FREQUENCY SHIFT BASED MULTIPLE ACCESS INTERFERENCE CANCELLER FOR MULTIRATE UMTS-TDD SYSTEMS

Transcription

1 FREUENCY SHIF BASED MULIPLE ACCESS INERFERENCE CANCELLER FOR MULIRAE UMS-DD SYSEMS Lus Gonçalves, Atlo Gamero Insttuto de elecomuncações, Campus Unverstáro de Santago, Avero, Portugal Departamento de Electrónca e elecomuncações, Unversdade de Avero, Portugal emal: lgoncalves@av.t.pt, amg@det.ua.pt Abstract Drect Sequence Spread Spectrum DS-SS sgnals exhbt cyclostatonary propertes whch mply a redundancy between frequency components separated by multples of the symbol rate. In ths paper we present a Multple Access Interference Canceller that explores ths property and apples to UMS-DD. hs frequency doman canceller operates n the spreaded sgnal n such way that the nterference and nose at ts output s mnmzed Mnmum Mean Squared Error Crterum. he performance s evaluated n two confguratons: one ncludng the Frequency Shft Canceller and the other concatenated wth Parallel Interference Canceller. he results are benchmarked aganst the performance of the conventonal detector and the conventonal detector. Keywords cyclostatonary, redundant, frequency shft,, DS-CDMA I. INRODUCION Drect sequence spread-spectrum DS-SS code dvson multple access CDMA has emerged as one of the most promsng technques to mplement varous rado communcaton systems. It presents sgnfcant advantages over me Dvson Multple Access DMA, namely frequency dversty, multpath dversty and more spectrum effcency on multcell systems 1], whch led to ts choce as the technology for thrd generaton cellular systems. he second generaton CDMA systems were based on the conventonal recever, whch s known to be lmted by the multple access nterference MAI and requre a very precse power control. o overcome these lmtatons and therefore enhance the capacty of CDMA systems, jont detecton of the receved DS-SS sgnals has been proposed to be used at the base staton BS or at the user equpment. he optmum jont detector 2] although well known requres however a prohbtvely hgh computatonal complexty, and consequently effort has been made to devse suboptmum algorthms wth good compromse between performance and complexty that can be mplemented wthout prohbtve costs n near future CDMA systems. hs communcaton fts n ths approach, and ams at presentng a moderate complexty MAI canceller operatng on the broadband DS sgnal. he Frequency Shft Detector can be used ether as standalone unt or t can be used pror to a multuser detector where t s ntended to produce sgnals clean enough so that the frst decsons of the can be consdered relable enough to be used by the subsequent stages. he DS-SS sgnal s a partcular case of a statonary random pulse ampltude modulaton of pulses symbols. hs knd of sgnals are known to have cyclostatonary propertes 3], 4]. hose propertes mply redundancy between frequency components separated by multples of the symbol rate. It s ths characterstc that we explore to propose a new MAI canceller. In prevous work 5] was defned and mplemented the for fxed spreadng factor. he results show that consderable hs work s beng supported by a PhD Grant from PRAXIS XXI, Fundacão para a Cênca e ecnologa and the projects ASILUM and VISEF mprovement s acheved aganst the and the even outperforms the for moderate to hgh values of E b /N 0. he was also tested as a precanceller to be used pror to a and the results have shown that nearly complete nterference cancellaton n a UMS-DD scenaro wth up to eght smultaneous users n the same slot. In the present paper we extend the work of 5] to nclude the general stuaton of UMS-DD where users wth dfferent spreadng factors can coexst n the same tme slot. hs enables the transmsson of servces wth dfferent rates by varyng the spreadng factor, but the structure of the spreadng channelzaton tmes scramblng n UMS-DD lead to codes that extend over several symbols. he paper s outlned as follows. In secton two we show that n a DS sgnal non overlappng frequency bands separated by a multple of the baud rate are lnearly related. hs result s used to present n secton three the archtecture of a MAI canceller that explores ths redundancy. In secton four we present smulaton results that llustrate the performance provded by the new canceller. Fnally n secton fve the man conclusons of ths work are outlned. II. HEOREICAL BACKGROUND he UMS-DD standard was desgned to accommodate several smultaneous transmsson rates. he dfferent rates are accomplshed by varyng the spreadng factor. he spreadng code s composed by the product between the channelzaton code and the scramblng code. he channelzaton code lasts for one symbol and ts number of chps s equal to the spreadng factor. he scramblng code lasts for sxteen chps the maxmum spreadng factor max or durng max symbols beng the spreadng factor. hen to construct the spreadng code the channelzaton code must be repeated max and multpled by the scramblng code. he spreadng code extends for more than one symbol n the case of max. In the case of UMS-DD, max = 16. hs leads to the followng representaton of the sngle user DS-SS sgnal: st = k max 1 a l kg l t l max k 1 where {a l k } are the sequences of nformaton symbols, 1 the symbol rate and g l t are the components of sgnature waveform. o obtan these components the spreadng code length equal to max s dvded n max sequences each equal to one component of length and then each one s pulse shaped and affected by channel estmates.

