MULTI USER DETECTION FOR CDMA- OFDM/OQAM SYSTEM COMBINED WITH SPACE TIME CODING

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1 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 MULTI USER DETECTION FOR CDMA- OFDM/OQAM SYSTEM COMBINED WITH SPACE TIME CODING Radhia GHARSALLAH 1 ad Ridha BOUALLEGUE 2 1 Naioal Egieerig School of Tuis, Iov Com Laboraory, Higher School of Commuicaios, Tuisia radhia.gharsallah@gmail.com 2 Iov Com Laboraory, Higher School of Commuicaios, Tuisia ridha.bouallegue@gmail.com ABSTRACT I his paper, we propose he combiaio of Muli Carrier (MC) OFDM/OQAM modulaio ad CDMAcalled MC-CDMA-OQAM sysem wih Space Time (ST) codig i a muli user coex. This combiaio akes advaages from mulicarrier modulaio, spread specrum ad spaial ime diversiy. Ideed, he use of OFDM has proved is abiliy o figh agais frequecy selecive chaels bu he iserio of guard ierval yields specral efficiecy loss ad sesiiviy o frequecy dispersio due o he use of recagular pulse shape. Thus, cyclic prefix OFDM is replaced by a advaced filerbak-based mulicarrier sysem OFDM/OQAM ha operaes wihou guard ierval. However, OFDM/OQAM provides orhogoaliy oly o he real domai, so rasmied symbols mus be real valued. I he oher had, he CDMA compoe has wo advaages: muliple access ierferece cacellaio ad providig orhogoaliy i he complex domai. From he orhogoaliy propery provided, he Alamoui ST code ca be combied wih MC-CDMA-OQAM sysem. The resulig MIMO-MC-CDMA-OQAM sysem improves he specral efficiecy of wireless sysem, comba chael fadig ad reduce arrowbad ierferece. Numerical resuls show he uiliy of his ew wireless commuicaio sysem; a sigifica BER versus Sigal o Noise Raio (SNR) was achieved hus Muliple access ierferece (MAI) is suppressed ad so supporig a large umber of users. KEYWORDS CDMA, OFDM/OQAM, ST codig, MIMO, Muli User Deecio 1. INTRODUCTION Icreasig he rasmissio rae ad providig robusess o chael codiios are owadays wo of he mai research opics for wireless commuicaios. Therefore, a sigifica ieres of lae has bee o develop sysems ha offer boh high capaciy ad high daa speed, alog wih MAI resisace. Ideed, much effor is doe i he area of Muliple Ipu Muliple Oupu (MIMO) sysems by usig several aeas eiher a he rasmiig side or a he receivig side. We ca exploi space ad ime diversiy by usig Space ime codes such as he famous Alamoui code [1]. I muli aeas commuicaio sysems, ivesigaios of N rasmi ad N receive aeas showed ha he capaciy of such sysems icreases liearly wih he r miimum of N ad N r [1], [6]. High daa raes are obaied by simulaeously sedig sigals from several rasmi aeas. To proec he iegriy of he rasmied iformaio, rasmi diversiy is obaied by iroducig redudacy amog he rasmied sigals over DOI : /ijwm

