Performance Analyss of Mult User MIMO System wth Block-Dagonalzaton Precodng Scheme Yoon Hyun m and Jn Young m, wanwoon Unversty, Department of Electroncs Convergence Engneerng, Wolgye-Dong, Nowon-Gu, Seoul, 447-1 orea. {yoonhyun, jnyoung}@kw.ac.kr Abstract. As the demand of hgh qualty servce n next generaton wreless communcaton systems, a hgh performance of data transmsson requres an ncrease of spectrum effcency and an mprovement of error performance n wreless communcaton systems. In ths paper, we propose a mult-user multple-nput multple-output (MIMO) orthogonal frequency dvson multplexng (OFDM) system wth cyclc delay dversty and block dagonalzaton precodng method to mprove system capacty wth mantanng good error performance for 802.11n WLAN system. he results of mathlab smulaton show the mprovement of system capacty n 802.11n wreless ndoor channel envronment. eywords: Mult-user MIMO, Cyclc delay dversty, Coded OFDM, Blockdagonalzaton, 802.11n WLAN system 1 Introducton Increasng demand of hgh performance n next generaton wreless communcaton systems s enabled by the use of system that combnes mult-user Multple-nput multple-output (MIMO) and orthogonal frequency dvson multplexng (OFDM) system. MIMO system provdes multple ndependent transmsson channels, thus, under certan condtons, leadng to a channel capacty that ncreases lnearly wth the number of antennas and mprovng system relablty n lmted frequency resources [1-3]. Also OFDM s an effcent technque for reducng the effect of nter-symbol nterference (ISI) and nter-carrer nterference (ICI) due to usng mult-carrer modulaton n frequency selectve fadng channel. hs technology accomplshes the hgh data rates and mproves the relablty n wreless communcatons [4]. Recently, we study mult-user MIMO system whch mproves the system capacty by usng the pre-codng method such as drty paper code (DPC) and block dagonalzaton (BD). DPC s an optmal scheme to acheve the system capacty of mult-user MIMO downlnk channel. DPC s not proper n practcal wreless communcaton systems because t needs data of other users and the perfect channel state nformaton, t means that system complexty s very hgh. Block dagonalzaton s smply mplemented due to low complexty than DPC. Block dagonalzaton s used to suppress co-channel nterference whch s happened by transmt of other users - 69 -
n same frequency and tme. Block dagonallazaton, whose complexty s lower than DPC complexty, changes mult-user MIMO channel to multple sngle-user MIMO channels [5-6]. In ths paper, we propose mult-user MIMO OFDM system wth cyclc delay dversty (CDD) scheme for 802.11n system. he 802.11n wreless local area network (WLAN) s one of the key technques n the next generaton wreless communcaton systems. And we use block dagonalzaton due to the co-channel nterference cancellaton and the low complexty. he proposed mult-user MIMO OFDM system mproves the system capacty by usng CDD scheme than mult-user MIMO OFDM system wthout CDD [7]. hs paper s organzed as follow: In Secton 2, we ntroduce block dagonalzaton of mult-user MIMO OFDM system and WLAN channel model. Secton 3, we explan the proposed scheme whch uses CDD scheme. In Secton 4, we compare the system performance between the mult-user MIMO OFDM system wth CDD scheme and wthout CDD scheme accordng to dfferent channel model. Fnally, we conclude the proposed scheme n Secton V. 2 Proposed System Model In the mult-user downlnk channel wth users and a sngle base staton, the base staton has N antennas, and the th user has n,r antennas. he total number of antennas at users s defned to be NR n. he L 1, R 1 transmt vector of the th user s defned by d whch s operated by IFF. he N t L pre-codng matrx of the th user s defned by whch s lyng on the null space of the channel matrx of other users. he n,r N t channel matrx from the base staton to the th user s defned by H. he parameters of the channel matrx are ndependently dentcally dstrbuted (..d) and complex Gaussan probablty varance (mean s 0, varance s 1). he L 1 receved matrx of the th user s defned by r. he receved matrx s defned as r H d n k k k1 Hd Hd n k k k1, k, (1) where the n,r 1 nose vector v s ndependently dentcally dstrbuted, mean of 2 element s 0, varance s and addtve Gaussan whte nose (AWGN). he second factor of (1) means the co-channel nterference whch s transmtted for other users n same frequency and tme. he block dagonalzaton s used to cancel co-channel nterference. he condton of co-channel nterference cancellaton s gven by H0, k. (2) k hroughout ths paper, t s assumed { H } k k s perfectly known at the base 1 staton n order to fnd the pre-codng matrces and the estmaton of SNR. o fnd, - 70 -
the pre-codng matrces are lyng on the null space of H whch s the (-1)n,r N channel matrx excepted the channel matrx of the user. H s defned as 1 1 1 H H H H H, (3) where ndcates transposton. he condton of co-channel nterference cancellaton, whch s (2), s satsfed to fne the null space of H by usng the sngular value decomposton (SVD). he SVD operaton of H s defned as 1 0 * H U V V, (4) where * means hermtan transpose. U s the (-1)n,r (-1)n,r untary matrx and left sngular matrx. s the (-1)n,r (-1)N dagonal matrx. V 1 s the frst N rght sngular vectors, and V 0 s the last N N rght sngular vectors. V 0 forms null space of H. N s rank of H. Let the number of columns of the precodng matrx equal n,r to reduce the complexty of detecton and maxmze the system capacty. It s needed that SVD of multply H by V 0, and defned as 0 1 0 * HV U[ 0][ V V ], (5) where 1 V s frst n,r rght sngular vectors. he pre-codng matrx of user s computed by usng 0 V and 1 V computed n (4) and (5), respectvely. he precodng matrx s defned as 0 1 V V. (6) computed by (6) satsfes the condton of the co-channel nterference cancellaton. If we defne the system channel matrx H and the pre-codng matrx as H [ H H H ], 1 2 [ ] 1 2 [ VV VV VV ]. 0 1 0 1 0 1 1 1 2 2 Multply H by converts mult-user MIMO channel to multple sngle-user MIMO channel. It s block dagonalzaton whch s defned as (7) H H 1 1 0 0 0 H 2 2 0 0 0 0 H. (8) he block dagonal pre-coded data of each user s summed and transmtted at the antennas of the base staton. So the receved sgnal vector s defned smplcty as - 71 -
