A Novel Time Domain Synchronous Orthogonal Frequency Division Multiple Access Scheme

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1 A ovel Tme Doman Synchronous Orthogonal Frequency Dvson Multple Access Scheme Lnglong Da Jan Fu Jun Wang Jan Song and Zhxng Yang Tsnghua atonal Laboratory for Informaton Scence and Technology Electroncs Engneerng Department Tsnghua Unversty Bejng PR Chna Abstract Tme-doman synchronous orthogonal frequency dvson multplexng (TDS-OFDM) outperforms prefxed- OFDM (CP-OFDM) n spectrum effcency at the expense of hgher complexty Up to now TDS-OFDM has only been used n undrectonal transmsson such as broadcastng applcatons The overwhelmng complexty of removng the mplctly superposed nterferences between the s and IDFT nformaton blocks caused by multple users hnders the applcaton of tme-doman synchronous orthogonal frequency dvson multple access (TDS-OFDMA) n mult-user communcaton scenaros To solve ths problem ths paper proposes a novel TDS-OFDMA scheme ncludng jont ty reconstructon and jont channel estmaton utlzng the new tme-space two-dmensonal frame structure Theoretcal analyss and computer smulaton show that the proposed scheme not only can acheve the purpose of multple access but also has an even better system performance n moble envronment than the conventonal sngle-user TDS- OFDM system I ITRODUCTIO Orthogonal frequency dvson multplexng (OFDM) has been wdely adopted n commercal systems such as terrestral dgtal vdeo broadcastng (DVB-T) [1] and the IEEE 80211a wreless local area network (WLA) [2] Recently there s a strong nterest n extendng the OFDM concept to mult-user communcaton scenaros For access scheme TDMA/FDMA/CDMA systems are proposed yet the orthogonal frequency dvson multple access (OFDMA) technology has not been adopted wdely OFDMA was orgnally suggested by Sar and Karam for cable TV (CATV) networks [3] and later adopted n the uplnk of the nteracton channel for dgtal terrestral televson (DVB-RCT) [4] More recently t has become part of the emergng IEEE standards for wreless metropoltan area networks (WMAs) [5] and s currently attractng vast research attenton from both academa and ndustry as a promsng canddate for next generaton broadband wreless networks [6] In most OFDMA systems prefx-ofdm (CP- OFDM) s used It uses a suffcently long prefx (CP) as the guard nterval (GI) to combat nter-carrer nterference (ICI) as well as to smplfy channel equalzaton n the frequency doman wth very low complexty [7] However t needs a large amount of extra tranng symbols or plots n order to facltate the synchronzaton and channel estmaton (CE) [8] whch decreases the spectral effcency of ths system Tme doman synchronous-ofdm (TDS-OFDM) s the essental technology of the Chnese natonal dgtal televson terrestral broadcastng (DTTB) standard [9] whch nserts pseudo-nose (P) sequence nstead of CP as the GI before the nverse dscrete Fourer transform (IDFT) block The P sequences are also used for synchronzaton and CE [10] Therefore ts spectral effcency ncreases a lot It has also been demonstrated that TDS-OFDM can provde even better performance than CP-OFDM [11] However the ty of the receved IDFT block s destroyed snce the and the IDFT block wll cause the nter-symbol nterference (ISI) to each other Therefore teratve nterference subtracton method [12] s adopted to remove the ISI between them and then reconstruct the ty of the receved IDFT block for CE and equalzaton Based on ths algorthm other mprovng methods are proposed to ncrease the accuracy and convergence speed [13] [14] When TDS-OFDM s appled to multple access scenaros we focus on TDS-OFDMA snce Rohlng [15] has demonstrated that OFDMA clearly outperforms other multple access technques lke OFDM-TDMA or OFDM-CDMA However the hgh complexty and dssatsfactory performance of those methods [12]-[14] make the nterference cancellaton n TDS- OFDMA systems almost mpossble snce the