Ravishankar Mahadevappa

Interference Cancellation Techniques for Multiuser Communication Systems

Date: Tue, 12 Nov 2002

Intersymbol Interference, together with Multiple Access Interference, causes significant deterioration in the performance of a CDMA system. To compensate for the loss, channel coding is often used, requiring additional bandwidth to accomodate more redundant information. In this dissertation we demonstrate the use of chip-level interleaving and recursive precoding in not only reducing the loss due to ISI but also realizing additional gain compared to the single user conventional CDMA system. A receiver for the chip-Interleaved CDMA system is designed along the lines of iterative (turbo) equalization, consisting of an {it a posteriori} probability (APP) equalizer and single user despreader. A low-complexity soft-input soft-output chip-level interference canceller is also presented as an alternative. Convergence behaviour of the former receiver is explained using extrinsic information transfer charts. Results show that significant gain can be achieved by using the modified system. In addition, a novel multiuser detection algorithm is presented, which uses the knowledge of constant error contours to eliminate errors overlooked by a decorrelating detector. Another contribution is the attempt towards computing the pdf of the decision variable output by a MAP equalizer. The pdf of state probability ratios are first expressed as solutions to a set of simultaneous integral equations. The pdf of the log APP ratio is then expressed as an integral involving the solutions. Due to the intractability of the integral equations, numerical solutions are plotted for a simple two-tap channel case.