A Novel TDS-FDMA Scheme for Multi-User Uplink Scenarios

Transcription

1 A Novel TDS-FDMA Schee for Multi-User Uplink Scenarios Linglong Dai, Zhaocheng Wang, Jun Wang, and Zhixing Yang Tsinghua National Laboratory for Inforation Science and Technology, Electronics Engineering Departent, Tsinghua University, Beijing, , P.R. China Abstract Tie doain synchronous OFDM (TDS-OFDM) with higher spectral efficiency than cyclic prefix OFDM (CP- OFDM) was originally proposed for downlink broadcasting transission. To support ulti-user uplink scenarios, this paper proposes a novel ultiple access schee called tie doain synchronous frequency division ultiple access (TDS-FDMA), wherein an unifor frae structure and its corresponding receiver algoriths are presented for both single-carrier and ulticarrier signal transission. Copared with typical OFDMA systes, TDS-FDMA has higher spectral efficiency. The ulti-user TDS-FDMA receiver has lower coplexity than the conventional single-user TDS-OFDM receiver, and it can achieve better bit error rate (BER) perforance under the slow to ediu tievarying channels. I. INTRODUCTION Orthogonal frequency division ultiple access (OFDMA) has been widely adopted as the ultiple access solution in any wireless counication standards [1]. OFDMA was firstly proposed for cable TV (CATV) networks [2] and then used in the uplink of the interaction channel for digital terrestrial television (DVB-RCT) [3]. IEEE e adopted OFDMA both in the uplink and downlink [4]. Recently, OFDMA attracts vast research attentions fro both acadeia and industry [5]. As the essential technology of the Chinese national digital television terrestrial broadcasting (DTTB) standard [6], tie doain synchronous OFDM (TDS-OFDM) outperfors cyclic prefix OFDM (CP-OFDM) in spectral efficiency at the cost of higher coplexity [7]. Instead of padding CP before the inverse discrete Fourier transfor (IDFT) block, TDS-OFDM adopts the pseudo-rando noise (PN) sequence as the guard interval (GI), which is also used for synchronization and channel estiation (CE) [7] [8]. Therefore, the cyclicity of the received IDFT block is destroyed due to the inter-sybolinterference (ISI) between the PN sequence and the IDFT block. The iterative interference cancellation algorith is proposed to reove the ISI [9]. Soe iproved ethods have been proposed either to increase the accuracy [10] or to reduce the coplexity [11]. However, those iterative ethods [9] [11] have high coplexity and unsatisfactory perforance due to the fact that CE and interference cancellation are utually conditional, i.e., perfect CE is needed for ideal interference cancellation, while perfect interference cancellation is required for ideal CE. Until now, TDS-OFDM has been used only in the downlink broadcasting transission. When TDS-OFDM is applied to the uplink ultiple access scenarios, if ultiple users are supported, the PN sequences fro ultiple users would cause the superposed interferences to the IDFT blocks, and the IDFT blocks fro ultiple users would also introduce the ixed interferences to the PN sequences. Such superposed interferences have to be reoved before restoring the orthogonality aong ultiple users. Unfortunately, the previously entioned iterative ethods [9] [11] can not be directly applied, since the superposed interferences caused by ultiple users are difficult to be eliinated. To solve this proble, based on the principle of TDS- OFDM and frequency division ultiple access (FDMA), we propose a novel uplink ultiple access schee called tie doain synchronous FDMA (TDS-FDMA) in this paper. The ain contribution of the proposed TDS-FDMA syste is the new frae structure. The key idea of the new frae structure lies in two aspects: 1) The specially designed frae structure akes the