An Improved Algorithm of Successive Interference Cancellation for STC-OFDM Systems

Transcription

1 Sensors & ransucers Vo. 66 Issue 3 March 04 pp Sensors & ransucers 04 by IFS Pubshng S. L. n Improve gorthm of Successve Interference Canceaton for SC-OFDM Systems We Shhong Zhao Wan Jang Hongun Schoo of Communcatons an Informaton Engneerng Chongqng Unversty of Posts an eecommuncatons CQUP Chongqng Chna e.: E-ma: Wesh@cqupt.eu.cn Receve: 5 January 04 /ccepte: 8 February 04 /Pubshe: 3 March 04 bstract: SC-OFDM systems effectvey overcome the savantage of nter symbo nterference an frequency seectve fang for Orthogona Frequency Dvson Mutpexng communcaton systems however there are st some ssues such as how to suppressng mutua nterference ue to mut-antenna transmttng. In ths paper an mprovng agorthm combnng wth channe frequency response recovery s propose whch recover the channe frequency response of the pot before successve nterference canceaton an reuce the effects of the channe frequency response estmaton. heoretca anayss an computer smuaton shows that the proposa agorthm can mprove performance of Bt Error Rate an suppress the nterference n the SC-OFDM communcaton systems. Copyrght 04 IFS Pubshng S. L. Keywors: OFDM Space-tme cong Channe frequency response recovery Successve nterference canceaton.. Introucton OFDM ue to ts reazaton an overcomng the frequency seectve fang has been wey use n gta transmsson an communcaton systems []. In recent years OFDM combnes wth Mutpe Input Mutpe Output (MIMO) an Space me Cong (SC) to acheve a SC-OFDM communcaton system whch s hgh capacty n a mte banwth hgh cong gan ant-mutpath an nter-symbo nterference (ISI) []. However mut-antenna transmsson cou resut n nterference such as Cochanne Interference (CCI) each other at recever by sgnas transmtte from fferent antennas of ether the same transmtters or other transmtters. Consequenty to sove the nterference probem s mmnent an the correaton etecton agorthm of the recever n MIMO system s partcuary mportant. Recenty researchers scovere ots of sgna etecton methos of MIMO systems whe fnng better performance sgna etecton agorthms of MIMO systems s becomng a research focus. Compare wth the maxmum ehoo etecton agorthm Successve Interference Canceaton (SIC) has ower computatona compexty whe ts etecton performance s ower than the maxmum ehoo etecton. hs agorthm etects each sgna of the mnmum eve of error probabty; the sgna etecte can be as the nterference sgna to emnate the mpact of the sgna eve whch s not etecte. he accuracy of the sgna whch was frst etecte cou nfuence the sgna etecton of next ayer. Detecton performance of the entre system rtce number P_98 5

2 Sensors & ransucers Vo. 66 Issue 3 March 04 pp was affecte by the etecton accuracy of the sgna whch was frst etecte. Frsty the aaptve channe frequency response recovery agorthm can reuce the evaton of sgna etecton [3 4]. In aton SIC agorthm can effectvey suppress CCI an mprove the system performance. Base on the prevous researches the combnaton of CFR recovery an SIC suppressng CCI n SC-OFDM systems was gven by ths paper. Recover the pot CFR before SIC an then etect the sgna by SIC. ccorngy theoretca anayss an computer smuaton show that the new agorthm can further mprove the system bt error rate an suppress the CCI of SC-OFDM system. In ths paper the certan organzaton s foowe as beow: he secton escrbes the moe of SC-OFDM system. he secton 3 anayzes the agorthm an evauates a theoretca expresson to estmate the system performance. s the secton 4 theoretca an smuaton curves are presente for comparson an vaaton. Fnay secton 5 presents the concusons.. System Moem smpe SBC-OFDM system s showe n Fg.. Here we conser two transmt antennas an one receve antenna for notaton smpcty. More receve antennas can be easy extene. For two transmt antennas ata symbo vector s efne as: X X X () = Fg.. System moe. For each K. he set K = {0... N } s the nex of subcarrers an N s the number of subcarrers. () enotes transpose. fter the SBC we can obtan X X S ( ) = () X X If cycc prefx s onger than the max channe eay sprea the sgnas can acheve fu synchronzaton. fter some manpuaton such as an Inverse fast Fourer transform (IFF) aton an remova of cycc prefx sera-to-parae converson an an FF before sgna etecton of recever we can obtan a foowng receve ata symbo vector at the -th subcarrer Y = Y Y ( ) = H S I W (3) H H H ( ) = (4) H H I I I (5) = W W W (6) In Eq. (4) = H s the CFR of a tme-varyng mutpath channe exstng between the -th transmt antenna an a receve antenna. In Eq. (5) I ( ) = I ( = ) I ( ) s the ICI cause by the tme-varyng channe exste between the -th transmt antenna an a receve antenna. he ICI can be moee as a ranom varabe wth zero mean an σ I. In Eq. (6) varance of whte Gaussan nose wth zero mean an varance of σ. W W s an tve Next step s to fee Y ( ) nto a near combner wth a combnng coeffcent of H ( ) resutng n 5

