Interference cancelling receivers with multiple antennas for CDMA systems.

摘要

The focus of this thesis is on low complexity multiuser receiving schemes for CDMA signals. Several interference cancelling receivers are introduced and it is shown that receivers with multiple antennas can take advantage of the spatial distribution of the users to achieve improved performance.; A single antenna multistage parallel interference canceller is developed, which without adding significant complexity improves on the performance of the conventional matched filter receiver, and in the limit, as the number of stages increases infinitely, becomes near-far resistant. It provides a simple structure for an approximate implementation of the decorrelating detector, where the number of stages can be determined by the performance requirements of the system.; 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 techniques can be used to reduce multiple access interference arriving from directions other than that of the desired user.; A successive interference cancelling scheme with multiple receiving antennas is formulated and the potential enhancement in performance by using antenna arrays is demonstrated. Due to the dynamic nature of cellular communications and severe multipath fading conditions, the array response vectors vary rapidly, making it impossible for the receiver to know their exact value at a given time. In a CDMA environment, the knowledge of the users' signature sequences can be exploited to design blind adaptive schemes for the antenna arrays. Two such schemes have been proposed for the adaptation of the combining as well as cancelling stages, the performances of these schemes are compared and it is shown that without requiring any additional spatial information, this receiver achieves better error performance than its single-element counterpart.; These schemes are extended to provide multipath diversity by constructively combining the replicas of the desired signal arriving from different directions and delays. The performance of one of the blind methods is shown to be close to that of the optimum spatio-temporal multipath combining receiver.