Spatial-temporal signal processing for multiuser CDMA communication systems

Among multi-access communications techniques, CDMA (Code Division Multiple Access) is interference-limited. Conventional single-user receivers suffer from the near-far 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. Vardú 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 first consider synchronous single-path Rayleigh fading channels. We develop a spatial-temporal decorrelator receiver employing the maximum likelihood criterion based on a novel discret time system model and analyze the decorrelator's asymptotic efficiency. It is shown that the spatial-temporal decorrelator is near-far resistant and that using a base-station antenna array significantly increases asymptotic efficiency 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 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 efficiency. 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 effects on the SAGE-based receiver are studied by simulation. Multipath diversity combining is shown to be effective 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.