On the limits of coded transmission over fading channels with CDMA

Abstract | In this paper we investigate the synchronous transmission over time–variant multi-path Rayleigh–fading channels employing Direct Se-quence Code Division Multiple Access (DS–CDMA). Assuming ideal knowledge of the actual channel state, we introduce an equivalent transmission model for the case of high processing gain. Based on this model an analytical solution for the spectral effi-ciency achievable by application of various nonlinear receivers can be given. Here, we consider the appli-cation of linear interference suppression by means of MMSE filters combined with successive cancellation and single user decoding. We show that in this way the system’s spectral efficiency approaches that of the AWGN channel if the number of users K is much larger than the spreading factorN. This holds for the flat Rayleigh fading channel as well as for frequency selective channels with a large number of propaga-tion paths. I