CE and interference cancellation in the proposed TDS-FDMA syste no longer utually conditional, so the conventional iterative ethods can be avoided, leading to the low coplexity of the joint cyclictiy reconstruction of the received IDFT blocks; 2) The preable in the frae structure enables tiing synchronization and joint channel estiation for all users to be realized by an one-step circular convolution. The reainder of this paper is organized as follows. Section II illustrates the overall architecture of the proposed TDS- FDMA syste based on the novel frae structure. The corresponding TDS-FDMA receiver algoriths are presented in Section III, together with the analysis of spectral efficiency and coputational coplexity. Section IV shows the siulation results to verify the feasibility and perforance of the proposed syste. We then conclude this paper in Section V. II. ARCHITECTURE OF THE TDS-FDMA SYSTEM In this section, we firstly present the novel frae structure of the uplink ulti-user TDS-FDMA syste, based on which the overall architecture is then outlined. A. Frae Structure Fig. 1 shows the frae structure for the th user of the proposed TDS-FDMA syste. Every user adopts the sae

2 Prefix Preable sequence One Superfrae, including one Preable and L Subfraes Subfrae 1 Subfrae i Subfrae L Postfix IDFT Block Postfix IDFT Block Postfix IDFT Block Postfix p,0 c p,0 X p,1,0 X p i,,0 X p L,,0,0 Copy Copy Copy Copy Copy Fig. 1. Frae Structure of the TDS-FDMA Syste. frae structure in the proposed TDS-FDMA syste. The basic block transission unit is the superfrae, which is coposed of one preable and L subfraes. The N g -point preable in the superfrae consists of three parts: the user-specific N p -point -sequence c,0, the K- point prefix and the K-point postfix p,0. Unlike the frae header of the TDS-OFDM syste where the cyclic prefix and cyclic postfix with different lengths and distinct contents are used, the prefix and the postfix in the TDS-FDMA syste are exactly the sae, both of which are the last K sybols of the -sequence c,0. Therefore, N g = N p +2K. For the ultiple users locating at different places, the user-specific -sequences between neighboring users hold a constant circular shift L s, which can be presented as c +1,0 = c ΞL s,0, (1) where c,0 and c +1,0 are the user-specific -sequences for the th user and the (+1)th user, respectively. c ΞL s eans the L s -sybol circular shift of the vector c. The N f -point subfrae s,i consists of the N-point IDFT block x,i and the K-point postfix p,i, and N f = N + K. The postfix p,i is not relative to the IDFT block x,i in every subfrae, but identical with the prefix and postfix in the preable. That is p,i = p,0 1 i L. (2) The nuber of subfraes L could be adaptively adjusted according to the coherent tie of the wireless channel. B. Syste Architecture At the transitter part, M users siultaneously transit their signals to a central base station. The generation of the IDFT block for the th user in the ith subfrae is as follows: the input frequency-doain data D,i = {D,i (k)} L 1 (N = M L ) are apped onto the sub-carrier set Γ assigned to the th user by the carrier assignent schee (CAS) unit, and an N-diensional vector X,i = {X,i (k)} N 1 is generated with entries { D,i (k) k Γ X,i (k) = 1 M. (3) 0 k/ Γ The sub-carrier sets {Γ } M ust be utually exclusive to guarantee the orthogonality, i.e., Γ i Γ j = if i j. The tie-doain IDFT block x,i = {x,i (n)} N 1 is then obtained by applying IDFT operation to the frequency-doain data X,i. After padding the postfix p,0 after x,i to construct L subfraes, the user-specific preable is inserted at the beginning of each superfrae. The superfrae is transitted over a user-specific ulti-path channel. The channel ipulse response (CIR) is odeled