2 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 N rasmi aeas (space) ad T ime periods (ime). Uder quasi-saic fadig chael, he maximum combied rasmi receive diversiy order equals o N N r [1]. A variey of Space-Time codes exiss i he lieraure. I [6], Alamoui proposed a ew modulaio scheme over N = 2 rasmi ad N receive aeas where a rae of oe symbol per chael use (PCU) wih 2 N diversiy was achieved [8]. The ML deecio of he Alamoui scheme ca be implemeed by a liear complexiy decorrelaor. O he oher had, mulicarrier modulaio (MCM) is becomig he appropriae modulaio for rasmissio over frequecy selecive chaels. The mos popular MCM is he Cyclic Prefix Orhogoal frequecy Divisio Muliplexig (CP-OFDM) ha explois frequecy diversiy by dividig he oal badwidh io M subcarriers ad rasmiig OFDM symbols over hese subcarriers. However, he iserio of he CP yields specral efficiecy loss. I addiio, he coveioal OFDM modulaio is based o a recagular widowig i he ime domai which leads o a poor (sic(x)) behavior i he frequecy domai. Thus CP-OFDM gives rise o wo drawbacks: loss of specral efficiecy ad sesiiviy o frequecy dispersio. I order o overcome hese wo problems, a varia of OFDM ha is called OFDM-OQAM has bee proposed [3]. OFDM/OQAM overcomes he wo drawbacks of OFDM bu i does o provide orhogoaliy o he complex domai, hus he idea is o use CDMA compoe for wo reasos; firsly o avoid muliple access ierferece ad secodly o provide orhogoaliy i he complex domai if he spreadig codes are well chose [4]. The origialiy of his paper cosiss o sudyig muli user deecio for MIMO-MC-CDMA-OQAM commuicaio sysem. This paper is orgaized as fallows; i secio 2 we prese he sysem model of MC-CDMA- OQAM sysem i order o describe his Muli carrier modulaio. I secio 3 we will prese our ew commuicaio scheme combiig ST codig wih spread specrum echique ad OFDM/OQAM. Secio 4 is reserved o ST decodig ad muli user deecio. I secio 5 we prese umerical resuls obaied by malab simulaio showig performace of differe muli user receivers of he proposed sysem. The las secio deal wih syhesis ad compariso bewee MC-CDMA ad MC-CDMA/OQAM i wo cases; wih ad wihou space ime codig ad we fiish by a coclusio. r 2. MC-CDMA-OQAM SYSTEM MODEL The block diagram i Figure 1 illusraes he MC-CDMA-OQAM rasmissio scheme relaive h o he k user. I cosiss o he combiaio of spread specrum wih mulicarrier modulaio OQAM [4]. The basic priciple of OFDM/OQAM is o divide he rasmissio badwidh io a umber of subbads. Like for OFDM, he rasmier ad receiver ca be implemeed by usig Fas Fourier rasform (FFT) algorihms. However isead of a sigle FFT or IFFT, a uiform filer bak based o a prooype filer is used. A he receiver side he dual operaios are carried ou. Differe kids of prooype fucios ca be implemeed such as Isoropic Orhogoal Trasform Algorihm or oher prooypes opimized i discree-ime usig he Time-Frequecy Localizaio crierio. Le us iroduce he CDMA echique used i our sysem. We deoe by Q he spreadig facor ad K he umber of users, each user k is disiguished by is spreadig code ck = c, k, c1, k, K, cq 1, k 1

3 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 Walsh Hadamard codes are chose because hey are characerized by heir perfec orhogoaliy. Figure 1 : MC-CDMA-OQAM sysem model We cosider he rasmissio of complex daa i a free disorsio chael. The spreaded resula sigal of K users ca be wrie as K m, = ckbk, m, k = x h h Wih bk, m, is he daa of he k user rasmied a ime over he m subcarrier. For each user, bk, m, = am, The basebad equivale of a coious ime Mulicarrier MC-CDMA-OQAM sigal is expressed as follows. (1) M 1 ( ) = m, ( τ ) υm, j 2 mf (2) m= s x g e π Wih : M he umber of subcarrier, g( ) The pulse shape, F = 1 T = 1 2τ he subcarrier spacig ad υm, a addiioal phase erm. The legh of he prooype filer is a muliple of he umber of subcarriers. We deoe by y he received sigal, he oupu of he polyphase filers is obaied as follows. y = y, g (3) m, m, The, we apply he despreadig block o obai sigal relaive o each user k. we ge yd The despreaded sigal is wrie as = M 1 c y, k k m, m= (4) 11