superposed nterferences caused by multple users are much more complcated Therefore there s no lterature addressng TDS- OFDMA system Amng at the problem mentoned above we propose a novel TDS-OFDMA scheme to cope wth the mplctly combned nterferences Takng advantage of the new twodmensonal frame structure n the proposed system not only the jont ty reconstructon for all users s completed va one-step add-subtract operaton so that sgnal separaton can be acheved n the frequency doman but also the jont channel estmaton and parttonng for all users s completed n the tme doman va crcular convoluton therefore the recever can restore the data for every user The remanng paper s organzed as follows Secton II llustrates the system model of the proposed TDS-OFDMA scheme Takng advantage of the new tme-space two- dmensonal frame structure presented n secton III for the proposed TDS-OFDMA scheme the jont ty reconstructon for all users and sgnal separaton n frequency doman /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs

2 Tx 1 User 1 S/P MAP CAS IFFT P / S Upconverter h 1 User M S/P MAP CAS IFFT d m D m m S/P MAP CAS IFFT c m P / S X x m P / S s m Upconverter Upconverter Tx m h m Tx M h M Rx r total Jont Cyclcty Reconstructon Jont Channel Eestmaton h total Sgnal separaton y ' total channel separaton Y ' m H m Eq and user separaton D User 1 m User M Fg 1 System model of the proposed TDS-OFDMA scheme s presented n secton IV and the jont channel estmaton method n the tme doman s derved n secton V respectvely Secton VI shows the smulaton results to verfy the feasblty and system performance of the proposed scheme We then conclude ths paper n secton VII II SYSTEM MODEL OF TDS-OFDMA Fg 1 shows a general dscrete-tme system model of the proposed TDS-OFDMA scheme where M users smultaneously transmt ther sgnals to a central base staton For the mth user n the proposed TDS-OFDMA system each block d m of L m nformaton symbols after seral-toparallel (S/P) converson s fed to the MAP unt for QPSK/m- QAM modulaton to generate the frequency-doman data D m = D m (n)} Lm 1 n the th sgnal frame Then the carrer assgnment scheme (CAS) unt maps D m onto the sub-carrers assgned to the correspondng user by extendng D m wth the nserton of L m zeros and results n an -dmensonal vector X m = X m (n)} 1 wth entres Dm (n) n Γ X m (n) = m 0 otherwse (1) where Γ m s the sub-carrer set of the mth user Clearly to avod that a gven sub-carrer s shared by dfferent users all these sets Γ m } M must be mutually exclusve eγ Γ j = φ for j After IDFT operaton of X m we get the tme-doman data symbols x m whose entry x m (n) can thus be wrtten as x m (n)= 1 1 X m (k)exp } j 2πnk 0 n 1 (2) k=0 The th basc TDS-OFDM sgnal frame of the mth user s m = s m (n)} f 1 s then obtaned as the seral concatenaton of the g -pont c m (wll be dscussed n detal n secton III) and data symbols x m and takes the form cm (n) 0 n s m (n) = g 1 x m (n) g n f 1 (3) where f = + g Each sgnal frame s parallel-to-seral (P/S) converted and then up-converted for transmsson After passng through a mult-path channel wth channel mpulse response (CIR) denoted by h m = h m (l)} lm 1 l=0 where l m s the maxmum delay spread the receved blocks correspondng to the P sequence c m and IDFT block x m can be represented by g m (n) and y m (n) respectvely g m (n) =c m (n) h m (n) 0 n< g +l m 1 (4) y m (n) =x m (n) h m (n) 0 n<+l m 1 (5) where denotes lnear convoluton Intutvely g m (n)} g+lm 2 n= g s the nterference form P sequence to IDFT block and y m (n)} +lm 2 n= s the nterference from IDFT block to the next Some teratve methods wth hgh complexty have been proposed to remove the ISI and then reconstruct the ty of the receved IDFT block for CE and equalzaton n the conventonal sngleuser TDS-OFDM system [12]-[14] However n the TDS- OFDMA system the nterferences caused by dfferent users are mplctly superposed at the recever dsmss the possblty of the nterferences cancellaton by the teratve nterference subtracton method that s already complex enough Therefore sgnal separaton for dfferent users s very dffcult n TDS- OFDMA system Confrontng the dffculty of nterference cancellaton n TDS-OFDMA system ths paper frstly proposes the new tme-space two-dmensonal frame structure takng both the tme dmenson and space (dfferent users locates at dfferent /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs

