Bit error rate optimization of DS-CDMA receivers

In search of acceptable cost versus performance trade-off points for DS-CDMA receivers linear tap-weight filters are considered. Based on stochastic approximation concepts a recursive algorithm is developed for the adaptive optimization of linear filters in the minimum Bit Error Rate (BER) sense. The recursive form is decision driven and distribution free. For AWGN channels, theoretical analysis of the BER surface of linear filter receivers identifies the subset of the linear filter space where the optimal receiver lies and offers a formal proof of guaranteed global optimization with probability one for the 2-user case. To the extent that the output of a linear DS-CDMA filter can be approximated by a Gaussian random variable, a minimum-mean-square-error optimized linear filter approximates the minimum BER solution. Numerical and simulation results indicate that for realistic AWGN DS-CDMA systems with reasonably low signature cross-correlations the linear minimum BER filter and the MMSE filter exhibit approximately the same performance. The linear minimum BER receiver is superior, however, when either the signature cross-correlation is high or the channel noise is non-Gaussian.