as an l order FIR filter h,i = {h,i (l)} l 1 l=0, and l is the axiu delay spread of the user-specific CIR h,i. At the receiver part, firstly, as presented in Section III.A, jointly cyclicity reconstructed signal y total,i of the received IDFT block for all M users is achieved via an one-step addsubtract operation. The orthogonal separation of the ultiple access signals can be then achieved in the frequency doain to obtain the user-specific frequency-doain signal Y,i { = Y,i (k) } N 1 for the th user, whose entries take the for Y,i (k)=h,i(k) X,i (k)+w,i (k) 0 k N 1. (4) where H,i ={H,i (k)} N 1 is the N-point DFT of the userspecific CIR h,i, W,i (k) is the coplex-valued additive white Gaussian noise (AWGN) in the frequency doain. Secondly, as derived in Section III.B, joint channel estiation can be realized by the circular convolution between one local -sequence and the received -sequence, whereby the equivalent total CIR ĥtotal,i is generated, which is the orderly concatenation of all the user-specific CIRs {h,i } M. Therefore, orthogonal separation of the CIRs for ultiple users can be achieved in the tie doain to get the user-specific CE result ĥ,i for the th user. After the user-specific signal Y,i and CE result Ĥ,i in the frequency doain for each user are obtained, the classical one-tap frequency doain equalization (FDE) can be carried out either by the zero-forcing (ZF) approach or alternatively the iniu ean square error (MMSE) equalizer with relatively good perforance and high coplexity [12] D,i = Ĥ,i Y,i Ĥ,i 2 +1/γ 1 M, 1 i L, (5) where γ is the signal-to-noise ratio (SNR), ( ) eans coplex conjugation, and D,i is the estiate of the frequency-doain transitted data D,i. It should be pointed out that the IDFT block in the proposed frae structure could be either the OFDMA type of ulticarrier (MC) signal or the single carrier FDMA (SC-FDMA) type of single-carrier (SC) signal [13], and the corresponding receiver algoriths based on the frae structure is applicable

3 to both cases. Therefore, an unifor frae structure and the corresponding syste architecture are provided in this paper for both the MC and SC uplink transission. III. TDS-FDMA RECEIVER DESIGN In this section, the TDS-FDMA receiver design issues based on the new frae structure are addressed, together with the analysis of the spectral efficiency and the coputational coplexity. A. Joint Cyclicity Reconstruction At the base station, the received tie-doain signal is the linear superposition of all M user-specific signals passing through different wireless channels. The following one-step add-subtraction operation between the received IDFT block, the postfix in the subfrae and the postfix in the preable would produce the jointly cyclicity reconstructed N-point signal y total,i of the IDFT block y total,i(n) = r,i (n)+r,i (n+n) r,0 (n+n p +K) r,i (n) 0 n K 1 K n N 1 where {r,i (n)} N 1 is the received IDFT block in the ith subfrae, {r,i (n+n)} K 1 is the received postfix in the ith subfrae, and {r,0 (n+n p +K)} K 1 is the received postfix in the preable, as shown in Fig. 2(a). y,i = x,i h,i = {y,i (n)} N+K 1 in Fig. 2(a) denotes the response of the IDFT block x,i, where eans the linear convolution. Assuing the wireless channel during one superfrae is quasi-static, i.e., {h,i } L i=1 =h,0, the postfix in the preable and the postfix in the ith subfrae would introduce the sae tail due to ulti-path dispersion, as shown by the shadows with the sae for in Fig. 2 (a). Therefore, we have y total,i (n) = M (6) y,i (n) 0 n N 1, (7) where { y,i(n)= y,i (n+n)+y,i (n) 0 n K 1 y,i (n) K n N 1. (8) The procedure to generate y,i = { y,i (n)} N 1 in (8) and the consequent process to obtain y total,i in (6) and (7) can be illustrated by Fig. 2(b) and Fig. 