A Space-Time Pre-RAKE Transmitter Diversity Method for W-CDMA Using Long Range Prediction 1

Transcription

1 A Spae-Tme Pre-RAKE Transmtter versty ethod for W-CA Usng Long Range Predton Sen Gunavd and Alexandra uel-hallen North Carolna State Unversty ept of Eletral and Computer Engneerng Center for Advaned Computng and Communaton an Campus rve Box 794, Ralegh, NC E-mal: {sgunav, Abstrat Transmtter dversty n the downlnk for Code vson ultple Aess (CA) systems provdes a means to aheve smlar performane gans as for the moble-staton reever dversty wthout the omplexty of a moble-staton reever antenna array Pre-RAKE preodng at the transmtter an be employed to aheve the multpath dversty wthout the need of the RAKE reever at the moble staton We examne feasblty of several transmtter dversty tehnques for the 3 rd generaton Wdeband CA (W-CA) systems We also nvestgate an optmal method to ombne transmtter dversty and preodng that aheves the gan of maxmum rato ombnng of all spae and frequeny dversty branhes We employ long range predton to enable these methods for rapdly tme varyng multpath fadng hannels I INTROUCTION Wdeband CA s a major anddate for the 3 rd generaton wreless ommunaton systems The uplnk (from moble staton to base staton) apaty of the proposed thrd generaton CA systems an be enhaned by varous tehnques nludng mult-antenna (dversty/antenna array) reepton and multuser deteton (U) The tehnques that nrease the downlnk (from base staton to moble staton) apaty have not been developed n reent years wth the same ntensty However, t s understood that the apaty demand mposed by the projeted data serves (eg Internet) burdens more heavly the downlnk hannel Hene, t s mportant to fnd tehnques that mprove the apaty of the downlnk hannel Bearng n mnd the strt omplexty requrements of termnals, and the haratersts of the downlnk hannel, an applaton of advaned detetors wth multple reeve antennas s not seen as the desred soluton to the downlnk apaty problem Transmtter dversty n the downlnk usng multple transmtter antennas provdes smlar performane gans as for the moble-staton reever dversty wthout the omplexty of a seond moble-staton reever antenna Transmtter based methods enable to shft sgnal proessng to the transmtter where power and omputatonal omplexty are more abundant, thus smplfyng reever unts fferent solutons have been proposed suggestng that multple antennas (or transmt dversty) at the base staton wll nrease downlnk apaty wth only mnor nrease n termnal mplementaton The proposed tehnques nlude Spae-Tme Codng [], elay versty, Orthogonal Transmt versty, Tme Swthed Transmt versty, Seletve Transmt versty (ST) and Transmt Adaptve Array (Tx AA) [2-5] Some of the transmtter dversty tehnques (eg Tx AA, ST) requre feedbak of the hannel state nformaton (CSI) from moble staton We refer to them as adaptve tehnques Wth the help of the feedbak, better performane s aheved then for non-adaptve methods The CSI s used to alulate the omplex weghts requred by Tx AA and to swth to the antenna wth hgher hannel power for ST CA tehnology s desgned to explot the multpath haratersts of the hannel However, the RAKE reever s requred to aheve multpath dversty gan For the downlnk, ths requrement nreases the omplexty and power onsumpton of the moble staton Pre-RAKE dversty ombnng tehnques [7,8,7] were proposed to overome ths problem Wth ths method, RAKE ombnng s performed before transmsson at the Base Staton so that the oble Staton an employ a smple mathed flter reever Whle Tx AA s optmal for flat fadng hannels, ts performane s lmted for multpath fadng hannels by the salar weghts oreover t requres a RAKE reever at the moble staton We nvestgate Spae-Tme Pre-RAKE dversty tehnque for multpath fadng hannels and ompare t wth the prevously studed tehnques suh as ST and Tx AA Ths transmtter dversty ombnng method an aheve the full beneft of maxmum rato ombnng (RC) of all spae