Nonlinear multiuser detector for synchronous coded CDMA on severely distorted frequency selective channels

In a synchronous CDMA system the orthogonality of the codes ensures no interference between the users. However, when strong multipath distortion is present, the orthogonality is destroyed. This paper treats both the analytical and the practical aspect of this problem. A new analytical method is presented for evaluating the performance of the optimal detector with known channel and random orthogonal code. For the practical receiver implementation, we propose a signal processing technique, based on the estimate-maximize algorithm, which iteratively seeks for the maximum likelihood solution. This nonlinear technique is shown to perform much better than the optimal linear solution. The algorithm lends itself to an efficient DSP or VLSI implementation and is combined with coding. A new analytical channel parameter is shown to accurately predict the performance to within ±0.5 dB. The results are also presented in the form of outage probabilities for random Rayleigh multipath against required fading margin