4 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 K 2b 1 M 1 M 1 ( p, ) yd, k = b, k j b, k ckck γ m, k = = 2b 1 p= m= { g } Wih ( p, ) m ( ) ( ) m p γ = I j A ( m p) g m, 1, A is he ambiguiy fucio of he prooype fucio g defied as j2π (, ) = ( τ ) ( ) F u Ag m g u g u e du (5) 3- SPACE TIME CODING IN MC-CDMA-OQAM SYSTEM OFDM/OQAM provides orhogoaliy i he real domai, a pseudo Alamoui code was proposed i [8]. This pseudo code is very complex so we hik o ake advaage from The CDMA-OFDM/OQAM combiaio ha resuls he orhogoaliy i he complex domai so we ca combie space ime codig o his muli carrier commuicaio sysem. We cosider 2 rasmi aeas ad 2 receive aeas where are assumed o be far eough apar such he complex fadig coefficies amog he aeas are ucorrelaed. We apply Alamoui ST codig scheme o each user k daa. Figure 2 : MC-CDMA/OQAM sysem wih space ime codig We doe by hi, j he complex chael coefficie bewee he h i rasmi aea ad he h j receive aea. The chael ampliudes are idepedes, zero mea complex Gaussia variables wih ui variace. The received code word is give by [6] : Y = HX (6) Wih X he rasmied code word, H is chael marix ad is a Addiive Whie Gaussia Noise. The chael bewee rasmi aea i ad receive aea j may be modeled by a complex h. Assumig ha fadig is cosa across wo cosecuive muliplicaive disorio ( ) symbols, we ca wrie i, j Form equaios (6) ad (7), we ca wrie j, ( ) = = α h h e θ i j i, j i, j i, j (7) y1 y3 h1,1 h1,2 s, k s 1, k 1 3 y y = h2,1 h 2,2 s 1, k s, k (8) 12

5 Wih Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 s ad, k s 1, k daa of h k user rasmied by he firs aea respecively a ime ad 1 afer spreadig ad passig hrough he polyphase filers. s 1, k ad s daa of, k h k user rasmied by he secod aea. y 1 ad y3 sigals received by he firs aea, y 2 ad y 4 sigals received by he secod aea. ( ) ( ) y1 = y1 = h1,1 s, k h1,2 s 1, k 1 y2 = y2 = h2,1 s, k h2,2s 1, k 2 y3 = y3 ( T ) = h1,1 s h 1, k 1,2s, k 3 y = y ( T ) = h s h s,,, are complex radom variables represeig oise ,1 1, k 2,2, k 4 (9) 4. SPACE TIME DECODING AND MUD 4.1. Space ime decodig We cosider sysem wih (CSI) Chael Sae Iformaio a he receiver. Combied sigals are give by: s% = α α α α s h h h h ( ) ( α α α α ) (1), k 1,1 1,2 2,1 2,2, k 1,1 1 1,2 2 2,1 3 2,2 4 s% s h h h h (11) 1, k = 1,1 1,2 2,1 2,2 1, k 1,1 2 1,2 1 2,1 4 2,2 3 These combied sigals are he se o he Maximum likelihood (ML) deecor. The ML decisio rule, a he receiver, is o choose s j, k if ad oly if 2K 2 2K ( i, i, j, k ) ( i, i, m, k ) d y h s d y h s j m i= 1 = 1 i= 1 = 1 Where d 2 ( x, y) = ( x y)( x y ) The oupu of he ST decoder will be se o he MC-CDMA-OQAM receiver. A his sage, he firs sep is o apply polyphase filerig ad he secod sep is o apply muli user deecor. The polyphase filerig, a he receiver, cosiss o usig M uiform filers like a he rasmier wih a simple FFT Muli user Deecio (MUD) We cosider K users commuicaig simulaeously, each of hem rasmi L daa. Collecively he mached filer oupu for all K users ca be expressed i a log vecor as [5]: = 1... K Wih Z ( i) Z ( i) Z ( i) ( ) ( ) Z = Z 1... Z L (12) 13

