MIMO Beamforming With Mutual Coupling, Limited Feedback and Coordination

Abstract

Multi-input, multi-output (MIMO) techniques use multiple antennas at both the transmitter and receiver to improve the performance of wireless communications systems over multipath fading channels. In recent years, MIMO techniques that employ transmit beamforming have been adopted in several new and emerging standards for situations where channel state knowledge is available at the transmitter. Most existing studies of MIMO beamforming assume that perfect channel knowledge is available at both the transmitter and receiver, and that the antenna elements in both the transmit and receive arrays are spaced sufficiently far apart so as to be essentially uncoupled. In practice, however, constraints on the physical size of antenna arrays may require elements to be spaced close together, leading to antenna coupling and signal correlation. The capacity of the feedback link from the receiver to the transmitter may also be limited, so that channel knowledge is necessarily imperfect at the transmitter. These challenges become all the more difficult in multiuser scenarios, when efficient coordination among several transmitters is required.

In this dissertation, we consider the analysis and design of MIMO beamforming techniques with antenna mutual coupling, limited feedback and multiuser coordination. We begin by introducing a circuit model of a compact wireless MIMO transceiver that incorporates the effects of antenna mutual coupling. We then use this model to derive new MIMO beamforming strategies appropriate for both single-user and multiuser systems. Through numerical examples, we illustrate the performance of the proposed beamforming techniques and their dependence on the properties of the antenna arrays, matching networks, channel estimation errors, and channel state feedback. Finally, we propose new asymmetric-rate coordinated beamforming strategies which improve both the individual rates of each user and the sum-rate subject to zero-interference constraints. These asymmetric-rate strategies can also be combined using time-division to create new, higher-rate symmetric beamforming strategies.