Multiple access interference and near-far effect cause the performance of the conventional single user detector in DS/CDMA systems to degrade. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have received considerable attention. In this paper we apply the classic and a new modified genetic algorithm for multiuser detection of DS/CDMA signals. It is shown that the classic genetic algorithm (GA) reaches an error floor at high signal to noise ratios (SNR) while the performance of proposed modified GA is much better than the classic one and is comparable to the optimum detector with much less complexity. The results hold true for AWGN and fading channels. We also describe another GA called as meta GA to find the optimum parameters of the modified GA. We compare the performance of proposed method with the other detectors used in CDMA.

The capture effect caused by some of power assignment, Rayleigh fading, and near-far effect, can improve the performance of random-access techniques in mobile radio environment. Moreover, if we consider the overall effect of them, the system performance can be noticeably improved. In this paper, the combined effect of fixed power assignment scheme, Rayleigh fading, and near-far effect on the performance of slotted ALOHA are studied. Plus, the performance degradation caused by AWGN is also investigated. The type of signal capture to be considered is the one that a signal involved in a collision survives if its power level exceeds the sum of the other signals participating in the same collision. Numerical calculations are used to demonstrate the degree of improvement and degradation.