3 Space (user) User (m+1) extenson extenson Fg 2 p L s m1 pm m-sequence p m the th sgnal frame IDFT Block IDFT Block xm 1 extenson m1 1 m 1 IDFT Block x m extenson L p p s IDFT Block x p L t m 1 pm the (+1)th sgnal frame m1 1 IDFT Block x m 1 Tme-space two-dmensonal frame structure of the proposed TDS-OFDMA system Tme places) dmenson nto consderaton whch wll be dscussed n detal n secton III At the recever of the TDS-OFDMA system on one hand as presented later n secton IV jontly ty reconstructed sgnal y total of the receved OFDM data for all the users s obtaned va one-step smple add-subtract operaton and then the multple access sgnal can be orthogonally separated n frequency-doman to acqure the frequency-doman sgnal Y m for each user whose entres take the form Y m(n)= H m (n) X m (n)+w m (n) 0 n 1 (6) where W m (n) s the complex-valued addtve whte Gaussan nose (AWG) n the frequency doman H m = H m (n)} 1 s the -pont DFT of the CIR h m and s gven by H m (n)= lm 1 l=0 h m (l) exp } j 2πnl 0 n 1 (7) On the other hand as derved later n secton V jont channel estmaton wll generate the equvalent total CIR ĥtotal and orthogonal separaton of the CIRs for multple users can be acheved n the tme doman to get the CE results of Ĥm for each user After the sgnal separaton and CE for each user are acheved channel equalzaton can be easly performed through a bank of one-tap dvders to restore the transmtted sgnal for each user [3]: D m = Y m Ĥ m 1 m M (8) where D m s the estmaton of the frequency-doman transmtted data D m for user m III TIME-SPACE TWO-DIMESIOAL FRAME STRUCTURE In ths secton the new tme-space two-dmensonal frame structure s proposed for the TDS-OFDMA system As shown n Fg 2 each sgnal frame s composed of an IDFT block and a The of the th sgnal frame for the m th user c m = c m (n)} g 1 s the L t -symbol ally extended verson of the m-sequence p m = p m (n)} p 1 : pm ( c m (n) = p L t + n) 0 n L t 1 p m (n L t ) L t n g 1 (9) where g = p + L t In the tme dmenson of the tme-space two-dmensonal frame structure the m-sequences between adjacent frames are nether dentcal nor holdng a specal phase offset sequence of as specfed n [9] but adopt a constant phase offset L t Inthe space dmenson the m-sequences between neghborng users smlarly hold a constant offset L s : p m+1 = p ΞL t m (10) p m+1 = p ΞL s m where p m s the m-sequence of user m n the th frame p ΞL denotes the L-symbol crcular shft of the vector p That s p ΞL =[p(l)p(l+1) p( p 1)p(0) p(l 1)] (11) The reason why we desgn such frame structure s to facltate the jont ty reconstructon and CE at the recever both of whch wll be presented n detal n the followng parts Frstly let us analyze the propertes of the tme-space twodmensonal frame structure n the tme dmenson From (9) and (10) the relatonshp between the of the (+1)th sgnal frame c m+1 and the m-sequence of the th sgnal frame p m should be pm (n) 0 n c m+1 (n) = p 1 p m (n p ) p n g 1 (12) Equatons (9) and (12) ndcate the last p samplesofthe GI of the th sgnal frame (c m ) are dentcal to the frst p symbols of the GI of the ( +1)th sgnal frame (c m+1 ) That s to say the IDFT block of the th sgnal frame x m s enclosed by two dentcal m-sequences p m (we refer to those three blocks as vrtual frame ) Ths feature s guaranteed by the fact that the phase offset L t n the tme dmenson between p m and p m+1 s the same as the extenson length of the n the tme-space two-dmensonal frame structure In the followng secton IV we wll show how ths useful desgned feature s used to facltate the jont ty reconstructon of the OFDM data at the recever IV JOIT CYCLICITY RECOSTRUCTIO Under the assumpton of deal synchronzaton at the recever the mplctly combned sgnal of the total M users r total (n) n the TDS-OFDMA system s the lnear superposton of the receved sgnal r m (n) correspondng to the vrtual frame for each user as shown n Fg 3(a) and can be expressed by r total (n) = M r m (n) 0 n +2 p 1 (13) /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs

