Interference Rejection in Wireless Communication Systems

Abstract: This thesis is concerned with the problem of interference rejection in wireless communication systems. Different solutions are proposed, depending on the type of system under study and the amount of knowledge available about the signal and the propagation environment.

When different users share the same frequency band and also transmit signals concurrently in time, it is necessary to separate these at the receiver. This can be accomplished by using an antenna array, or assigning different code sequences to the users (DS-CDMA). The mathematical model for the received signal is structurally similar in both cases. An algorithm is proposed which iterates between estimating the vector channel and the transmitted symbols. By exploiting structure inherent in the digitally modulated signals, the method is able to demodulate each user's symbol stream reliably. The performance of the method is evaluated analytically as well as by numerical simulations, and is shown to compare favorably to other alternative methods in terms of performance as well as complexity.

Further, by relaxing the important assumption of time-and phase synchronization in the DS-CDMA case, the problem becomes one of channel parameter estimation and symbol detection. A method for solving this in a joint fashion is suggested, based on maximum-likelihood principles and a large sample size. It is shown how imperfect knowledge about the transmitted symbols significantly improves the quality of the parameter estimates, which can further be used to reduce the probability of symbol errors.

The performance in a multipath scenario is largely dependent upon properly combining the different scaled and delayed signal copies in the receiver. A method is proposed that estimates the number of multipath components from a specific user, based on the statistics of a criterion function, and using a simple hypothesis test.

In cases where the interference is narrow-band in relation to the signal-of-interest, a method employing a filter bank is proposed to accomplish time-and frequency localization of the interference, followed by thresholding. It is shown that the rejection of this interference prior to conventional matched filter reception, significantly improves system performance.