and frequeny dversty branhes wthout usng the RAKE reever at the moble staton A smlar sheme was studed n [7] for tme dvson duplex (T) applatons ST, Tx AA and pre-rake systems all requre knowledge of the reeved CSI However, n pratal rapdly varyng fadng hannels, the fed bak CSI s not up-to-date whh results n performane degradaton Long range predton an be used to foreast the fadng profle of the hannel and mprove performane [9,5,6] The superor performane of ths algorthm relatve to onventonal methods s due to ts muh longer memory span obtaned by usng lower samplng rate gven fxed model order In ths Ths researh s funded by NSF grants CCR and CCR-876

2 Base Staton oble Staton Codng & Interleavng Spreadng Tx versty, Pre-RAKE Channel emodulator eoder Predton Fgure W-CA System odel paper, we use long range fadng hannel predton to mprove performane of losed-loop antenna dversty methods for rapdly varyng fadng hannels (see also [9, 5]) In the next seton, we revew the W-CA system model, the struture of the hannel and Tx AA, ST and pre-rake preodng tehnques The Spae-tme pre- RAKE s desrbed n Seton 3 Seton 4 summarzes the long range predton method, and smulaton results and omparsons are presented n Seton 5 II W-CA SYSTE OEL, TRANSITTER IVERSITY AN PRECOING TECHNIQUES For smulaton of performane mprovements wth long range predton, a omputer smulaton envronment s reated based on proposed W-CA features [] The blok dagram of the system s shown n Fgure Half- rate onstrant length 9 onvolutonal odng s used wth generator polynomal parameters 56 and 753 n otal form The mnmum dstane of the ode s 2 The nterleavng depth s ms Orthogonal odes are obtaned usng the tree struture, explaned n [] The hannels assoated wth dfferent antenna elements are assumed to be d multpath Raylegh fadng modeled by the Jakes model wth 9 osllators [] The mpulse response of the th L antenna s gven by h ( t) = h ( t) δ ( t jt ), where L j j= s the number of multpath omponents and T s the hp nterval The path weghts are d, and the transmtter power s normalzed to unty Eah hannel s orrupted by Addtve Whte Gaussan Nose (AWGN) wth power spetral densty (PS) N o /2, and Quadrature Phase Shft Keyng (QPSK) modulaton s employed In the dsusson of the performane of varous transmtter dversty tehnques n ths seton, we wll assume a sngle user unoded system wth the sgnature waveform that s orthogonal to ts shfts by the hp nterval T We also assume that the transmtter has the perfet knowledge of reeved hannel oeffents In the smulaton results of Seton 5, the self-orthogonalty assumpton s removed, sne the W-CA sgnature waveforms are employed, the data s oded and rapd hannel varaton s taken n the aount n the performane evaluaton Tx AA and ST are losed loop transmt dversty tehnques and they requre feedbak from the moble staton It s assumed that multple transmt antennas are loated at the Base Staton (BS) and oble Staton (S) employs sngle antenna reepton Tx AA s llustrated n Fgure 2 For Tx AA, eah antenna transmts oherently wth the same data and ode but wth antenna-spef weghtng Eah transmtter antenna has a separate plot sgnal, whh enables the S to ndvdually estmate the hannels used by eah antenna [] The goal of ths sheme s to hoose the weghts appled at the BS so that the total power reeved by the S s maxmzed Tx Base Staton Spread Sgnal w w 2 w h (t) h 2 (t) h (t) Fgure 2 Tx AA Shemat oble Staton Predton Assume antennas wth flat fadng hannels efne h=[h h 2 h ] as a row vetor, where h s the fadng hannel oeffent for the hannel between the th transmtter antenna and the S Then the Tx AA weghts are [4]: H h w = () H hh H where h s the Hermtan of the vetor h If there are L paths for eah antenna, then the weghtng vetor whh maxmzes the total reeved power at the S s the egenvetor v max, orrespondng to the largest egenvalue, λ max of the x matrx H H H [5], where H=[h h 2 h ] and h =[ h h h (L-) ] T s the vetor that ontans L multpath oeffents between the th antenna and the S For flat fadng ase, the performane of the Tx AA system s equvalent to the optmal dversty ase, e axmal Rato Combnng (RC) wth the order of (number of antennas) However Tx AA annot aheve the optmal performane n multpath fadng oreover, t requres Rx

