Adaptive Co-Channel Interference Suppression Technique for Multi-User MIMO MC DS/CDMA Systems

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1 Int. J. Counications, Networ an Syste Sciences, 009,, 8-86 oi:0.436/ijcns Publishe Online Deceber 009 ( Aaptive Co-Channel Interference Suppression Technique for Multi-User MIMO MC DS/CDMA Systes Prabagarane Nagarajane, Arvin Sai Sarathi Vasan, Lashi Krishnan, Anan Venatasway Departent of Electronics an Counication Engineering, SSN College of Engineering, SSN Institutions, Chennai, Inia Departent of Electrical an Coputer Engineering, University of Marylan, College Par, Marylan, USA E-ail: {arvin88, Receive Septeber, 009; revise October 0, 009; accepte Noveber 6, 009 Abstract In this paper, an aaptive co-channel interference suppression technique for ulti-user MIMO MC DS/CDMA syste is envisage. MC DS/CDMA offers any avantages lie flexibility, robustness, low PAPR an spectral efficiency. In spite of these avantages, perforance of MC DS/CDMA syste is greatly ipaire by interference. Coon interferences, which egrae the perforance of the syste, are MAI an CCI. Mitigating these interferences can irectly increase the capacity of the syste. In this wor, an aaptive co-channel interference suppression technique base on single-stage an two-stage MMSE IC is consiere for ulti-user MIMO MC DS/CDMA syste. Siulation results show that, at low SNR two-stage MMSE IC outperfors single-stage, while at high SNR, single-stage provies better BER perforance. Base on this, a selection criterion has been propoune for iprove syste perforance as a whole in interference liite environent. Also, aaptive selection criterion resulte in better error perforance. Keywors: CCI, MIMO, MC DS/CDMA, MMSE IC, ML Decoing. Introuction Multi-carrier transission has recently gaine enorous attention for proviing high ata rate counications on both forwar an reverse channels. Multi-carrier transission is realize through OFDM.OFDM perfors well in frequency selective channels []. One of the ost proising single carrier transission schees is CDMA, which is robust to noise []. Also in the recent past, ultiinput ulti-output (MIMO) has prove to provie very high capacity without any increase in banwith or power [3]. Cobining these techniques leas to MIMO MC DS/CDMA syste, which is expecte to eet the eans of future broaban (4G) wireless wie-area networs. Though MIMO MC DS/CDMA possesses any avantages, it still suffers fro the traitional ipairents of conventional CDMA systes lie MAI an CCI [4 6]. Of late, yria research concentration has been on proposing techniques for itigating MAI (Multiple Access Interference) an ulti-path, but very little wor has been carrie out on CCI (Co-Channel Interference) cobating techniques. In this paper, we investigate the perforance of single-stage an two-stage CCI cancellation techniques for a MIMO MC DS/CDMA syste. MIMO is realize by eploying space-tie bloc coes. At the receiver, we have eploye two-stage MMSE (Miniu Mean Square Error) IC (Interference Cancellation) with ML (Maxiu Lielihoo) ecoing [4]. We have consiere a ulti-user MIMO MC DS/CDMA syste, with asynchronous co-channel users in the uplin. Each of the asynchronous co-channel users is equippe with N T transit antennas an they counicate to a single base station, equippe with N R receive antennas. In this ulti-user environent, x N T interfering signals will be arriving at the base station. Conventional interference itigation technique fro - cochannel users requires N T x ((-) +) receive antennas, so as to suppress the co-channel interferences. But by eploying STBC the sae can be achieve by using N R receive antennas such that N R, which exploits the spatial an teporal structure. Also in this wor we consier two co-channel users, each equippe with two transit antennas an the base station equippe with two receive antennas. The perforance of MMSE IC an ML ecoing algorith over MC DS/CDMA syste with each MC DS/CDMA transitter in turn carrying ultiuser ata is assaye for both ownlin an uplin. The paper is organize as follows: Section introuces the Copyright 009 SciRes.

