Space -time transmit precoding and interference suppression for a wireless downlink

The capacity of modern wireless communication systems is limited by the mutual interference between simultaneous users, and the dispersive and time-varying nature of the communication channels. This thesis explores strategies for exploiting transmit antenna arrays to compensate for these effects. Such techniques are referred to as space-time transmission methods because they exploit the additional spatial dimension provided by the antenna array. These methods are particularly promising for increasing the capacity of the downlink (base-to-mobile) of cellular communication systems, for which it is desirable to reduce the complexity of mobile receivers by compensating for the wireless channel at the base station to the extent possible. The methods used depend on the amount and quality of channel feedback available to the transmitter, and a number of different assumptions regarding the latter are considered in this thesis.;The first topic considered in the thesis is interference suppression using space-time transmit filters for a multiuser system, assuming perfect channel feedback. The filter design is formulated as an optimization problem, and an iterative algorithm that converges to the global optimum is derived. Next, the effect of imperfections in the channel feedback is investigated, focusing on a single receiver. Conditioned on the feedback, the space-time channel estimate at the transmitter is modeled as a random vector with a complex Gaussian distribution, which is consistent with standard models of Rayleigh and Rician fading channels. Optimum strategies that maximize the long term information transfer rate are derived under a variety of assumptions regarding this distribution. It is shown that the simple strategy of beamforming is close to optimal in many situations of interest. Finally, methods for the generation of channel feedback are devised. These methods employ Wiener filtering at the transmitter and the receiver for channel estimation and prediction from information derived from the pilot and feedback, respectively. Furthermore, these schemes employ the novel approach of directly sending analogue values back to the transmitter, thus avoiding quantization error. The new schemes are shown to provide better channel estimates than quantization-based feedback schemes currently being standardized for third generation cellular systems.