4 The followng add-subtract operaton of receved sgnal r total would produce a new sequence y total (n) y total(n) = rtotal (n+ p )+r total (n++ p ) r total (n) 0 n p 1 r total (n+ p ) p n 1 (14) Substtute (13) nto (14) we have y total(n) = M r m (n+ p )+r m (n++ p ) r m (n) M r m (n+ p ) 0 n p 1 p n 1 (15) Snce the s the ally extended verson of the m-sequence the tals part of user m (shadow area wth vertcal lnes n Fg 3(b)) n r m (n)} l m 1 and r m (n)} p+l m 1 should be the same assumng the n= p CIR varaton durng one sgnal frame s neglgble and l m L t p Therefore (15) leads to y total(n) = M y m (n) 0 n 1 (16) where y m(n) = ym (n+)+y m (n)+v m (n) 0 n l m 1 y m (n)+v m (n) l m n 1 (17) As shown n Fg 3(b) y m (n) shares the exactly same form as that of the IDFT block n CP-OFDM systems Therefore t s the ty reconstructed sgnal of the receved OFDM data for user m Intutvely the new sequence y total (n) Fg 3(c) has the ty property just lke that n the CP-OFDMA system snce s the lnear combnaton of y m (n)} M where each y m (n) holds the ty property As a result jont ty reconstructon for all users } s completed The -pont DFT of 1 y total = y total (n) wth entres Y total(n) = 1 1 s Y total y total(k)exp k=0 j 2πnk } (18) Accordng to the same CAS at the transmtter the frequency-doman sgnal Y m = Y m (n)} 1 for channel equalzaton and data detecton n (8) s selected out of Y total for user m Y Y m(n) = total (n) n Γ m 1 m M (19) 0 n/ Γ m Because the sub-carrer set for each user s mutually orthogonal the lnearly combned sgnal for multple users n the tme doman s orthogonally separated n the frequency doman User 1 User M y 1 1 y 1 m y 1 M L t g p p 1 rm np 2 1 m ( ) p r n np 1 m ( ) p r n n0 r1 y1 y ( ) 1 1 n rm ym y ( ) m 1 n rm ym y ( ) M 1 n (a) Receved sgnal for M users y' m y' 1 l 1 1 y' m l 1 y' M lm m 1 y' total lmax 1 p g (b) Cyclcty reconstructon for user m (c) Jont ty reconstructon for M users Fg 3 Jont ty reconstructon of the mplctly combned data for all the users n the TDS-OFDMA system After sgnal separaton s acheved by jont ty reconstructon we must obtan the CE for data restoraton for each user whch s the object of the followng secton V JOIT CHAEL ESTIMATIO AD SEPARATIO To get the CE for each user let s secondly analyze the feature of the tme-space two-dmensonal frame structure n the space dmenson In Fg 2 the m-sequences for dfferent users locatng at ther own places satsfy (10) thus p m = p Ξ (m 1) Ls 1 1 m M (20) Snce m-sequence has very good auto-correlaton property and satsfes p m p m p δ(n) 1 m M (21) where denotes crcular convoluton and δ(n) denotes the Drac functon /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs

5 h User Wndow 1 User Wndow 2 L s : h1 : h2 : hm User Wndow M User 1 n the proposed mult user system User 2 n the proposed mult user system User 1 n the conventonal sngle user system User 2 n the conventonal sngle user system SER L s Tme 10 3 Fg 4 Orthogonal separaton of the CIRs n the tme doman and User Wndow n the proposed TDS-OFDMA system 10 4 The cross-correlaton functon between p j and p k s p j p k p δ[n (j k) L s ] 1 j k M (22) The receved m-sequence for each user ntrnscally enjoys the ty property because of the ally extended structure n Fg 2 Therefore the actual receved m-sequence at the recever n the th frame q = r total (n)} p 1 whch s the lnear superposton of