3 Tx Channel Rx Pre-RAKE Flter Spreadng & Salng ss(t) p(t) r(t) Flter mathed to ss(t-(l-)t ) h * L-(t) h o (t) n(t) Predton h * L-2(t) h (t) h * (t) h L- (t) Fgure 3 Pre-RAKE versty Combnng the RAKE reever at the S In the ST method, the dedated hannel of a gven user s swthed to the desgnated antenna wth the largest reeved power The deal performane of ths tehnque mathes that of the seleton dversty at the reever ST swthng frequeny determnes the rate at whh antenna seleton sgnal s fed bak to the BS, hene the swthng between the transmt antennas fferent swthng rates result n dfferent BER performane of the system The lower bound for the performane of ths tehnque s aheved when the antenna swthng s done for every symbol transmtted (ST Every bt) Ths bound s omputed as follows Suppose the RAKE reever wth RC s employed at the S Then the umulatve dstrbuton funton of the fadng oeffent at the output of the RAKE reever s: F where s b ( ) =! p b ( ) L p b ( ) p b ( ) d d d (2) L / = e s the pdf for sum of L ( ) ( L )! L paths hannel powers and s the average Sgnal to Nose Rato (SNR) per path, whh s assumed to be dental ε for all paths and defned as b = E(h j), where ε s the b N bt energy, No/2 s the AWGN power, h j s the fadng oeffent for any of the paths and E(ï) s the expetaton operaton Then the probablty of error s: STLx b BPSK ) P s ( e) = P ( ) P ( d (3) s where P ( ) s the probablty densty funton and found b by dfferentatng (2) The P BPSK ( ) = Q( 2 ) s the probablty 2 of error for the BPSK system and Q(ï) s the error funton defned as Q 2 t / 2 ( x) = e dt In partular, the 2x x probablty of error for ST Every bt wth flat fadng d hannels and antennas an be found from [6]: k P ST( e) = ( ) (4) k= 2( k + ) k k + + For multpath fadng hannels, frequeny dversty an be aheved usng the RAKE reever Pre-flterng of the transmtted sgnal at the base staton an aheve the same performane as RAKE wth RC by employng a sngle mathed flter at the moble staton [7,8,7] Ths transmtter preodng tehnque s known as pre-rake versty Combnng In Fgure 3, the pre-rake method s llustrated for a multpath sngle antenna fadng hannel wth mpulse L response h ( t) δ ( t ) = T As seen n Fgure 3, the oeffents of the pre-transmt flter are the onjugates of the hannel path oeffents and the order of the pre-rake oeffents s the reverse order of the hannel oeffents III SPACE-TIE PRE-RAKE TRANSITTER IVERSITY ETHO When Tx AA s used for the multpath fadng hannel, the resultng system annot aheve the optmum performane, e t annot aheve the gan of the RC of all spae and frequeny dversty branhes [5] Ths s due to the salar weghts used by the Tx AA We nvestgate a Spaetme pre-rake method that optmally ombnes multpath powers assoated wth all transmtter antennas usng pre-

4 RAKE preodng and the approprate salng For antenna m, we flter the spread sgnal s(t) so that the transmtter pulse shape s: L * p ( t) = hm, L j ( t) s( t jt ) (5) L 2 j= h ( t) m= j= m, j where s(t) s the sgnature sequene and h m, j ( t) s the fadng oeffent orrespondng to the j th path of the m th antenna Assume the reever employs a sngle flter mathed to s(t-(l- )T) The desred sgnal ours at t-(l-)t and s gven by: L 2 hm, j( t) b+ z (6) m= j= where b s the transmtted BPSK symbol and z s the fltered nose Ths s equvalent to RC wth Lx dversty branhes The probablty of error s then [2]: Lx Lx Lx + k P ( ) = ( µ ) (+ µ ) 2 k= k 2 where ε (7) µ =, and + s the average SNR per path IV LONG RANGE PREICTION TECHNIQUE It was shown n [9,5,6] that long range predton based on the nmum ean Square Error rteron (SE) an be used to aurately estmate