Precoding and Beamforming for Multi-Input Multi-Output Downlink Channels

This dissertation presents precoding and beamforming schemes for multi-user wireless downlink channels when multiple antennas are employed at both the transmitter and the receivers. In the first part of the thesis, we will discuss transmitter processing without channel information which is applicable in both flat and frequency selective (when orthogonal frequency-division multiplexing (OFDM) is applied) fading channels. This leads to methods for designing signature matrices for transmitters that use any combination of the spatial, temporal and frequency dimensions, with good performance provided by low-complexity receivers. In the rest of the thesis, we pose the problem when the channels between the base station and each user are known perfectly at the base station. A non-linear precoding scheme is designed to minimize the mean-squared error between the transmitted and received data with a per-user power constraint. We also develop methods that are able to provide user-specific signal-to-interference-noise ratios (SINRs) with minimal total transmit power, through the extension of a so-called uplink-downlink duality result. Our study indicates that channel knowledge at the transmitter leads to substantial reductions in required power for providing given levels of SINRs to users.