Some multiple access interference cancellation techniques for asynch. DS/CDMA systems

Abstract

The performance of successive interference cancellers (SICs) and parallel interference cancellers (PICs) for asynchronous DS/CDMA systems in multipath Rayleigh fading channels is examined and their implementation is considered. These interference cancellers regenerate multiple access interference (MAI) and intersymbol interference (ISI) and subtracted them from the input signal. The simulation results indicate that power control is necessary even in systems with these interference cancellation schemes and that the PIC can perform better than the SIC for practical systems with power control. These interference cancellers require a sophisticated respreading technique which in turn requires chip rate operations and large memory. To overcome this difficulty, we propose a new interference cancellation scheme called the symbol-by-symbol based multistage adaptive interference canceller (SAMIC). The SAMIC adaptively estimates and removes both MAI and ISI at the output of the RAKE receiver. It requires neither respreading nor the evaluation of partial cross-correlation. In addition, its complexity is independent of the number of fingers of the RAKE receiver. Consequently, the SAMIC is considerably simpler to implement than existing interference cancellation techniques. Computer simulation results demonstrate that the SAMIC can perform much better than conventional RAKE receiver and also show that when the number of fingers is less than the number of multipaths the SAMIC outperforms the conventional successive and parallel interference canceller with respreading. These interference cancellers such as SIC, PIC and SAMIC use tentative decision values for the regeneration and estimation of the MAI and ISI and the effect of the interference cancellation highly depends on the reliability of the tentative decision. In order to improve the performance of the interference cancellers, we consider the design of soft decision functions for each stage of the interference canceller...