all receved m-sequences for M users also holds the ty property and takes the form M q = p m h m + v (23) where the vector v s the AWG n the th frame Use a local m-sequence p 1 to do crcular convoluton wth q we can get the rearranged total CIR ĥtotal ( M ) ĥ total = q p 1 p m h m p 1 (24) M p h m δ[n (m 1) L s ] Intutvely equaton (24) shows that h m s shfted by (m 1) L s symbols n the tme doman If l max = max(l m } M ) L s and M L s p the shfted h m } M are non-overlappng n the tme doman whch means that the CIRs correspondng to M dfferent users are orthogonally separable by each User Wndow n Fg 4 The functon of the mth User Wndow s w m w m = Ls n=1 δ[n (m 1) L s ] 1 m M (25) Multply w m wth ĥtotal and then we get the CE ĥm for the mth user (ĥtotal ) ĥ m = 1 w m p ( Ls )( M ) δ[n (m 1) L s ] h m δ[n (m 1) L s ] n=1 h m 1 m M (26) SR (db) Fg 5 SER performance vs SR for each user n the proposed TDS- OFDMA system It s worthwhle to pont out that any local m-sequence can be used for jont CE e p m (1 m M) and the crcular convoluton operaton can also be replaced by crcular correlaton or FFT to mprove the computatonal effcency After ĥm s obtaned for each user sgnal restoraton can be completed va (8) for each user VI PERFORMACE EVALUATIO Smulatons are performed to verfy the feasblty and the performance of the proposed TDS-OFDMA system The followng parameters are assumed: 1) M =2and avalable = 3780 sub-carrers wth each sub-carrer modulated n QPSK constellaton are equally assgned to two users; 2) Subcarrer spacng s 2 khz; 3) A baseband symbol rate of 756 M symbol/s; 4) The length of the m-sequence s p = 255; 5) Phase shft of the m-sequence n the tme dmenson s L t = 153 and L s = 76 n the space dmenson; 6) Two typcal mult-path channel models Brazl A and Brazl D [16] are used for system smulaton; 7) The channel for user 1 s Brazl A and Brazl D for user 2 ndependently For the purpose of comparson we also provde the system symbol error rate (SER) performance of the conventonal sngle-user TDS-OFDM system wth the same channels where all the avalable sub-carrers are used by only one user Fg 5 presents the SER performance both for the conventonal sngle-user TDS-OFDM system and the proposed TDS- OFDMA system wthout codng and decodng The smulaton results ndcate that each user of the proposed TDS-OFDMA system can approach almost the same SER performance as the sngle-user system It also shows that the SER performance has deterorated a lot for channel Brazl D snce the mult-path dstorton for Brazl D s more severe than that for Brazl A Fg 6 shows the SER comparson of those two systems n Brazl D Raylegh fadng channel (the maxmum Doppler shft of the channel s 30 Hz) It s obvous that the proposed /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs

6 SER The proposed mult user system The conventonal sngle user system SR (db) Fg 6 SER performance comparson between the conventonal sngle-user TDS-OFDM system and the proposed TDS-OFDMA system system holds better SER performance n moble envronment when compared wth the conventonal sngle-user system For example the requred SR of the conventonal sngle-user TDS-OFDM system s 25 db f the target SER s 0001 whle the SR needed of the proposed system decreased to 22dB for the same SER target The ratonale behnd the better performance s that the jont ty reconstructon proposed n ths paper does not need any channel nformaton at all Consequently t avods the conventonal teratve nterference cancellaton between the GI and OFDM data whch s not only complex but also accumulates errors caused by mperfect CE durng teraton More errors of the CE caused by moble envronment would lead to more severe performance degradaton It should be ponted out that compared wth the conventonal TDS-OFDM system the requrements of l max L s and M L s p on the proposed jont CE method of the proposed TDS-OFDMA scheme n secton V lead to two mplementaton strateges