Optimal Performance for DS-CDMA Systems with Hard Decision Parallel Interference Cancellation

We study a multiuser detection system using code division multiple access (CDMA). We show that applying multistage hard decision parallel interference cancellation (HD-PIC) significantly improves performance compared to the matched filter system. In (multistage) HD-PIC, estimates of the interfering signals due to other users are used iteratively to improve knowledge of the desired signal. We use large deviation theory to show that the bit-error probability (BEP) is exponentially small and investigate the exponential rate of the BEP after several stages of HD-PIC. We propose to use the exponential rate of the BEP as a measure of performance, rather than taking the signal-to-noise ratio, which is not reliable in multiuser detection models. We show that the exponential rate of the BEP remains fixed after a finite number of stages, resulting in an optimal hard decision system. When the number of users becomes large, the exponential rate of the BEP converges to (log 2)/2−1/4. We provide intuition concerning the number of stages necessary to obtain this asymptotic exponential rate. Finally, we give Chernoff bounds on the BEP’s. These estimates show that the BEP’s are quite small as long as k = o(n/ logn) when the number of stages of HD-PIC is fixed, and even exponentially small when k = O(n) for the optimal HD-PIC system. Keywords—Code division multiple access, hard-decision parallel interference cancellation, large deviations, exponential rate, performance measures, optimal hard-decision system.