2(c), respectively. Siilar to the IDFT block after reoving the CP in typical CP based OFDM/OFDMA systes, y,i is the cyclicity reconstructed signal of the ith received IDFT block for the th user, and joint cyclictiy reconstruction for all M users is achieved via the one-step add-subtraction operation in (6) to obtain y total,i }. N 1 Denoting Y total,i {Y = total,i (k) as the N-point DFT of y total,i, then the user-specific frequency-doain signal Y,i ={ Y,i (k)} N 1 for channel equalization in (5) for the User 1 User, 1, User M. r 1,0 r 1,1 r 1,i g 1,0 q y 1, i 1 1,i K N p K K N K r,0 r i, g,0 q i y i r M,0 r Mi, g M,0 q M y, i 1 Mi, (a) Received signal for M users ( ) N r 1 i, n ( ) K r 1 i, n N r 1,0( ) K n Np K y ' i, (b) Cyclicity reconstruction for user y ' 1, i y ' i, y ' Mi, y ' total, i (c) Joint cyclicity reconstruction for M users Fig. 2. Joint cyclictiy reconstruction of the received IDFT block for all M users in the TDS-FDMA syste. th user could be selected out of Y total,i, according to the sae CAS at the transitter defined by (3) { Y Y,i(k) = total,i (k) k Γ 1 M. (9) 0 k/ Γ Because the sub-carrier sets for all users are utually orthogonal and the cyclicity property of the IDFT block has been reconstructed, the linearly superposed signal in the tie doain is orthogonally separated in the frequency doain. B. Joint Channel Estiation This part addresses the second design eleent of the TDS- FDMA receiver: channel estiation for the uplink ultiple users for coherent detection. The actual received -sequence g ={r total,0 (n)} K+Np 1 n=k at the TDS-FDMA receiver intrinsically inherits the cyclicity property due to the cyclic prefix in the preable and takes the for g = c,0 h,i, (10) where denotes the circular convolution. Using a local -sequence c 1,0 to do circular convolution with the received -sequence g, we can get the equivalent total CIR ĥtotal,i, which is the sequential concatenation of r 1,L

4 all the user-specific CIRs {h,i } M ( M ) ĥ total,i = g c 1,0 = c,0 h,i c 1,0 = N p h,i δ[n ( 1) L s ]. (11) In (11), we have utilized the good autocorrelation property of the -sequence and the circular phase shift feature of the - sequences in the preable denoted by (1), whereby the crosscorrelation between c j,0 and c k,0 (c j,0 = c Ξ (j k)ls k,0 ) could be written as c j,0 c k,0 = N p δ[n (j k) L s ] 1 j, k M. (12) Intuitively, equation (11) shows that the user-specific CIR h,i is shifted by ( 1)L s sybols. If l ax = ax {l } M L s and ML s N p, the shifted CIRs {h,i } M are orthogonally separable in the tie doain. In addition, the uplink tiing synchronization can be also realized by circular convolution in (11), whereby the tiing errors for all users are less than half of the sapling period under ulti-path channels [14]. C. Spectral Efficiency Table I copares the spectral efficiency of the proposed TDS-FDMA syste and the DVB-RCT syste in the 2K ode (N = 2048) with three types of burst structure (BS) [3]. Because DVB-RCT can update the channel state inforation (CSI) every 6 OFDMA signal fraes with the pilot insertion schee specified in [3], we select L =5to guarantee the equivalent CSI updating speed for TDS-FDMA. GI Length TABLE I SPECTRAL EFFICIENCY OF AND TDS-FDMA. BS1 BS2 BS3 TDS-FDMA K=N/ % 77.00% 77.89% 84.22% K=N/ % 72.73% 73.56% 78.44% It is clear that the increase of the spectral efficiency by about 5% 9% can be achieved for the TDS-FDMA syste. The reason for the increased spectral efficiency is that, the frequency-doain pilots and the tie-doain CP result in the decrease of the spectral efficiency for typical OFDMA systes like DVB-RCT, while the preable and the postfix only in the tie doain lead to the spectral efficiency loss for the proposed TDS-FDMA syste. D. Coputational Coplexity Regarding to cyclicity