Downlink Assisted Uplink Zero Forcing for TDD Multiuser MIMO Systems
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Research Article Downlink Assisted Uplink Zero Forcing for TDD Multiuser MIMO Systems Petri Komulainen, Antti T¨olli, Matti Latva-aho, and Markku Juntti Centre for Wireless Communications, University of Oulu, P.O. Box 4500, 90014 Oulu, Finland Correspondence should be addressed to Petri Komulainen, [email protected] Received 1 February 2009; Revised 11 May 2009; Accepted 19 July 2009 Recommended by Bruno Clerckx This paper proposes practical coordinated linear transmit-receive processing schemes for the uplink (UL) of multiuser multipleinput multiple-output (MIMO) systems in the time division duplex (TDD) mode. The base station (BS) computes the transmission parameters in a centralized manner and employs downlink (DL) pilot signals to convey the information of the beam selection and beamformers to be used by the terminals. When coexisting with the DL transmit-receive zero forcing, the precoded DL demodulation pilots can be reused for UL beam allocation so that no additional pilot overhead is required. Furthermore, the locally available channel state information (CSI) of the effective MIMO channel is sufficient for the terminals to perform transmit power and rate allocation independently. In order to reduce the UL pilot overhead as well, we propose reusing the precoded UL demodulation pilots in turn for partial CSI sounding. The achievable sum rate of the system is evaluated in time-varying fading channels and with channel estimation. According to the results, the proposed UL transmission strategy provides increased rates compared to single-user MIMO transmission combined with user selection as well as to UL antenna selection transmission, without being sensitive to CSI uncertainty. Copyright © 2009 Petri Komulainen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction In order to attain all the capacity gains available in multipleinput multiple-output (MIMO) communication systems, channel state information in the transmitter (CSIT) should be utilized. CSIT is available in time division duplex (TDD) systems, provided that the channel does not change significantly between the receive and transmit periods. Due to the channel reciprocity, the receiving node can estimate the state of the channel during one frame, and use that knowledge for the purposes of MIMO transmission in the next one. CSI can be estimated from pilot symbols that are known to the receiver. The pilots are also necessary for performing coherent demodulation in the receiver side. In order to keep the pilot overhead as low as possible, it is desirable that the same pilot symbols are a useful reference for both reception and transmission. In a cellular multiuser MIMO system, the downlink (DL) comprises a broadcast channel (BC), whereas the uplink (UL) is a multiple access channel (MAC). The channel reci-
procity leads into duality properties between the BC and MAC
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