Fg. 1. Mult-user MIMO OFDM system wth CDD R HDN, r1 d1 n H r d n. (9) he co-channel nterference s canceled by block dagonalzaton. So each user receves the desred data whch s canceled the data of other users. 1/2 X Rtx Rrx Hd, (10) he block dagram for the conventonal L -branch dversty recever s shown n Fg. 1. In order to guarantee that the sgnals between each par of recever antennas fade ndependently, each recever antenna s suffcently separated n space. Its assumed that the search step sze s / 2. All correlators are assocated wth the c same phase of the local dspreadng code. And the outputs X l, l 0,1,..., L 1, of the L correlators are combned nto one decson varable V. hs combnng scheme s EGC. hen the decson varable V s compared to a threshold n order to make a decson whether codes are algned or not. he threshold value s determned by usng the cell averagng-constant false alarm rate (CA-CFAR) algorthm [8]. he search mode employs a seral search strategy. Whenever the decson varable V exceeds the threshold, the system decdes that the correspondng delay of the locally generated PN sequence s the correct one and enters nto the verfcaton mode. If V does not exceed, the phase of a locally generated PN sequence s rejected. hen, another phase s chosen to update the decson varable and above operaton s repeated. hs conventonal structure requres addtonal hardware unts accordng to ncreasng dversty branches so that both the cost and complexty ncrease. he proposed recever employs delay dversty schemes whch can ncrease dversty order wth low cost and complexty. he receved sgnal from one of the two - 72 -
antenna element s ntentonally delayed by the amount of the predetermned value. In order to avod overlappng wth the ntentonally delayed sgnals from the delay branches, should be larger than the maxmum excess path delay for the meanngful multpath sgnals from the normal branches. In ths recever, half of L branches are for the normal branches and the remanng L / 2 branches are for the delay branches. he proposed DDR has L dversty branches. he sgnals of the normal and delay branches are nput to the correlators n sequence. hen, the correlator outputs are combned nto one decson varable V usng EGC scheme. And then, V s compared wth the threshold n order to determne whether codes are algned or not. he proposed recever does not requre addtonal hardware unts and space compared wth the conventonal dversty recever, shown n Fg. 1. herefore, for the proposed recever, we can save cost and reduce complexty. 4 Smulaton Results We analyze the proposed scheme by a computaton smulaton. he smulaton parameters are presented n table I. We smulate the error performance accordng to IEEE 802.11n ndoor channel model. he computaton smulaton s performed under the base staton and users know perfect channel state nformaton. Each user use one antenna, the number of user s 2 and the number of base statons antennas s 2. If user uses multple antennas, we can use jont demodulaton processng to ncrease the BER performance, whle ths processng ncreases the system complexty. he error performances of mult-user MIMO OFDM wth CDD are depcted n Fg. 2. BER graphs shows the 3~4dB gan compared wth mult-user MIMO OFDM system wthout CDD. Fg. 2. BER performance of mult-user MIMO OFDM system wth CDD (QPS and Codng rate 1/2) - 73 -
5 Conclusons Mult-user MIMO OFDM systems wll be used to mprove the spectrum effcency whch s very mportant because frequency resource s lmted and wreless communcaton systems wll be ncreased n next generaton wreless communcatons. So, n ths paper, we propose a mult-user MIMO OFDM system wth CDD and block dagonalzaton precodng method to mprove BER performance wth wreless WLA) channel model C and D for 802.11n WLAN system. he results of mathlab smulaton show the mprovement of BER performance n 802.11n wreless ndoor channel envronment. So the proposed scheme of ths paper can be appled to many systems n the advanced wreless communcatons such as bologcal data transmsson and home network systems. Acknowledgments hs work was, n part, supported by the Natonal Research Foundaton of orea(nrf) grant funded by the orea government(mes) (No. 2011-0029329), and n part, supported by the ME (he Mnstry of nowledge Economy), orea, under the IRC (Informaton echnology Research Center) support program supervsed by the NIPA (Natonal I Industry Promoton Agency)" (NIPA-2012-(C1090-1211- 0005)). References 1. M. Jankraman, Space-me Codes and MIMO Systems, Artech House Publshers, 2004. 2. D. se and P. Vswanath, Fundamentals of Wreless Communcaton, Cambrdge Unversty Press, 2005. 3. A. Goldsmth, S. A. Jafar, N. Jndal, and S. J. Select. Areas Commun., vol. 21, no. 5, June 2003, pp. 684-702. 4. L. Hanzo, M. Munster, B. J. Cho, and. eller, OFDM and MC-CDMA for Broadband Mult-User Communcatons, WLANs and Broadcastng, John Wley & Sons, 2003. 5. -antenna and mult- - 1316. 6. Q. H. Spencer, C. B. Peel, A. L. Swndle-user -67. 7. IEEE rans. Veh. echnol., vol. 1, pp. 505 509, May. 2004. 8. Select. Areas Commun., vol. 5, 1987, pp. 128-137. - 74 -