Blind space-time interference cancellation for DS/CDMA systems

As the demand for wireless communication grows, it is critical to search for techniques to efficiently exploit the limited bandwidth and discriminate between the users. CDMA has been found to be a promising multiple-access technique to mitigate fading and to provide efficient use of bandwidth. Unlike FDMA and TDMA, which are band-limited, CDMA is interference-limited. Therefore, for a CDMA system designer, one major objective is to eliminate interference of the desired signal. Multiuser detection exploits the structure of the multiuser interference in order to increase the efficiency and eliminate the effect of MAI (Multi-Access Interference). Included in the various multiuser detection schemes are the interference cancellation receivers; those are the simplest to implement. As the number of users in a system with limited bandwidth grows, it becomes necessary to assign signature waveforms which have relatively larger correlations. This motivates the use of multiple antennas at the receiver to provide increased discrimination between the users by exploiting their spatial separation. In addition, beamforming as well as diversity combining techniques can be used to reduce multiple access interference arriving from directions other than that of the desired user. Due to the dynamic nature of cellular communications and severe fading conditions, the array response vectors vary rapidly, making it impossible for the receiver to know their exact values at a given time. Therefore, it is desirable for a multiple antenna receiver to be able to detect and follow the array response vectors blindly in an adaptive fashion. The focus of this thesis is blind space-time multiuser detection schemes for DS/CDMA communication systems. Different types of interference cancelling receivers are presented and compared. Several adaptive algorithms in CDMA environments are proposed. The performance on convergence rate, and the steady state beam pattern of these schemes, are also investigated and compared. The interference cancelation scheme is then combined with smart antennas. A serial interference cancelling scheme with multiple receiving antennas is formulated and potential enhancement in performance when using antenna arrays is demonstrated. The antenna arrays are either correlated or independent. It is shown that without requiring any additional spatial information, this receiver achieves better error performance than its single-element counterpart. These schemes are then extended to provide multipath diversity by constructively combining the replicas of the desired signal arriving from different directions and delays. By exploiting the Two Dimensional RAKE (2D-RAKE) receiver, the performance improves dramatically.