5] F. Ling, Coherent etection with reference-sybo base estiation for irect seuence CDMA upink counications, Proc. IEEE Vehicuar ech. Conference, ew Jersey, pp. 4-43, May 993. [6] Y. Hona an K. Jaa, Channe estiation base on tie-utipexe piot sybos, IEICE echnica Report, RCS96-7, pp. 3-35, Aug [7] C. I. Bang an M. H. Lee, An anaysis of piot sybo assiste 6QAM in the Rayeigh faing channe, IEEE rans. Consu. Eect., Vo. 4, pp.38-4, ov [8] P. Schra Anaysis an optiization of piot-channe assiste BPSK for DS-CDMA systes, IEEE rans. Coun., Vo. 46, o. 9, pp. -4, Sept [9] H. Anoh, M. Sawahashi an F. Aachi, Channe estiation fiter using tie-utipexe piot channe for coherent rake cobining in DS-CDMA obie raio, IEICE rans. Coun., Vo. E8-B, pp.57-56, Juy 998. [] S. Abeta, M. Sawahashi an F. Aachi, Perforance coparison between tie-utipexe piot channe an parae piot channe for coherent rake cobining in DS-CDMA obie raio, IEICE rans. Coun., Vo. E8-B, pp.47-45, Juy 998. [] S. Abeta, M. Sawahashi an F. Aachi, Aaptive channe estiation for coherent DS-CDMA obie raio using tie-utipexing piot an parae piot structures, IEICE rans. Coun., Vo. E8-B, pp.55-53, Sept.999. [] S. akaoka an F. Aachi, Piot-aie aaptive preiction channe estiation in a freuency-nonseective faing channe, IEICE rans. Coun., Vo. E85-B, pp , Aug.. [3] S. Haykin, Aaptive fiter theory, 3r eition, Prentice Ha, 996. [4] F. Aachi, ransit power efficiency of fast transit power controe DS-CDMA reverse ink, IEICE rans. Funaentas, vo. E8-A, pp. 4-48, Dec