Opportunistic Scheduling and Beamforming for MIMO-OFDMA Downlink Systems with Reduced Feedback

📝 Abstract

Opportunistic scheduling and beamforming schemes with reduced feedback are proposed for MIMO-OFDMA downlink systems. Unlike the conventional beamforming schemes in which beamforming is implemented solely by the base station (BS) in a per-subcarrier fashion, the proposed schemes take advantages of a novel channel decomposition technique to perform beamforming jointly by the BS and the mobile terminal (MT). The resulting beamforming schemes allow the BS to employ only {\em one} beamforming matrix (BFM) to form beams for {\em all} subcarriers while each MT completes the beamforming task for each subcarrier locally. Consequently, for a MIMO-OFDMA system with $Q$ subcarriers, the proposed opportunistic scheduling and beamforming schemes require only one BFM index and $Q$ supportable throughputs to be returned from each MT to the BS, in contrast to $Q$ BFM indices and $Q$ supportable throughputs required by the conventional schemes. The advantage of the proposed schemes becomes more evident when a further feedback reduction is achieved by grouping adjacent subcarriers into exclusive clusters and returning only cluster information from each MT. Theoretical analysis and computer simulation confirm the effectiveness of the proposed reduced-feedback schemes.

💡 Analysis

Opportunistic scheduling and beamforming schemes with reduced feedback are proposed for MIMO-OFDMA downlink systems. Unlike the conventional beamforming schemes in which beamforming is implemented solely by the base station (BS) in a per-subcarrier fashion, the proposed schemes take advantages of a novel channel decomposition technique to perform beamforming jointly by the BS and the mobile terminal (MT). The resulting beamforming schemes allow the BS to employ only {\em one} beamforming matrix (BFM) to form beams for {\em all} subcarriers while each MT completes the beamforming task for each subcarrier locally. Consequently, for a MIMO-OFDMA system with $Q$ subcarriers, the proposed opportunistic scheduling and beamforming schemes require only one BFM index and $Q$ supportable throughputs to be returned from each MT to the BS, in contrast to $Q$ BFM indices and $Q$ supportable throughputs required by the conventional schemes. The advantage of the proposed schemes becomes more evident when a further feedback reduction is achieved by grouping adjacent subcarriers into exclusive clusters and returning only cluster information from each MT. Theoretical analysis and computer simulation confirm the effectiveness of the proposed reduced-feedback schemes.

📄 Content

arXiv:0802.4291v1 [cs.IT] 28 Feb 2008 Opportunistic Scheduling and Beamforming for MIMO-OFDMA Downlink Systems with Reduced Feedback Man-On Pun, Kyeong Jin Kim and H. Vincent Poor Abstract— Opportunistic scheduling and beamforming schemes with reduced feedback are proposed for MIMO- OFDMA downlink systems. Unlike the conventional beamforming schemes in which beamforming is implemented solely by the base station (BS) in a per-subcarrier fashion, the proposed schemes take advantages of a novel channel decomposition technique to perform beamforming jointly by the BS and the mobile terminal (MT). The resulting beamforming schemes allow the BS to employ only one beamforming matrix (BFM) to form beams for all subcarriers while each MT completes the beamforming task for each subcarrier locally. Consequently, for a MIMO-OFDMA system with Q subcarriers, the proposed opportunistic scheduling and beamforming schemes require only one BFM index and Q supportable throughputs to be returned from each MT to the BS, in contrast to Q BFM indices and Q supportable throughputs required by the conventional schemes. The advantage of the proposed schemes becomes more evident when a further feedback reduction is achieved by grouping adjacent subcarriers into exclusive clusters and returning only cluster information from each MT. Theoretical analysis and computer simulation confirm the effectiveness of the proposed reduced-feedback schemes. I. INTRODUCTION Orthogonal frequency-division multiple-access (OFDMA) has recently attracted much attention as a promising tech- nique for future broadband wireless communications. In an OFDMA downlink system, the base station (BS) transmits data to several active mobile terminals (MTs) simultaneously by modulating each MT’s data onto an exclusive set of or- thogonal subcarriers. In addition to its robustness to multipath fading and high spectral efficiency, OFDMA is particularly attractive due to its flexibility in allocating subcarriers to different MTs based on their different quality of service (QoS) requirements and channel conditions in a dynamic fashion [6]. Furthermore, the recent advancement in multiple-input multiple-output (MIMO) techniques has inspired considerable research interest in MIMO-OFDMA. However, perfect channel state information (CSI) is usually required at the BS in order to fully harvest the advantages provided by MIMO in the down- link transmission, which incurs formidable feedback overhead from MTs. This problem becomes particularly challenging in MIMO-OFDMA since the required feedback amount is Man-On Pun and H. Vincent Poor are with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544. Kyeong Jin Kim is with Nokia Inc, 6000 Connection Dr., Irving TX 75039. This research was supported in part by the Croucher Foundation under a post-doctoral fellowship, and in part by the U. S. National Science Foundation under Grants ANI-03-38807 and CNS-06-25637. proportional to the number of subcarriers. To circumvent this obstacle, an opportunistic scheduling and beamforming scheme has been proposed for single-carrier (SC) multiple- input single-output (MISO) systems in [5] as an effective means of achieving the asymptotic sum-rate capacity by exploiting multiuser diversity with limited channel feedback. Some extensions of [5] have been developed for SC-MIMO systems [1], [3]. In particular, [1] has proposed a singular value decomposition (SVD)-based scheme to schedule data transmission to the MT whose channel matrix has the right singular vectors aligning with beams in the common codebook shared by the BS and MTs. Most recently, an opportunis- tic scheme has been proposed for single-input single-output (SISO)-OFDMA by grouping the adjacent subcarriers into exclusive clusters [4]. Assuming the subcarriers in each cluster have approximately the same channel conditions, [4] has demonstrated good throughput performance by only feeding the average cluster SNRs from each MT back to the BS. In this work, we first propose a novel beamforming tech- nique for MIMO-OFDMA systems. By properly decomposing the time-domain channel response matrix into a product of subcarrier-dependent components and subcarrier-independent components, we can divide the beamforming task into the subcarrier-independent part implemented by the BS and the subcarrier-dependent part accomplished by each MT locally. Next, this novel beamforming technique is proposed to be incorporated into the design of opportunistic and beamforming schemes, which results in opportunistic and beamforming schemes that only require feedback of one beamforming matrix (BFM) index and Q supportable throughputs for a MIMO-OFDMA system with Q subcarriers. For comparison purposes, opportunistic and beamforming schemes employing the conventional SVD technique are also proposed by extend- ing [1] to MIMO-OFDMA systems. While the SVD-based schemes can achieve the optimal throughput performance, they incur feedback overh

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