6 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 = 1, 2,..., K he vecor coaiig spreadig codes of K users, A = I ( K) he ideiy marix of size K ad B he vecor coaiig he daa of K users. From equaio (12), we ca wrie: We deoe by C [ c c c ] Z = CC AB Le w be a liear rasformaio vecor for he muli user deecor. The decisio vecor is (13) d = w Z (14) The Zero Forcig (ZF) deecor i [2] has a liear rasformaio equivale o he iverse of he correlaio marix R = CC The decisio vecor is he, The decisio vecor has covariace marix w = 1 R ( ) 1 d = R RAB ( )( ) E R R = σ R 1 1 H 2 1 Which ca resuls, i oise power ehaceme, creaig a gap bewee he sigle user error performace ad he Zero Forcigr error probabiliy. Aoher liear deecor wih he same srucure as ZF deecor based o he opimizaio of he miimum mea-squared error (MMSE) crieria: ( %) ( %) w = mi E b b b b (18) The soluio of he equaio above is 2 1 ( σ ( ) ) 1 w = R A A While he sigle user mached filer combas whie oise exclusively ad he Zero Forcig elimiaes Muliple Access Ierferece (MAI) disregardig backgroud oise, he MMSE liear deecor forms a compromise bewee he wo, akig he releva imporace of oise ad ierferig users io accou. Differely o liear MUD here is ieraive MUD such Successive Ierferece Cacellaio (SIC) receiver [7] which is less complex. (15) (16) (17) (19) 5. SIMULATION AND NUMERICAL RESULTS I his secio, we prese umerical resuls of he MC-CDMA/OQAM commuicaio sysem. 14

7 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus MUD for MC-CDMA-OQAM usig ST codig ZF SIC BER Eb/N[dB] Figure 3: Muli user deecio for Space ime codig MC-CDMA/OQAM commuicaio sysem. We assume ha boh he mobile ad he base saio have wo rasmi ad wo receive aeas, we cosider K users commuicaig simulaeously, each user has his spreadig code of legh Q=16. We use Quadraure Ampliude Modulaio QAM 4. The legh of fas Fourier Trasform is 64. I Figure 3 muli user deecio is preseed. I represes Bi Error rae (BER) versus Sigal o Noise Raio (SNR). Simulaio resuls improve ha liear muli user deecors provide beer performace gai i compariso wih oliear deecors. However, oliear deecors ca aed he compromise performace ad simpliciy Space ime codig wih Muli Carrier modulaio MC-CDMA-OQAM MC-CDMA BER Eb/N[dB] Figure 4: Space ime codig wih muli carrier modulaio: Compariso bewee MC- CDMA/OQAM ad MC-CDMA. Figure 4 shows a compariso bewee he classic MC-CDMA ad MC-CDMA/OQAM. MC- CDMA is based o he combiaio of CDMA ad OFDM wih cyclic prefix bu MC- CDMA/OQAM is based o he combiaio of CDMA ad he advaced muli carrier modulaio OFDM/OQAM. Simulaio resuls improve ha MC-CDMA/OQAM provide similar BER as MC-CDMA. Furhermore MC-CDMA/OQAM provides beer daa rae sice ha operaes wihou guard ierval. I Figure 5 we compare he wo muli carrier modulaios MC-CDMA ad MC-CDMA/OQAM i, boh, Sigle Ipu Sigle Oupu (SISO) ad Muliple Ipu Muliple Oupu (MIMO) chaels. Boh SISO ad MIMO chaels aed good performace i erm of BER versus 15

