Efficient Transmission Schemes for Multiuser MIMO Downlink with Linear Receivers and Partial Channel State Information
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Research Article Efficient Transmission Schemes for Multiuser MIMO Downlink with Linear Receivers and Partial Channel State Information Mohsen Eslami1, 2 and Witold A. Krzymien´ 1, 2 1 Electrical 2 TRLabs,
& Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 Edmonton, Alberta, Canada T5K 2M5
´ [email protected] Correspondence should be addressed to Witold A. Krzymien, Received 18 August 2009; Accepted 10 May 2010 Academic Editor: Alex B. Gershman ´ This is an open access article distributed under the Creative Commons Copyright © 2010 M. Eslami and W. A. Krzymien. Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Downlink of a multiuser MIMO system is considered, in which the base station (BS) and the user terminals are both equipped with multiple antennas. Efficient transmission schemes based on zero-forcing (ZF) linear receiver processing, eigenmode transmission, and partial channel state information (CSI) at the BS transmitter are proposed. The proposed schemes utilize a handshaking procedure between the BS and the users to select (schedule) a subset of users and determine the precoding matrix at the BS. The advantage of the proposed limited feedback schemes lies in enabling relatively low-complexity user scheduling algorithms and high sum-rate throughput, even for a small pool of users. For large user pools and when the number of antennas at each user terminal is at least equal to the number of antennas at the BS, we show that the proposed scheme is asymptotically optimum.
1. Introduction Increasing demand for broadband wireless services calls for much higher throughputs in future wireless communication systems. It has been shown that with the use of multiple antennas at the transmitter (Tx) and the receiver (Rx), the capacity of a point-to-point communication link increases linearly with min{M, N } where M is the number of Tx antennas and N is the number of Rx antennas [1, 2]. Recently, there has been a great interest in multiuser multiple-input multiple-output (MU-MIMO) systems and transmission strategies that would enable similar capacity gains in multiuser environment [3–5]. In a multiuser downlink with the base station (BS) equipped with multiple antennas, multiple users can be served simultaneously. In fact, it has been shown that to obtain the MU-MIMO downlink sum capacity, transmitting to several users simultaneously must be considered [6]. Since the number of users in the system is usually greater than the maximum number that can be served simultaneously through spatial multiplexing, user selection is required. User selection (or scheduling) favours users, which experience better propagation condition while being sufficiently separated in space. Such user scheduling
leads to multiuser diversity gain [7, 8], which increases with increasing number of users awaiting transmission. It has been shown that the capacity of the MU-MIMO downlink can be achieved by dirty paper coding (DPC) [
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