imp - efficient receiver algorithms for dft spread ofdm systems

3216 IEEE TRANSACTIONS ON WIRELESS COMMUNICA TIONS, VOL. 8, NO. 6, JUNE 2009 Ef ficient Receiver Algorithms for DFT-Spread OFDM Systems Narayan Prasad, Shuangquan Wang, and Xiaodong Wang Abstract —For the 3GPP LTE uplink transmissions, the DFT- spread OFDM technique has been adopted as the air interface in order to reduce the peak-to-average-power ratio (PAPR). In this scheme, each data symbol is spread over many tones by a Discrete Fou rier T ransf orm (DFT) operatio n at the trans mitte r bef ore being sent to the Ortho gona l Freq uency Divisio n Multip lexing (OFDM) mod ula tor. Mor eove r, mor e tha n one user can be scheduled over the same frequency and time resource block (RB) via spac e-div ision multip le-access (SDMA). The con vent ional receiver technique for such DFT-spread OFDM systems involves tone-by-tone single-tap equalization followed by an inverse DFT opera tion. In this paper , we pro pose a mor e powe rful receiver technique for DFT-spread OFDM systems that consists of an ef ficient linear pre-filter and a two-symbol soft output demodulator. The proposed method can be applied to both single-user per RB (DFT -S-OFDMA) and multip le users per RB (DFT -S-OFDM- SDMA) systems and it offers significant performance gains over the conventional method, especially in the high-rate regime, with little attendant increase in computational complexity. Index T erms —DFT-spread OFDM, OFDMA, SDMA, log like- lihood ratio, interference suppression, linear filter. I. I NTRODUCTION D ISCRETE Four ier Transform-Spre ad-Ort hogon al Fre- quency Division Multiple Access (DFT-S-OFDMA) has emer ged as the pre fer red uplink air int erf ace for the nex t generat ion cellula r sys tems suc h as those envis age d by the 3GPP LTE [1]. DFT-S-OFDMA is essentially a modi fied form of OFDMA where scheduled users transmit their data simul- taneously on non-overlapping (orthogonal) sets of subcarriers (frequencies). The key difference from OFDMA is that each user spreads its coded and modulated information bits using a DFT matri x and the spread (prec ode d) symbols are then mapped to its allocated subcarriers. The main advantage of this multiple access technique is that it results in consider- ably lower PAPR at each transmitter (user) compared to the classical OFDMA technique [2]. Upcoming cellular systems will employ antenna arrays at the base station (a.k.a Node-B) and possibly at the user equipment (UE) as well. A promising scheme, also adopted in the 3GPP LTE, that is enabled due to the use of antenna arrays at the base station, is the SDMA scheme which is sometimes referred to as the virtual Multi- Input Multi-Output (MIMO) scheme [1]. In SDMA multiple Manuscript received July 16, 2008; revised December 10, 2008 and Febru- ary 27, 2009; accepted February 28, 2009. The associate editor coordinating the review of this paper and approving it for publication was D. Huang. X. Wang is with the Electrical Engineering Dept., Columbia Univ., New York, NY 10027 (e-mail: [email protected]). N. Prasad and S. Wang are wit h NEC Laborat ories Ameri ca, 4 Ind e- pend enc e Way , Prin ceto n, NJ 0854 0 (e-mail: {shuangquan , prasad}@nec- labs.com). Digital Object Identifier 10.1109/TWC.2009.080941 single-antenna users are scheduled over the same frequency and time resource blocks in order to boost the system through- put. Since different users are geographically separated, their channel responses seen at the base-station antenna array will be indepen den t and hence capa ble of sup por tin g high rat e communications. Henceforth, the DFT-S-OFDM based uplink employing SDMA will be referred to as the DFT-S-OFDM- SDMA uplink. In DFT-S-OFDM systems, which encompass both DFT-S- OFDMA and DFT-S-OFDM-SDMA, as a consequence of the DFT spreading operation at the transmitter, the signal arrives at the base-station with substantial intersymbol interference 1 and the received suf ficient statistic s can be modele d as the channel output of a large MIMO system. The conventional receiver techniq ue involves tone-b y-ton e singl e-tap equaliz ation followed by an inverse DFT operation [3]. Such a simple receiver suf fices for the DFT-S-OFDMA case in the low-rate regime when there is enough receive diversity and where the available frequency diversity can be garnered by the underlying outer code. Howe ver , it results in a degra ded performance at higher rates as well as with SDMA. Unfortunately, the large dimension of the equivalent MIMO model in DFT-S-OFDM systems precludes us from leveraging the sphere decoder [4], [5] which has an exponential complexity in the problem dimension [6]. Furth ermore , the string ent complexity constrain ts in practi- cal systems also rule out the near-optimal MIMO receivers developed for the narrowband channels, see for instance [7]– [9]. Other promising equalizers for the DFT-S-OFDM systems are the decision feedback equalizers (DFEs). The two most promising DFEs are the hybrid DFE proposed in [3], where the feedforward filter is realized in the frequency domain and the feedba ck filter is realized in the time domai n, and the iterative block DFE with soft decision feedback proposed in [10], where even the cancelation is performed in the frequency domain. However, even the DFE whose iterative process does not incl ude deco din g the out er code, is sub stan tial ly mor e complex and has higher latency especially in the SDMA case, tha n the con ven tion al rece iver and hen ce is not consid ered in this paper . We rema rk howe ver , tha t the iterative sof t cancelation can be readily added to the receivers considered her e in order to obt ain fur ther per for mance improv emen ts albeit with higher complexity and latency. In this paper we consider receiver design for such DFT- S-OFDM systems. To realize our goal of obtaining ef ficient receivers, we first propose a very ef ficient implementation of a soft-output demodulator for the narrowband MIMO model wit h two input symbols. W e then design a novel receiv er , 1 Note that inter-block interference is avoided due to the use of cyclic pre fix. 1536-1276/09$25.00 c 2009 IEEE Authorized licensed use limited to: VELLORE INSTITUTE OF TECHN OLOGY. Downloaded on July 28, 2009 at 06:29 from IEEE Xplore. Restrictions apply.

imp - efficient receiver algorithms for dft spread ofdm systems

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