2 he Fourer ransform of ths sgnal s: Sf = k = Notce that f then max 1 max 1 a l kg l fe j2πfl e j2πfmaxk/ G l fe j2πfl k A l f = k A l f + Defnng the vectors: a l ke j2πfmaxk/ 2 a l ke j2πfmaxk/ 3 max = A lf Z 4 af = A 0 f... A max/ 1f ] 5 gf = G 0 f... G max/ 1fe jmax/ 12πf ] we can wrte 2 n a matrx form: 6 Sf = gf a f 7 where means transpose. In the same way f we defne: sf = S f S f +... S f + 1 where Sf se f 0, S f = Sf s defned n 2 and 7. Defnng: g f = G 0 f + max... G max/ 1 f + e ] 8 9 jmax/ 12π f+ max where {0, 1,..., max / 1} and: G G l f se f 0, lf = ] G l f s defned n 2. hen: g 0 f s f =.. g max/ 1f a f = Gf a f 12 and then: s = G a f 13 s f = Gf G 1 s 14 hs means that we can relate the sgnal nformaton n frequency bands spaced by a multple of the baud rate by a lnear transformaton. he converson method between redundant bands expressed by 14 s the base of the and represents a generalzaton of the technque developed n 5] where the use of a fxed spreadng factor resulted a scalar transformaton. III. PRINCIPLES OF HE CANCELLER Let the representaton of sgnal n the tme doman at the nput of the canceller be rt = U u=1 s u t + nt where U s the number of users and nt s statonary nose wth power spectral densty η n f. We consder wthout loss of generalty that user one u = 1 s the user of nterest. he canceller operates n the frequency doman and n a practcal mplementaton or n a smulaton system the tme to frequency doman converson s performed dgtally through a FF. In the followng dervaton we adopt a contnuous sgnal notaton. he objectve and desgn crtera for the frequency doman canceller s to mnmze the overall dsturbance MAI+nose subject to the condton that s 1 t must not be dstorted. Let Rf and V f be the fourer transform of rt and vt representaton on the tme doman of the output sgnal of the respectvely, 1 the spreadng factor of the user one and G u f the matrx gven by 12 Gf where we have added the subscrpt u to ndcate that t refers to user u. We defne the vectors rf and vf as: rf = vf = where: R f R f R f V f V f V f Rf se f 0, R f = 1 1 max ] 15 ] max V f have a dentcal defnton as R f where Rf s replaced by V f. he uses the nformaton of bands to reconstruct bands m for the user of nterest accordng to 14 whle mnmzng the overall dstorton. In the frequency doman the output sgnal s related wth the nput one through the followng relaton: v 1 = α,m G 1 1 G 1 1 f + 1 r f where α,m s a set of complex vectors wth sze max 1 whose selecton s ntended to mnmze the Multple Access Interference and nose at the output. desgnates an element by element multplcaton between matrces or vectors. Let s uf be the matrx gven by 12 where we have added the subscrpt u to ndcate that t refers to user u. Let a u f be the matrx gven by 5 where we have added the subscrpt u to ndcate that t refers to user u. After some processng we obtan from 18: v f + + U m = s n f + 1 α,m G 1 f + α,m m G f + h 1 u f

3 A FF B Path Algnment C Frequency Shft Canceller D DownSamplng E F G Matched H DownSamplng IFF Flter Parameters Fg. 1. Recever wth where h u f = z W u,z fa u,zf 20 If the spreadng factor of the nterferng sgnal u s equal to the sgnal of nterest the sum n 20 has only one term and W u,1 f = G u f, a u,1 f = a u f and h u f = s u f. If the spreadng factor of the nterferng user s not equal to the spreadng factor of the user of nterest then h u f s u f because of ncompatblty between dmensons of matrces n 19. If the spreadng factor of a nterferng user u s less than the user of nterest then we must decompose s u f = G u f a uf n a sum of several products of matrx tmes vector W u,z fa u,z f to obtan h uf takng care to get a good representaton of the sgnal n queston. If the spreadng factor of the partcular user u s greater than