of the proposed TDS-OFDMA scheme On one hand f the same transmsson effcency s requred the maxmum delay spread the proposed scheme can support wll decrease whch s nversely proportonal to the number of actve users M On the other hand f the capablty to combat the same delay spread s needed the of longer length should be used and thus leads to a certan loss of system transmsson effcency VII COCLUSIO A novel TDS-OFDMA scheme s proposed n ths paper together wth the new tme-space two-dmensonal frame structure The correspondng recever desgn ncludng jont ty reconstructon of the receved OFDM data and jont CE for all users are also derved Theoretcal analyss and computer smulaton show that the proposed system frstly overcomes the major dffculty that the mxed nterferences between dfferent frame bodes and GIs caused by multple users are hardly to be elmnated n TDS-OFDMA systems and t also provdes the multple access capablty wth even better system performance n moble envronment compared wth the conventonal sngle-user TDS-OFDM system at the expense of longer P length Therefore the proposed TDS- OFDMA scheme could be another baselne technology for the bdrectonal mult-user applcatons such as nteractve dgtal TV and cellular wreless systems ACKOWLEDGMETS We would also lke to thank Arthur Lous Alanz for readng ths paper Ths work was supported n part by Program for Changjang Scholars and Innovatve Research Team n Unversty (PCSIRT) and n part by Boeng Company under the Grant o REFERECES [1] Dgtal Vdeo Broadcastng (DVB): Frame Structure Channel Codng and Modulaton for Dgtal Terrestral Televson (DVB-T) ETSI ETS Mar 1997 [2] A Doufex S Armour and M Butler A comparson of the HIPER- LA/2 and IEEE 80211a wreless LA standards IEEE Commun Mag vol 40 no 5 pp May 2002 [3] H Sar and G Karam Orthogonal frequency dvson multple access and ts applcaton to CATV networks Eur Trans Commun vol 45 pp ov 1998 [4] Dgtal Vdeo Broadcastng (DVB); Interacton Channel for Dgtal Terrestral Televson (RCT) Incorporatng Multple Access OFDM ETSI ETS Mar 2002 [5] IEEE Standard for Local and Metropoltan Area etworks Part 16: Ar Interface for Fxed Broadband Wreless Access Systems IEEE Oct 2004 [6] G Parsaee and A Yaral OFDMA for the 4G cellular networks n Proc Canadan Conference on Electrcal and Computer Engneerng May 2004 pp [7] L Rugn P Banell and G Leus Smple equalzaton of tme-varyng channels for OFDM IEEE Commun Lett vol 9 no 7 pp July 2005 [8] D Hu L He and L Yang Jont plot tone and channel estmator desgn for OFDM systems IEEE Commun Lett vol 10 no 10 pp Oct 2006 [9] Framng Structure Channel Codng and Modulaton for Dgtal Televson Terrestral Broadcastng System (n Chnese) Chnese atonal Standard GB Aug 2006 [10] J Song Z Yang and L Yang Technque revew on Chnese dgtal terrestral televson broadcastng standard and measurements on some workng modes IEEE Trans Broadcast vol 53 no 1 pp 1 7 May 2007 [11] Z Yang L Tong and L Yang Outage probablty comparson of CP- OFDM and TDS-OFDM for broadcast channels n Proc IEEE Global Communcatons Conference (GLOBECOM 02) ov 2002 pp [12] J Wang Z Yang C Pan and J Song Iteratve paddng subtracton of the for the TDS-OFDM over broadcast channels IEEE Trans Consumer Electron vol 51 no 11 pp ov 2005 [13] S Tang K Peng and K Gong ovel decson-aded channel estmaton for TDS-OFDM systems n Proc IEEE Internatonal Conference on Communcatons (ICC 08) May 2008 pp [14] F Yang J Wang and Z Yang ovel channel estmaton method based on reconstructon for Chnese DTTB system IEEE Trans Consumer Electron vol 54 no 4 pp ov 2008 [15] H Rohlng and R Gruned Performance comparson of dfferent multple access schemes for the downlnk of an OFDM communcaton system n Proc IEEE 47nd Vehcular Technology Conference (VTC 97) May 1997 pp [16] SET/ABERT Dgtal Televson Systems-Brazlan Tests-Fnal Report AATEL SP May /09/$ Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE "GLOBECOM" 2009 proceedngs