Studies in multiple -antenna wireless communications
Wireless communications systems are used today in a variety of milieux, with a recurring theme: users and applications regularly require higher throughput. Frequency bandwidth is a limited resource; multiple antennas enable higher through-put and/or more robust performance than single-antenna communications, with no increase in power or bandwidth. Systems are required which achieve the full potential of this “space-time” communication channel under the significant challenges of time-varying fading, multiple users, and the choice of appropriate coding schemes. This dissertation is focused on solutions to these problems. For the single-user case, there are many well-known coding techniques available; in the first part of this dissertation, the performance of two of these methods are analyzed. In the second part the focus is on multi-user systems for which capacity-approaching coding schemes were not previously available. In addition to analyzing current modulation schemes, new capacity-approaching multi-user coding techniques are developed.
Trained and differential modulation are simple coding techniques for single-user time-varying channels. The performance of these coding methods is characterized for a channel having a constant specular component plus a time-varying diffuse component. A first-order auto-regressive model is used to characterize diffuse channel coefficients that vary from symbol to symbol, and is shown to lead to an effective SNR that decreases with time. The effective SNR of differential modulation is shown to be higher than that of trained techniques. Pairwise probability of error expressions are derived which accurately describe performance for unitary modulation over a Rician channel. A lower bound on the capacity of trained modulation is found for the specular/diffuse channel. This bound is maximized over the training length, training frequency, training signal, and training power. Trained modulation is shown to have higher capacity than differential coding, despite the effective SNR penalty of trained modulation versus differential methods.
The second part of the dissertation considers the multi-user, multi-antenna channel. Precoding with the channel inverse is shown to provide capacity that approaches a constant as the number of users and antennas simultaneously increase. Regularizing the inverse is shown to have linear growth in sum-rate, but its performance is still below the sum-capacity of the multi-user channel at high SNR. (Abstract shortened by UMI.)