the user of nterest then we must dvde the sgnature waveform n the same number of components of the user of nterest see Secton II generatng G u f of the same dmenson of G 1 f. In that stuaton h u f s equal to s u f. Let x u,,m,z f be the vector lne whch elements are of the frst lne of G 1 1 G 1 1 f + 1 W u,z f + 1 see 19 and 20. Let α,m 0 be the frst element of each α,m and N 0 f the frst element of n f. he frst lne of the vector v f V 1 = S 1 + U α 0,m z 1 m 1 n 19 s: α 0,m x u,,m,z fa u,z f + N Let η n f be the power spectral densty of the nose at the nput of the canceller. Defnng the vector lne: η n f = η nf η nf max... ] 22 η nf max where: η η nf n f se f 0, 1 = 1 max 0 se f / 0, max he power spectral densty of the nose N 0 1 n 21 s: η out 1 = α,m 0 2 Mf η n f + 24 where Mf s the vector lne wth elements w 1j 2 whch w 1j s the 1, j element of G 1 1 G 1 1 f + 1. Let Xu,,m,z e f be a element of x u,,m,zf see 21wth { e 1, 2,..., max 1 }. he desgn crtera mples that the weghts α,m 0 are dmensoned so that: C U α 0,mXu,,m,z 2df e f z e f + Bt f η out 1 df 25 s mnmzed to the condton that α,m 0 = 1. C s equal to the number of symbols of the user of nterest n each burst tmes 1 max. In 25 Bt s the burst duraton tme. Notce that the power of each symbol n UMS-DD s one. hs process must be repeated for all the lnes of v f + m 1 and for all bands m. In the prevous approach we consder that f 0, 1 see 17. We can dvde ths nterval n several ntervals and apply the procedure presented n ths secton for each of the ntervals ncreasng the performance. he shorter each nterval the better the performance s. IV. APPLICAION OF HE CANCELLER O UMS-DD In ths secton we present some numercal results llustratng the performance of the proposed detector confguratons wth UMS-DD sgnals. he proposed confguratons are more sutable to be mplemented n the uplnk because the detectors requre knowledge of the spreadng codes of the actve users. o evaluate the canceller performance a smulaton chan was mplemented. Bascally ths smulaton chan s composed by a transmtter, a transmsson channel and a recever. A. ransmtters he transmtters are complant wth the 3GPP specfcatons for UMS-DD. B. Model he channel model used n ths work was the Geometrcal Based Sngle Bounce Ellptcal Model GBSBEM proposed by Lbert 6]. hs model was developed for mcrocell and pcocell envronments. he propagaton channel s characterzed by L paths for each user, one n lne of sght plus L 1 arrvng from remote reflectors located randomly wthn an ellpss where the base staton and the moble unt are at the foc. Each path s characterzed by complex constant and a delay. he delay s unformly dstrbuted between zero and the maxmum delay spread. he phase of the complex constant s unformly

4 A Frequency Shft Canceller H J Parameters Fg. 2. Recever ncludng plus Sngle User Eb/No db Fg. 4. BER for users wth spreadng factor of eght, n the frst scenaro Sngle User Eb/No db Fg. 3. BER for users wth spreadng factor of sxteen, n the frst scenaro dstrbuted n 0, 2π. he ampltude of the complex constant follows a Raylegh dstrbuton. he channel parameters are assumed to be constant wthn each burst. C. Recevers Fgure 1 depcts the basc confguraton for the detector that ncludes the Frequency Shft Canceller. If we remove the Frequency Shft Canceller block the detector s a conventonal. he Path Algnment and Downsamplng are blocks whose operatons are done n frequency doman despte the fact that the names reflect the correspondent tme doman operatons. he path algnment ncludes delay algnment and maxmum rato combnng of the spreaded sgnal. he sgnal n A fgure 1 has a resoluton of four samples per chp and the frst downsamplng has the same factor. he second downsamplng takes a factor equal to the spreadng