reconstruction and channel estiation, Table II copares the coputational coplexity of the conventional single-user TDS-OFDM receiver and the proposed ulti-user TDS-FDMA receiver. To be consistent with the TDS-OFDM syste [6], the paraeters are configured as N = 3780, N p = 255. J is the iteration nuber in TABLE II COMPLEXITY COMPARISON BETWEEN TDS-OFDM AND TDS-FDMA. Operation Wang [9] Tang [10] Yang [11] Proposed 512-point FFT/IFFT point FFT/IFFT 2J 2(J +1) 3(J +1) point FFT/IFFT 2J 5(J +1) point FFT/IFFT 2J Table II. Note that the 255-point circular convolution for the joint channel estiation in (11) is ipleented by 512-point FFT/IFFT for higher coputing efficiency. We can see fro this table that the coputational coplexity of the proposed ulti-user TDS-FDMA receiver is only 6.1% of the traditional single-user TDS-OFDM receiver adopting Wang s ethod [9] with J = 3, and 6.0% of the Tang s ethod [10], 35.1% of the Yang s ethod [11], respectively. IV. SIMULATION RESULTS AND DISCUSSIONS Siulations are carried out to verify the feasibility and the perforance of the proposed TDS-FDMA syste without channel coding and interleaving. The ajor syste paraeters are configured as below: 1) Multi-carrier signal with the bandwidth of 8 MHz in the ultra high frequency (UHF) band at 770 MHz; 2) M =4, N = 3780, N p = 255, K =64, L =5; 3) The odulation schees were chosen to be QPSK and 16QAM; 4) The ulti-path channel odel, Vehicular-A as defined by ITU [15], was used. 4) The axiu Doppler spread f d of 5 Hz, 20 Hz, and 50 Hz with the corresponding velocity of 7 k/h, 28 k/h, and 70 k/h in the UHF band, are used respectively. Fig. 3 and Fig. 4 copare the bit error rate (BER) perforance of the ulti-user TDS-FDMA syste with that of the single-user TDS-OFDM syste over the Vehicular-A Rayleigh fading channel, for the QPSK and 16QAM schees, respectively. In TDS-FDMA, the BER is averaged aong active users. The ulti-user TDS-FDMA syste achieved a superior BER perforance over the single-user TDS-OFDM syste. For exaple, in the QPSK case, an SNR iproveent of about 1.8 db was achieved by the ulti-user TDS-FDMA for the target BER of 10 3 when f d = 5 Hz, while the SNR gain was increased to be about 5 db when f d = 20 Hz. However, the SNR gain achieved by the TDS-FDMA over the TDS-OFDM becae negligible for the high Doppler spread of f d =50Hz. Therefore, we can conclude that higher BER perforance could be achieved for the proposed ultiuser TDS-FDMA syste under slow to ediu tie-varying channels. The reasons for the perforance iproveent both under static and fading channels lie in two aspects. Firstly, the received -sequence in the new frae structure for joint CE is iune to the interferences caused by the IDFT block, leading to ore accurate CE in obile environents. Secondly, the joint cyclicity reconstruction does not need any CSI at all, and

5 BER Proposed, Doppler = 5 Hz Proposed, Doppler = 20 Hz Proposed, Doppler = 50 Hz TDS OFDM, Doppler = 5 Hz TDS OFDM, Doppler = 20 Hz TDS OFDM, Doppler = 50 Hz SNR (db) Fig. 3. BER perforance coparison of the proposed ulti-user TDS- FDMA syste and the conventional single-user TDS-OFDM syste over the Vehicular-A Rayleigh fading channel with the QPSK odulation schee. BER Proposed, Doppler = 5 Hz Proposed, Doppler = 20 Hz Proposed, Doppler = 50 Hz TDS OFDM, Doppler = 5 Hz TDS OFDM, Doppler = 20 Hz TDS OFDM, Doppler = 50 Hz SNR (db) Fig. 4. BER perforance coparison of the proposed ulti-user TDS- FDMA syste and the conventional single-user TDS-OFDM syste over the Vehicular-A Rayleigh fading channel with the 16QAM odulation schee consequently, it avoids the conventional iterative interference cancellation between the PN sequence and the IDFT block. Thus it avoids the accuulated errors due to the iperfect CE