8 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 SNR. I addiio, MIMO chael gives he possibiliy of exploiig he space ad ime diversiy by rasmiig differe daa symbols simulaeously he hig ha icreases he daa rae i compariso wih SISO chael BER Eb/N[dB] Figure 5: Compariso bewee MC-CDMA/OQAM ad MC-CDMA wih ad wihou space ime codig. 6. CONCLUSION I his paper, we have sudied ad evaluaed muli user deecio i MC-CDMA-OQAM sysem i presece of MIMO chael. The idea is o use mulicarrier modulaio i order o comba chael effecs. Two kids of muli carrier modulaios are compared; cyclic prefix OFDM ad OFDM/OQAM. OFDM/OQAM operaes wihou guard ierval so i elimiaes specral efficiecy loss prese i OFDM. The idea of combiig he CDMA compoe wih muli carrier modulaio has wo advaages; he firs oe is o elimiae muliple access ierferece ad he secod oe is o ake advaage from he orhogoaliy i he complex domai provided by CDMA echique. The resulig is a MC-CDMA/OQAM commuicaio sysem ha provides orhogoaliy i he complex domai. So we ca use space ime codig i order o exploi spaial ad ime diversiy. The ST code used is he Alamoui code. Numerical resuls improve he performace gai of mulicarrier modulaio i wo cases; wih ad wihou space ime codig i muli user evirome. Boh liear ad ieraive muli user deecors applied o 4 he proposed sysem aed a BER of 1 for a Sigal o Noise Raio (SNR) approximaely equal o 1 db. As perspecive, we hik o exed his work o a large umber of rasmi ad receive aeas ad o use oher ST coders. REFERENCES [1] S.M. Alamoui A Simple Trasmi Diversiy Techique for Wireless Commuicaios, IEEE Joural o seleced areas i commuicaio, VOL. 16, NO. 8, OCTOBER 1998 [2] R. Lupas ad S. Verdú, Near -far resisace of muliuser deecors i asychroous chaels, IEEE Trasacio i commuicaios., vol. 38, pp , Apr.199. [3] H. Bolcskei, Orhogoal frequecy divisio muliplexig based o offse QAM, i Advaces i Gabor Aalysis, Birkhauser, Boso, Mass, USA,

9 Ieraioal Joural of Wireless & Mobile Neworks (IJWMN) Vol. 4, No. 4, Augus 212 [4] C. Lélé, P. Sioha, R. Legouable, ad M. Bellager, CDMA rasmissio wih complex OFDM/OQAM, EURASIP Joural o Wireless Commuicaios ad Neworkig, vol. 28, Aricle ID 74863, 12 pages, 28. [5] P. Pael ad J. M. Holzma, Aalysis of a simple successive ierferece cacellaio scheme i a DS-CDMA sysem, IEEE Joural o seleced areas i commuicaios., vol. 2, pp , Jue [6] H. Huag, H. Viswaaha, ad G. J. Foschii, Muliple Aeas i Cellular CDMA Sysems: Trasmissio, Deecio, ad Specral Efficiecy, IEEE Trasacios o Wireless Commuicaios., vol. 1, pp , July 22. [7] T. Muharemovic, E. N. Oggosausi, A. G. Dabak, ad B. Aazhag, Hybrid liear-ieraive deecio algorihms for MIMO CDMA sysems i mulipah chaels, i Proc. IEEE I. Cof. Acousics, Speech, ad Sigal processig, 22 (ICASSP '2), vol. 3, pp , 17 May 22. [8] H. Li, C. Lélé, ad P. Sioha, A pseudo alamoui rasceiver desig for OFDM/OQAM modulaio wih cyclic prefix, i Proceedigs of he IEEE Workshop o Sigal Processig Advaces i Wireless Commuicaios (SPAWC 9), pp. 3 34, Perugia, Ialy, Jue 29. Auhors Radhia GHARSALLAH (M 4) received he Egieerig Degrees from he Naioal Egieerig School of Tuis (ENIT). I Jui 25, she received he maser degree o commuicaios sysems from ENIT. Sice Jauary 28, she was a uiversiy assisa i he high school of compuig. She is currely workig oward he Ph.D. degree i Telecommuicaios sysems a he high school of commuicaios of Tuis i Iov Com laboraoory. RidhaBouallègue (M 98) received he Ph.D Degrees i elecroic egieerig from he Naioal Egieerig School of Tuis. I Mars 23, he received he Hd.R degrees i muliuser deecio i wireless commuicaios. From Sepember 199 he was a graduae Professor i he higher school of commuicaios of Tuis (SUP COM), he has augh courses i commuicaios ad elecroics. From 25 o 28, he was he Direcor of he Naioal egieerig school of Sousse. I 26, he was a member of he aioal commiee of sciece echology. Sice 25, he was he laboraory research i elecommuicaio Direcor sa SUP COM. From 25 he served as a member of he scieific commiee of validaio of hesis ad Hd.R i he higher egieerig school of Tuis. His curre research ieress iclude wireless ad mobile commuicaios, OFDM, space-ime processig for wireless sysems, muliuser deecio, wireless mulimedia commuicaios, ad CDMA sysems. 17