Multiple antenna downlink: feedback reduction, interference suppression and relay transmission

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Tang, Taiwen

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Multiple input multiple output (MIMO), which uses multiple antennas at both the transmitters and the receivers, is an emerging technology that greatly improve the data rate of wireless systems. This dissertation focuses on the downlink of a cellular system, where a base station transmits data to multiple users. It proposes design solutions on how to combat the impairments in the system and improve the data rate or error rate performance with multiple antennas at the base station and/or the users. In the downlink, the scheduling decisions, which determine how to transmit data from the base station to the users, usually depend on users’ channel conditions, e.g., the gains or the coefficients of the channels. Reducing the system resource, e.g., transmission time or radio frequency band, used for feeding back the channel information from the downlink MIMO users, which works with scheduling algorithms and different MIMO transmitter and receiver configurations, has been recently recognized as an important design issue. The first part of this dissertation proposes a scheduling and feedback strategy that exploits multiuser diversity with significant reduction of the amount of resource for feedback of channel information. The second contribution of this dissertation studies the problem of interference suppression. Combatting interference from adjacent base stations that use the same radio frequency band (cochannel interference) requires signal processing techniques at the transmitters and/or the receivers. In particular, in orthogonal frequency division multiplexing (OFDM) systems, dealing with cochannel interference that is not synchronized in carrier frequency and propagation time poses challenges to the receiver design. This dissertation studies the design problem for suppressing the asynchronous cochannel interference at an OFDM receiver. In the third part of this dissertation, a technique for improving coverage for the downlink users under heavy signal attenuation is investigated. The basic idea is to use relays that are deployed at fixed locations in the downlink to receive and forward signals from the base station to mobile users. This dissertation studies the design of different signal processing components for the system where both the base station and the relay have multiple antennas.