A novel variable group hybrid interference cancellation for CDMA systems

The CDMA system is multiple access interference (MAI) limited. The conventional matched filter (MF) receiver is non optimal as the MAI is treated as useless noise. This leads to the proposal of the optimal multiuser receiver which has very good performance but is too complex to be practical. Sub-optimal detectors such as interference cancellation (IC) receivers seem promising due to their structural simplicity and good performance. The variable group hybrid interference cancellation (VGHIC) receiver proposed in this thesis combines the advantages of the parallel IC (PIC) and successive IC (SIC) in a novel way. In the VGHIC, users are grouped such that those with similar powers are selected into the same group. The PIC is used within the members of each group while the SIC is applied between groups of different powers. Theoretical analysis as well as simulations show that the VGHIC has an overall performance advantage over other IC receivers. Improvements have also been proposed to further enhance the performance of the VGHIC. They include the ranking of users, use of the averaged correlation samples, partial combining of correlation samples and partial cancellation. These techniques are found to be effective. Due to the adaptive nature of the VGHIC structure, it exhibits different processing delay profiles depending on the power spread of received signals. A simple method of limiting the processing delay has been proposed. It can reduce the processing delay by 40% at the expense of 4.5% reduction in user capacity. When phase and timing estimation errors are introduced, the VGHIC has better performance than other IC schemes even with high estimation errors. The VGHIC with RAKE receiver is investigated using realistic channel conditions and the results verify that it outperforms the conventional RAKE receiver. The major advantage of the VGHIC over other IC schemes is the ability to adapt its structure according to changing power profile of the received signals. Hence, the VGHIC is able to perform consistently in different environments. Moreover, its structure can be easily changed by modifying the design parameter. All these features make the VGHIC a viable option for increasing the capacity of CDMA systems.