3 Sensors & ransucers Vo. 66 Issue 3 March 04 pp Z = Z Z = = ψ S ϒ Ω H Y (7) () represents Hermtan transpose of a matrx. g = H = = = ( ) g g H H = g = H 3 = ϒ = ϒ ϒ = H I H W Ω = Ω Ω = ψ ( ) g g = g g (8) 3. gorthm Descrptons an nayss Frst recover the pot CFR so the error of ata CFR estmate by nterpoaton w be smaer an then combne wth the SIC the ata w be etecte better. Fg. shows the boc agram of the agorthm an the foowng s the agorthm anayss: Y(K) Y = Y(K) Z(K) X(K) X(K) Z(K) X(K) ˆX (K) Z (K) X (K) X (K) Z (K) X (K) ˆX (K) (K) (K) Fg.. he boc agram of the agorthm. ) Pot CFR recovery For convenence of anayss we efne the foowng notatons: λ the poston of the -th pot n an OFDM symbo; ˆ n H λ the estmate CFR of the -th pot n the n -th OFDM symbo; n the fference between Hˆ ( λ n ) an ˆ Hˆ λ Hˆ λ H n( λ ) = n n n ; ( n ) n ; ( n ) the rea part of of n. If CFR H n R I the magnary part s varyng sowy wth subcarrer nex. Hence the fference between the aacent pot CFRs shou be sma. hus we can use the foowng equaton to etect the affecte pot CFRs an recover them. ( ) ( ) Hˆ ( λ ) otherwse n Hˆ λ Hˆ λ f R > ε an R > ε; n n n n H ( λ ) = or εan ε n I > I > n n H n( λ ) (9) s the recovere pot CFR an ε s a preetermne thresho. he rea an magnary parts of pot CFRs are examne nepenenty to etect the affecte pots. If scontnuty exts n any of the rea or magnary part a corrupte pot s etecte. he scontnuty s chece among the aacent three pots by usng thresho ε. If the - th pot s affecte by CCI ts CFR s recovere as the average of the -th an the -th pot CFRs. ) Estmate the ata CFR: Usng the recovery pot CFR estmate the ata CFR by nterpoaton we can obtan H an cacuate ψ. 3) Obtan the st temporary symbo from (7): = { } X Z (0) { } s the har ecson. 4) Set seecton parameters: () f g ( ) < g ( ) = f g g = 0 ese 0 ese 53