2 syste oel we have consiere throughout this wor, Section 3 escribes the iprove MMSE IC with ML ecoing algorith, Section 4 escribes the two-stage MMSE IC for MIMO MC DS/CDMA syste, Section 5 expouns the perforance results an Section 6 provies the conclusions.. Syste Moels N. PRABAGARANE ET AL. 83 ere the syste oel that we have consiere coprises two asynchronous co-channel users in a MIMO MC DS/CDMA syste in which each transitting terinal is a MC DS/CDMA transitter with MIMO support realize through Space Tie Bloc Coes (spatial iversity). The two co-channel users counicate with two receive antennas at the receiver, which perfors interference cancellation an then etects the transitte signal of each user. Figure shows the MIMO MC DS/ CDMA transitter.the user ata at each transitting terinal after appropriate constellation apping is ultiplie with a spreaing coe an the sprea sybols of each ata are ulti-carrier oulate. Then the su of all the carriers of the th user coposes the output of the th user signal S (t). The total output S (t) is the su of all the user signals. After PA (high power aplification), the final signal is transitte. The transitte signal corresponing to the nth ata sybol of the th user is S n M Q 0 q 0, q ( t ) C p ( t qt nt ) e c j f t where, is the user nuber, is the carrier nuber, q is the chip nuber, T c is chip uration an T is sybol uration an equals to Q T c. Q is the length of user specific spreaing coe. is the ata of th sub-carrier an th user, c,q is the qth spreaing coe of th carrier of th user, p (t) is Root raise cosine pulse of th carrier an f is the th carrier frequency. When the total nuber of users is K, the total transitte signal corresponing to the nth ata sybol is S n ( t ) K M Q 0 0 q 0 C, q e () j ft () This oulate strea is then passe through a STBC encoer which groups the sybols accoring to a specific STBC pattern (G) an then transits the sybols through ultiple transit antennas. The receive vector at the first receive antenna for the transitte sybols an is expresse as r r * * h h n n where, h an h enotes the channel faing an n an n represents the aitive white Gaussian noise (3) Figure. MIMO MC DS/CDMA transitter. (AWGN). Let u, u an v, v represent the sybols transitte over two consecutive sybol urations by the corresponing two co-channel asynchronous users. As shown in Figure, let h, h, g an g represent the channel faing between all the transit antennas an the first receive antenna at the base station respectively an h, h, g an g represent the channel faing between all the transit antennas an the secon receive antenna at the base station respectively. Now, the receive vector at the receive antenna Rx over the two sybol tie perio is expresse as r h u h u g v g v (4) n * * * * r hu hu gv gv n (5) The receive vector at the receiver antenna Rx is given by r hu hu gv gv n (6) * * * * r hu hu gv gv n (7) 3. Iprove MMSE IC ML Decoing The overall receive signal fro each of the receive antenna is expresse as T T r R R T (8) The channel faing coefficient between each transitter an the receive antenna is rearrange for each cochannel user to for a generic channel atrix. For etecting the first co-channel user ata, the corresponing channel atrix is represente as G (9) G an for the secon user, it is given by ~ G (0) G The MMSE weight atrix is calculate base on the generic channel atrix an the weight atrix value if- Copyright 009 SciRes.

3 84 N. PRABAGARANE ET AL. Figure. MIMO MC DS/CDMA with two co-channel asynchronous users an aaptive base station receiver syste oel. fers for each co-channel user. For each of the two users consiere here in this wor, the MMSE weight atrix is copute fro the expression [4,5] ~ ~ ~ M I 4 an M I 4 () where, M is the weight atrix for the first user an M ~ is the weight atrix for the secon user. represent the eritian transpose of the channel atrix an σ is the noise variance. To suppress the interferences fro other co-channel users, the inverse of the weight atrix is ultiplie with the coluns of the channel atrix that represents the channel faing for a particular cochannel user i.e. w M h an ~ ~ w M h where, h =first colun of or ~ h =secon colun of or ~ The MMSE Interference cancellation receiver suppresses both co-channel interferences an noise coponents, which eans that the ean square error or variance between the transitte sybols an the estiate is reuce. The axiu lielihoo (ML) etection is use to etect the transitte sybols for the corresponing user. The ML ecoing estiates the sybols by eterining the iniu Eucliian istance of all possible transitte sybols fro the receive constellation [4], given by Uˆ arg in { w uˆ U r uˆ ˆ s w rv w rvˆ 4. Two Stage Interference Cancellation (6) The two stage interference cancellation procees with first ecoing the ata fro the two terinals. Then, assuing each ecoe value is correct, the ata corresponing to the other user is estiate using Maxiu lielihoo estiation. i.e. first assuing the first terinal ecoe ata is correct base on single stage MMSE IC, the secon user ata is estiate fro the ecoe ata. This is carrie out by first calculating x an x. () x R. cˆ 0 an x. ˆ R c0 (7) where, is the ata ecoe, corresponing to the w r uˆ } (3) an where, ˆ ~ arg in { ˆ ~ V w r v w r vˆ } (4) vˆ V ĉ 0 first user. The secon user ata is estiate fro the first user ecoe ata by using the axiu lielihoo estiation given by 0 sˆ S s ˆ arg in{ x G s x G s } (8) 0 where, S taes all possible values in the signal constellation. The reliability corresponing to the estiate ata is given by the expression s0 x Gs ˆ0 x Gsˆ0 (9) Siilarly, the first user ata base on single stage MMSE IC is estiate by coputing y an y. y R G. sˆ an y R G. sˆ (0) ŝ is the ata ecoe corresponing to the secon user. The first user ata is estiate fro the secon user ecoe ata by using the expression [4] where, Uˆ an V ˆ represent the two co-channel users ˆ estiate ata. û, û, ˆv an ˆv arg in{ c yc y c } () taes all possible cˆ C values of the users signal constellation. The reliability of The corresponing value of reliability for the estiate ata the ecoe signals are copute by is 0 ˆ c w ru w ruˆ (5) ˆ c y c y cˆ () Copyright 009 SciRes.

