In multiuser MIMO downlink communications, it is necessary to design schemes that are able to suppress co-channel interference (CCI). This work examines several issues related to suppressing CCI in multi-user scenarios through the use of precoders at the transmitter. First, the work proposes precoder designs by choosing beamforming coefficients based on maximizing a performance criterion defined as the signal-to-leakage-plus-noise ratio (SLNR) for all users simultaneously. Compared with zero-forcing solutions, the proposed method does not impose any condition on the relation between the number of transmit and receive antennas and it also avoids noise enhancement. Simulations illustrate the superior performance of the scheme.
This work also develops dynamic antenna scheduling strategies for downlink MIMO communications, where a subset of the receive antennas at certain users are selectively disabled. The proposed method improves the signal-to-leakage-plus-noise (SLNR) ratio performance of the system and it relaxes the condition on the number of transmit-receive antennas in comparison to traditional zero-forcing and time-scheduling strategies. The largest value that the SLNR can achieve is shown to be equal to the maximum eigenvalue of a certain random matrix combination, and the probability distribution of this eigenvalue is characterized in terms of a Whittaker function. The result shows that increasing the number of antennas at some users can degrade the SLNR performance at other users. This fact is used to propose an antenna scheduling scheme that leads to significant improvement in terms of SNR outage probabilities. Compared with non-closed-form iterative solutions, the proposed beamforming scheme coupled with antenna selection performs within less than 1 dB of the iterative scheme.
Finally, the thesis proposes an efficient channel estimation scheme in terms of feedback rate in order to enable proper channel tracking at the transmitter and to facilitate the beamformer design. The proposed scheme exploits the channel statistics by using space and time correlation in a joint manner in order to track the channel at the transmitter. Simulation results show improvements compared to a system with space-time block coding.
Acknowledgment This work was supported in part by the National Science Foundation under grants CCF-0208573 and ECS-0401188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.