Spatial - Temporal Signal Processing

Among multi-access communications techniques, CDMA (Code Division Multiple Access) is interference-limited. Conventional single-user receivers su er from the nearfar problem in CDMA cellular communications systems. One method to suppress multi-access interference is digital beamforming by using base-station antenna arrays. However, using beamforming alone cannot solve the near-far problem. Verd u demonstrates that multi-user signal detection can be used to eliminate multi-access interference by utilizing all active users' spreading codes at the base-station. This thesis addresses the incorporation of array signal processing with multi-user signal detection in the CDMA terminal to base-station uplink. In particular, this thesis proposes a method jointly estimating the unknown channel array response vectors and detecting the bits from all users. We rst consider synchronous single-path Rayleigh fading channels. We develop a spatial-temporal decorrelator receiver employing the maximum likelihood criterion based on a novel discrete-time systemmodel and analyze the decorrelator's asymptotic e ciency. It is shown that the spatial-temporal decorrelator is near-far resistant and that using a base-station antenna array signi cantly increases asymptotic e ciency for either the spatial-temporal decorrelator or the conventional single-user detector. We formulate the expectation-maximization (EM) and the space alternating generalized expectation-maximization (SAGE) algorithms based on the discrete-time model and obtain two receiver structures for joint channel array response vector estimation and bit sequence detection. The receiver's convergence rate is analyzed. We have observed that using base-station antenna array accelerates the SAGE-based receiver's ii convergence and improves channel estimation performance. The BER performance of the SAGE-based receiver is shown to be near-far resistant. A synchronous equivalent discrete-time system model is formulated for asynchronous multipath channels. Based on this model, we exploit multipath diversity by incorporating maximal-ratio combiner into the spatial-temporal decorrelator. It is shown that unlike antenna arrays, using multipath diversity combining does not improve detector's asymptotic e ciency. We exploit the SAGE algorithm to decouple the multi-user signals for bit sequence detection and again decouple the multipath signals to estimate the channel array response vector for each path of each user for given time delays. Timing error e ects on the SAGE-based receiver are studied by simulation. Multipath diversity combining is shown to be e ective in improving the receiver's bit error rate (BER) performance. Finally, we extend the techniques developed for single-rate systems to multi-rate systems with base-station antenna arrays over asynchronous multipath fading channels. An iterative multi-user receiver for dual-rate systems is derived. It is shown that unlike the conventional single-user receiver, the proposed receiver's BER relative performances for high-rate and low-rate users are similar. We observed that the BER of high-rate users converges to the derived lower bound as a function of the number of iterations faster than that of low-rate users. iii Acknowledgements It is a great pleasure to thank my supervisor, Dr. Steven Blostein, for his excellent supervision and nancial support throughout this research. Without Dr. Blostein's constructive suggestions and knowledgeable guidance in statistical signal processing area, this work would not have been successfully completed. I would like to thank my defence committee members, Dr. S. G. Akl (Computer Science), Dr. N. C. Beaulieu, Dr J. C. Cartledge and Dr. D. Falconer (Carleton University) for taking time to review this thesis. Prof. Falconer also provides helpful suggestions to the nal version of this thesis. I am indebted to my parents for their continuous encouragement and expectation. Special thanks give my mother for her coming Canada to take care of my baby son. Finally, I would like to express my appreciation to my wife, Ying, for her understanding, patient and support. I am grateful to my son, Andrew, for the happiness he brings to my family. This work was supported by the Canadian Institute for Telecommunications Research and the School of Graduate Studies and Research at Queen's University. iv