4 N. PRABAGARANE ET AL. 85 The receiver coputes the overall reliability for the two users i.e. 0 c0 s 0 an c s.the ecision is ae on the sets of sybols copute by coparing the two reliabilities. The coparison is ae as [4] if ( 0 ) (,) cs ˆˆ ( cˆ, sˆ ) 0 0 else (,) cs ˆˆ ( cˆ, sˆ ) (3) The syste illustrate in Figure, consists of a receiver with a switch between a single an ouble stage MMSE IC unit, CSI (Channel State Inforation) an a ecision unit. When the channel is slowly varying, the receiver etects the sybols base on single stage technique. When the channel variation is rapi, two stage MMSE IC is eploye to etect the sybols. At present, perfect channel nowlege is assue at the receiver. The entire etection taes place base on the instantaneous SNR available at the receiver. Table. Siulation paraeters. PARAMETERS Spreaing coe VALUES Walsh aaar Nuber of Uplin 5000 channel realizations Downlin 0000 FFT Size 8 Cyclic Prefix /8 Spreaing Factor 6 or 3 Data Moulation Channel Moel BPSK Rayleigh Nuber of Transit antennas Nuber of Receive antennas 5. Perforance Analysis In this section, we present the perforance of aaptive co-channel suppression technique for a ulti-user MIMO MC DS/CDMA syste. The siulation results for the single stage an two stage interference cancellation techniques are shown in Figures 3 an 4. The channel oel consiere is quasi-static Rayleigh faing channel, which is built on the classical unerstaning of Doppler shift an elay sprea. The oulation schee eploye is BPSK, as it provies the best syste throughput for MIMO realization base on STBC. The nuber of channel realizations consiere for uplin an ownlin is 5000 an 0000 respectively for each value for SNR. Table suarizes the siulation paraeters. Siulation results ivulge that, at low SNR value, two stage interference cancellation techniques perfor well whereas at high SNR value single stage MMSE IC with ML ecoing provies better BER perforance. ence, a trae off can be ae in selecting the interference cancellation techniques at the receiver when the SNR winles. This can result in better perforance of MIMO MC DS/CDMA syste in an interference liite environent as switching of IC can be ae in an aaptive anner. Figure 3 shows the uplin perforance of MIMO MC DS/CDMA syste with single stage IC, two stage IC an aaptive IC for two co-channel users. Each user ata is sprea by a spreaing factor of 3. ere in each MC DS CDMA syste one user is accooate. It can be iscerne that aaptive IC outperfors both single stage an ouble stage. The sae perforance can also be realize over the ownlin channel. Figure 4 eluciates Figure 3. Perforance of aaptive co-channel interference schee for co-channel users over an uplin counication channel for MIMO MC DS/CDMA syste. Figure 4. Perforance of aaptive co-channel interference schee for 4 co-channel users over ownlin counication channel for MIMO MC DS/CDMA syste. Copyright 009 SciRes.

5 86 N. PRABAGARANE ET AL. the perforance of the sae syste with four co-channel users, with each user sprea by a spreaing factor of 6 over ownlin counication channel. ere also aaptive switching schee provies better BER perforance. 6. Conclusions In this wor, we consiere a two stage MMSE co-channel interference cancellation receiver for MIMO MC DS/ CDMA systes. MC DS /CDMA can be realize as a proinent air interface for 4G Broaban counications; however, capacity of such systes is liite by interference. Mitigating the various interferences can result in confronting the future generation wireless networs nees. In this paper we have analyze a two stage IC technique for a ulti-user environent. Results of our analysis reveal that a trae off coul be ae in selecting the IC techniques for itigating CCI. It coul be iscerne that at low SNR values two stage has resulte in better perforance because of its iterative nature while at high SNR values, single stage perfors better. Also, it is expoune fro our analysis that the aaptive interference cancellation receiver has resulte in better suppression of CCI. 7. References [] L. anzo, L-L. Yang, E-L. Kuna, an K. Yen, Single an ulti-carrier DS-CDMA ulti-user etection, space-tie spreaing, synchronization an stanars, IEEE Press, 003. [] M. K. Sion an M. S. Alouini, BER perforance of ulti-carrier DS-CDMA systes over generalize faing channels, IEEE International Conference, 999. [3] E. Bigieri, R. Calerban, A. Constantinies, A. Golsith, A. Paulraj, an. Vincent Poor, MIMO wireless counications, Cabrige University Press, 007. [4] Anan. V, Arvin. S, an Lashi Krishnan, Investigations on the perforance of MIMO assiste Multi Carrier DS/CDMA syste with ulti-user etection for 4G obile counications, Dissertation, SSN Institutions, 009. [5] Prabagarane Nagarajane, Arvin Sai Sarathi Vasan, an Lashi Krishnan, A robust space tie co-channel interference itigation an etection technique for ulti-user MIMO ulti-carrier DS/ CDMA systes, Proceeings of IEEE International Conference, Wireless Vitae, 009. [6] S. Kono an L. B. Milstein, Perforance of ulti-carrier DS CDMA systes, IEEE Transactions on Counications, Vol. 44, No., February 996. Copyright 009 SciRes.