the future hannel state nformaton at least several mllseonds ahead for rapdly tme varyng fadng hannels In ths paper, SE predton of ndvdual omplex hannel oeffents assoated wth all paths s employed [9] It s based on lnear predton method wth autoregressve (AR) hannel modelng Assume we sample the hannel at a rate f s =/T s, where f s f d, f d s the maxmum oppler shft The hannel s assumed to be omplex Raylegh fadng proess (t) Let =(T s ) Our am s to predt n based on p prevously observed hannel samples n-p, n-p+,, n-2, n- We ompute the (pxp) autoorrelaton matrx R wth * oeffents Rj = E[ n n j ] and (px) autoorrelaton * vetor r wth oeffents r = E[ nn+ ] Then the mnmum mean square error predton of n s : p n= d n- (8) = where d={d }, =p and d=r - r For the smulatons, p=5 and the observaton nterval of 2 samples are used to ompute the autoorrelaton of n Ths observaton nterval an be sgnfantly redued and matrx nverson an be avoded f adaptve long range predton s used [5] For W-CA the samplng rate f s s hosen as the slot rate of 6kHz Ths results n 625 ms delay for alulatng the hannel state nformaton In addton, t s desrable to obtan future CSI further than one slot ahead when the k antenna swthng rate s lower than the slot rate (eg 4 Hz) [6, 9, 5] The predton algorthm desrbed above s used to obtan predted values of the urrent and future hannel oeffents gven the delayed hannel samples For the ST ase, these oeffents are used to hoose the antenna wth the largest reeved power For Tx AA, they are used to alulate the weghts of eah antenna Fnally, for pre-rake and Spae-tme pre-rake, they are utlzed n flterng the spread sgnal pror to transmsson Both Tx AA and Spaetme pre-rake methods requre the knowledge of the CSI for every symbol transmtted Sne the CSI s fed bak from the S at 6kHz, t s neessary to use nterpolaton to obtan the ntermedate oeffents of the hannel For these methods, the begnnng of the urrent slot and the begnnng of the next slot are predted usng the past values of the hannel Then these values are used to ompute the ntermedate values between two slots, so that the CSI at the transmsson rate s obtaned V SIULATION RESULTS We removed the deal assumptons of Seton 2 and 3, and used omputer smulatons to evaluate the bt error rate (BER) of the shemes desrbed above The W-CA ommunaton envronment ntrodued n Seton 2 s employed wth 2 GHz arrer frequeny, 6mph vehle speed, 496 ps hp rate, and the bt rate of 28 kbps [] The data was oded We assume that there s no estmaton error at the S The results are for a sngle user where multple aess nterferene (AI) s modeled as whte Gaussan nose In all urves, RC, no dversty and Every bt results are obtaned by smulaton usng the prefet CSI Fgure 4 shows the omparson between ST for 6 KHz swthng rate wth RAKE reever (no pre-rake dversty) and Spae-tme pre-rake ST 6 KHz and Spae-Tme pre-rake methods wth predton are ompared wth the same tehnques, but when delayed CSI s used to selet antenna or to ompute pre-rake weghts Predton results n ~ ½ - db gan for all methods In [6,9,5] t s demonstrated that predton s even more helpful when swthng frequeny s redued to 4Hz It s observed that Spae-Tme Pre-RAKE performs better than ST for all swthng rates Spae-Tme Pre-RAKE provdes around db gan over ST 6kHz Sne t uses perfet CSI and s not affeted by multpath nterferene, RC wth the same order of dversty as Spae-Tme Pre-RAKE s the lower bound for all methods In a pratal system lke W-CA, orthogonal odes that are used to separate users from eah other ntrodue multpath nterferene Beause of ths nose and predton errors, Spae-tme pre-rake system annot aheve ts deal BER gven by the 8-path RC Fgure 5 ompares the BER performane of spae-tme pre-rake and Tx AA wth the RAKE reever [5] The predton method desrbed above s employed to enable pre- RAKE flterng and to alulate the weghts requred for Tx AA Fgure 5 demonstrates that Spae-tme pre-rake system aheves better performane than Tx AA for multpath fadng hannels In the Fgures, the BER s alulated n terms of the reeved SNR per oded bt Ths SNR was evaluated theoretally for ST Every bt and no dversty urves,