and residual ISI during the iteration process in high-speed obile environents. However, when the channel is varying too fast, e.g.,f d is up to 50 Hz, the preable based CSI updating speed becoes relatively low which counteracts the perforance gain explained above, so no obvious perforance gain can be achieved. This proble can be solved by decreasing L at the cost of spectral efficiency, which is unavoidable for reliable transission in high-speed environents. V. CONCLUSION The ajor difficulty of eliinating the superposed interferences fro ultiple users when TDS-OFDM is extended to ulti-user uplink scenarios is resolved by the proposed TDS- FDMA schee in this paper. It can achieve higher spectral efficiency by about 5% 9% than typical OFDMA systes like DVB-RCT. Based on the novel frae structure, the conventional coplex iterative interference cancellation ethods are avoided, and the corresponding receiver algoriths can be realized with lower coplexity for both the MC and SC uplink transission. TDS-FDMA also achieves better BER perforance than the signal-user TDS-OFDM schee. Due to its low coplexity and good perforance copared with conventional TDS-OFDM syste, TDS-FDMA schee could be adopted as a potential uplink solution to the Chinese next generation broadcasting standard. REFERENCES [1] M. Sternad, T. Svensson, T. Ottosson, A. Ahlen, A. Svensson, and A. Brunstro, Towards systes beyond 3G based on adaptive OFDMA transission, Proc. IEEE, vol. 95, no. 12, pp , Dec [2] H. Sari and G. Kara, Orthogonal frequency division ultiple access and its application to CATV networks, Eur. Trans. Coun., vol. 45, pp , Nov [3] Digital Video Broadcasting (DVB); Interaction Channel for Digital Terrestrial Television (RCT) Incorporating Multiple Access OFDM, ETSI Std. ETSI ETS , Mar [4] IEEE Standard for Local and Metropolitan Area Networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systes, IEEE Std. IEEE , Oct [5] H. Chen, X. Zhang, and W. Xu, Next-generation CDMA vs. OFDMA for 4G wireless applications, IEEE Trans. Wireless Coun., vol. 14, no. 3, pp. 6 7, June [6] Fraing Structure, Channel Coding and Modulation for Digital Television Terrestrial Broadcasting Syste (in Chinese). Chinese National Standard, GB , Aug [7] J. Song, Z. Yang, and L. Yang, Technique review on Chinese digital terrestrial television broadcasting standard and easureents on soe working odes, IEEE Trans. Broadcast., vol. 53, no. 1, pp. 1 7, May [8] J. Wang, Z. Yang, C. Pan, M. Han, and L. Yang, A cobined code acquisition and sybol tiing recovery ethod for TDS-OFDM, IEEE Trans. Broadcast., vol. 49, no. 3, pp , Sept [9] J. Wang, Z. Yang, C. Pan, and J. Song, Iterative padding subtraction of the PN sequence for the TDS-OFDM over broadcast channels, IEEE Trans. Consuer Electron., vol. 51, no. 11, pp , Nov [10] S. Tang, K. Peng, and K. Gong, Novel decision-aided channel estiation for TDS-OFDM systes, in Proc. IEEE International Conference on Counications (ICC 08), May 2008, pp [11] F. Yang, J. Wang, and Z. Yang, Novel channel estiation ethod based on PN sequence reconstruction for Chinese DTTB syste, IEEE Trans. Consuer Electron., vol. 54, no. 4, pp , Nov [12] A. Gorokhov and J. P. Linnartz, Robust OFDM receivers for dispersive tie-varying channels: equalization and channel acquisition, IEEE Trans. Coun., vol. 52, no. 4, pp , Apr [13] H. G. Myung, J. Li, and D. J. Goodan, Single carrier FDMA for uplink wireless transission, IEEE Veh. Technol. Mag., vol. 1, no. 3, pp , Sept [14] L. Dai, Z. Wang, J. Wang, and Z. Yang, Joint channel estiation and tie-frequency synchronization for uplink TDS-OFDMA systes, IEEE Trans. Consuer Electron., vol. 56, no. 2, pp , May [15] Guideline for Evaluation of Radio Transission Technology for IMT Recoendation ITU-R M.1225, 1997.