factor. In H we have the sgnal before symbol decson. Notce that the Dscrete Fourer ransform s made only once n each slot and burst. he second detector confguraton to be evaluated s the detector composed by plus fgure 2. he n fgure 2 corresponds to the whole recever chan of fgure 1. he provdes the sgnal for the symbol decson nsde the. hese two confguratons are benchmarked wth the conventonal and conventonal sngle stage. D. Results he smulatons were made wth the parameters shown n able I. he channel parameters estmates were assumed to be perfect. Sngle User Eb/No db Fg. 5. BER for users wth spreadng factor of four, n the frst scenaro Followng we present some results for two scenaros. Frst Scenaro: Eght users: Four wth spreadng factor of sxteen, two wth spreadng factor of eght and two wth spreadng factor of four Full load system. he users wth spreadng factor eght and four have powers above the users of spreadng factors of sxteen of 3 and 6dB respectvely. Second Scenaro: Four users: wo wth spreadng factor of sxteen, one wth spreadng factor of eght and one wth spreadng factor of four Half load system. he users wth spreadng factor eght and four have powers above the users of spreadng factors of sxteen of 3 and 6dB respectvely. Plots of the BER acheved are shown n Fgures 3-8. Fgures 3, 4 and 5 refer to the frst scenaro and apply to users wth spreadng factors of 16, 8 and 4 respectvely. In each plot we compare the followng confguratons:,

5 Sngle User Eb/No db Fg. 6. BER for users wth spreadng factor of sxteen, n the second scenaro ABLE I SIMULAION PARAMEERS SEINGS Spreadng Factor 16, 8, 4 Number of aps 2 GBSBEM Velocty 50 Km/h Path Loss 3.7 Burstype 1 Maxmum Delay Spread 2.0 µs Degrees of Freedom of 16, 8, 4 Number of samples per chp 4 Lne of Sght Dstance 300m,, + and Sngle User. he BER shown for a gven spreadng factor s the average of the varous users wth the same spreadng factor. he same knd of comparsons are presented n Fgures 6-8 for the second scenaro. he results show that sgnfcant gans are obtaned by the and + over the recevers. For the case of half loaded system the + nearly acheves the sngle user performance lmt. It was notced that the results presents some sensvty n relaton wth the specfc scramblng code used. hs ssue s currently under study. V. CONCLUSIONS In ths communcaton we presented a new lnear canceller operatng n the frequency doman that takes advantage of frequency redundancy of the DS-SS sgnals. hs Multrate Canceller was appled n a UMS-DD smulaton chan n two scenaros: one n a full loaded system and one n a half loaded system. In both scenaros sgnfcant performance gans were obtaned. When Sngle User Eb/No db Fg. 7. BER for users wth spreadng factor of eght, n the second scenaro Sngle User Eb/No db Fg. 8. BER for users wth spreadng factor of four, n the second scenaro operatng n concatenaton wth a the nearly acheves sngle user performance n the case of a half loaded system. REFERENCES 1] Ryuj Kohno, Reuven Medan and Laurence B. Mlsten, Spread Spectrum Access Methods for Wreless Communcatons, IEEE Communcatons Magazne, January ] Sergo Verdu, Mnmum Probablty of Error for Asynchronous Gaussan Multple-Access s, IEEE ransactons of Informaton heory, January ] Gardner, Wllam A., Cyclostatonarty n Communcatons and Sgnal Processng, IEEE PRESS, ] Gardner, Wllam A., Explotaton of Spectral Redundancy n Cyclostatonary Sgnals, IEEE Sgnal Processng Magazne, Aprl ] Lus Gonçalves, Adão Slva, Atlo Gamero, Frequency Shft Based Multple Access Interference Canceller for DS-CDMA Systems, Vehcular echnology Conference 2001 Fall, October 2001, Atlantc Cty, USA. 6] J. C. Lbert,. S. Rappaport, Smart Antennas for Wreless Communcatons: IS-95 and hrd Generaton CDMA Applcatons, Prentce Hall, 1999.