5 BER 6 BER No dversty ST 6 KHz no predton ST 6 khz predton ST Everybt Spae Tme Pre RAKE no predton Spae Tme Pre RAKE predton 8 Path RC Path RC Tx AA no predton Tx AA predton Spae tme pre RAKE no predton Spae tme pre RAKE predton 8 Path RC Reeved SNR per oded bt (db) Fgure 4 Comparson of ST and Spae-tme pre-rake, 4-path fadng hannel, 2 transmtter antennas, f dm =2Hz and omputed from smulatons for other methods The omparson n terms of reeved SNR per bt allows to ompare dretly the dversty advantages of varous ombnng methods over dfferent multpath hannels It s observed that employng Spae-Tme Pre-RAKE wth long range predton aheves near-optmal performane for a rapdly tme varant multpath hannel wth transmsson antenna array However, ths gan s aheved at the expense of sgnfant omplexty Whle the feedbak load s hgh, the RAKE flterng and predton s performed at the BS, where ths omplexty an be afforded The Tx AA method an redue feedbak load and feed bak a sngle omplex weght per antenna, provded that predton and the RAKE reever are performed at the S Thus, the omplexty at the S s hgher for Tx AA than for Spae-tme pre-rake oreover, Tx AA does not provde sgnfant performane gan over ST for a modest number of antennas The omplexty of ST s the lowest sne t only requres the feedbak of the antenna seleton bts to hoose the antenna wth the greatest hannel power ST an be easly ombned wth pre-rake, whle stll retanng lower omplexty than spae-tme pre- RAKE [7] Thus, the methods desrbed n ths paper provde a varety of performane/omplexty trade-offs, wth ST beng the smplest, but the least power effent method, and Spae-tme pre-rake beng more omplex, and ahevng near-optmal performane VI CONCLUSION A Spae-tme pre-rake transmtter dversty tehnque was ompared to ST and Tx AA transmtter dversty methods for realst W-CA hannels The performane of the Spae-tme Pre-RAKE method approahes the performane of RC for all spae and frequeny dversty branhes It s shown that all losed loop methods depend on the long range predton to approxmate the deal performane n the rapdly fadng envronment Reeved SNR per oded bt (db) Fgure 5 Comparson of Tx AA and Spae-tme pre- RAKE, 2 transmtter antennas, 4 paths, f dm =2Hz VII REFERENCES [] V Tarokh, N Seshadr, AR Calderbank, Spae-tme odes for hgh data rate wreless ommunaton: performane rteron and ode onstruton, IEEE Transatons on Informaton Theory, Vol 44, Issue: 2, pp , arh 998 [2] A Hottnen and R Whman, Transmt versty by Antenna Seleton n CA ownlnk, IEEE 5 th Internatonal Symposum on Spread Spetrum Tehnques and Applatons, Vol 3, pp , 998 [3] Ratola, A Hottnen, R Whman, Transmsson dversty n wdeband CA, IEEE 49th Vehular Tehnology Conferene, Vol 2, pp , 999 [4] K Rohan, Harrson, K Kuh, A omparson of base staton transmt dversty methods for thrd generaton ellular standards IEEE 49th Vehular Tehnology Conferene, Vol, pp , 999 [5] C Frank, Optmal Transmt Array Weghtng for S- CA wth Channel Feedbak, Conferene on Informaton Senes and Systems, Vol, pp WA , New Jersey, 2 [6] S Gunavd, A uel-hallen, Performane Analyss of Seletve Transmt versty for W-CA Usng Long Range Predton, IEEE Conferene on Thrd Generaton Wreless and Beyond, San Franso, ay 2 [7] R Esmalzadeh, Nakagawa, Pre-RAKE versty Combnaton For ret Sequene Spread Spetrum Communatons Systems, IEEE Internatonal Conferene on Communatons, vol, pp , 993 [8] R Esmalzadeh, E Sourour, Nakagawa, Pre-RAKE versty Combnng In Tme vson uplex CA oble Communatons, Sxth IEEE Internatonal Symposum on Personal, Indoor and oble Rado Communatons, pp , 995 [9] S Hu, T Eyeoz, A uel-hallen and H Hallen, Transmtter antenna dversty and adaptve sgnalng usng

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