4 Sensors & ransucers Vo. 66 Issue 3 March 04 pp ) Obtan the n temporary symbo from: X X = () r = N 3N 3N η K o = = (9) 6) Cance the CCI from (7): { } { } = Z Z g X (3) Z Z g X = g ( ) an g ( ) are estmaton vaues of g ( ) an g ( ) respectvey. 7) Obtan the 3 r temporary symbo from: X Z (4) { } = 8) Detect the fna ata symbo by usng: = X X X (5) In forwar error correcton the Vterb agorthm s use for econg a commony use upper boun of the probabty of bt error s [5] P < b BP (6) = free s the number of the nformaton bts per tres branch s the output weght of a specfc path P s the probabty that the ecoer seects a coe sequence that s a Hammng stance from the correct coe sequence an B represents the tota nformaton weght of a coe sequences of weght an represents the sum of a possbe bt errors that can occur when the a-zero coe sequence s transmtte. Fnay free s the mnmum Hammng stance between a pars of non-zero paths. he probabty P for har ecson econg s P = ( ) s o p p = s even / / P = p p p p / = he probabty of bt error after the canceaton s gven by [6] ( ) (7) -th P = Q r (8) fter canceaton the varabe of the ecson varabe contone on as foows K No η = η = = 3N 3N for synchronous (0) K s the tota number of actve users. s the amptue of -th user. N / o s the se ban power spectra ensty. N = / c s the bt pero an c s the chp pero. whch s the orere set of amptues of K users s assume to be Rayegh strbute wth unt mean square vaue.e. ts probabty ensty functon s gven by f ( x) = xe x an ts cumuatve ensty functon s F x = e x so the pf of the orere gven by s enote by f ( K ) ( ) x an s obtane as foows: K! K f ( x ) = F!! x F x f x hen P = Q f x x 0 E η = 0 E x f x x () () K N o E η E η for synchronous = = = 3N 3N (3) 4. Computer Smuatons an nayss In orer to ustrate the BER performance of the propose agorthm base on CFR recovery an SIC some parameters are shown by abe. abe. Parameters. Parameter Vaue FF 04 B K 3 free N o c Mouaton 0-8 W/Hz 8Mc/s QPSK 54

5 Sensors & ransucers Vo. 66 Issue 3 March 04 pp Fg. 3 shows the BER curves wth the SNR s changes. Due to ntrouce pot CFR recovery before the SIC agorthm the ata CFR estmate w brng a smaer error. So BER of the new agorthm s ower than the tratona n Rayegh fang for fferent users. It s obvous that the fewer the number of users the ower the error rate. he BER of 0 users s ower than 0 users n Rayegh fang. Fg. 4 shows the BER curves wth the number of users changes. It can be seen the BER s growng arger wth the ncrease of users n Rayegh fang. n the BER of the gven agorthm s growng ower wth the ncrease of SNR uner Rayegh fang cose to the BER of Gaussan state. hs ncates that the gven agorthm s more favorabe than the tratona metho to emnate CCI to a certan extent Rayegh-tratona 0users-Rayegh-propose 5users-Rayegh-propose 0users-Rayegh-propose Gaussan-propose Rayegh-tratona 5B-Rayegh-propose 0B-Rayegh-propose 5B-Rayegh-propose 0B-Rayegh-propose 5B-Rayegh-propose Gaussan-propose BER BER snr user Fg. 3. BER vs. SNR. Fg. 4. BER vs. User. 5. Concusons he combnaton of SC an OFDM technoogy s wey use n broaban wreess communcaton system. It not ony aevates rawbacs of each other but aso mproves the performance of hgh spee transmsson whch has two nherent funamenta mparments mut-path fang an ISI. he gven agorthm base on the combnaton of pot CFR recovery an SIC agorthm can further nhbt the CCI an the BER performance of the system has mprove to some extent. 6. References []. K. Zheng B. Fan J. H. Lu et a. Interference coornaton for OFDM-base muthop LEavance Networs IEEE Wreess Communcatons Vo. 8 Issue 0 pp []. S. H. L D. F. Huang K. B. Letaef Z. C. Zhou Pre- DF processng for MIMO-OFDM systems wth space-tme-frequency cong IEEE Wreess Communcatons Vo. 6 Issue 007 pp [3]. Y. H. Zeng W. H. Lam. S. Ng Sembn channe estmaton an equazaton for MIMO space-tme coe OFDM IEEE ransactons on Crcuts an Systems Vo. 53 Issue 006 pp [4]. P. eera W. Utschc J. Nosse G. Bauch Rate baancng n mutuser MIMO OFDM systems IEEE ransactons on Communcatons Vo. 57 Issue pp [5]. H. J. Jang Z. F. Zhang J. B J. Huang Improve anayss of co-channe nterference suppresson for WH-SC-OFDM systems n Proceengs of the 7 th Internatona Conference on Wreess Communcatons Networng an Mobe Computng Wuhan Chna September 0. [6]. S. Q. Chen W. J. Wang S. Jn X. Q. Gao Sorte QR ecomposton base etecton for MU-MIMO LE upn n Proceengs of the IEEE 7 st Vehcuar echnoogy Conference ape awan 6-9 May Copyrght Internatona Frequency Sensor ssocaton (IFS) Pubshng S. L. rghts reserve. ( 55