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In this correpsondence, we study a lightly loaded code-division multiple-access (CDMA) system with and without multistage hard- and soft-decision parallel interference cancelation (HO-PlC and SD-PlC). Throughout this paper we will only consider the situation of a noiseless channel, equal powers and random spreading codes. For the system with no or a fixed number of steps of interference cancelation, we give a lower bound on the maximum number of users such that the probability for the system to have no bit-errors converges to one. Moreover, we investigate when the matched filter system, where parallel interference cancelation is absent, has bit errors with probability converging to one. This implies that the use of HD-PIC and SD-PlC significantly enhances the number of users the system can serve. [ABSTRACT FROM AUTHOR]

We study a multiuser detection system for 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 are used iteratively to improve knowledge of the desired signal. We use large deviation theory to show that the bit-error probability (REP) is exponentially small when the number of users is fixed and the processing gain increases. We investigate the exponential rate of the REP after several stages of HD-PIC. We propose to use the exponential rate of the REP as a measure of performance, rather than the signal-to-noise ratio (SNR), which is often not reliable in multiuser detection models when the system is lightly loaded. We show that the exponential rate of the REP 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 REP converges to (log 2)/2 - 1/4. We provide guidelines for the number of stages necessary to obtain this asymptotic exponential rate. We also give Chemoff bounds on the REPs. These estimates show that the REPs are quite small as long as k = o(m/ log n) when the number of stages of HO-PlC is fixed, and even exponentially small when k = O(n) for the optimal HD-PlC system, and where k is the number of users in the system and n is the processing gain. Finally, we extend the results to the case where the number of stages depends on k in a certain manner. The above results are proved for a noiseless channel, and we argue that we expect similar results in a noisy channel as long as the two-sided spectrum of the noise decreases proportionally to n. [ABSTRACT FROM AUTHOR]