On the Limits of Coded Transmission over Fading Channels with CDMA

Alexander Lampe, and Johannes B. Huber Telecommunications Institute II, University of Erlangen Cauerstraße 7/NT, D–91058 Erlangen, Germany Phone: +49-9131-85 27114, Fax: +49-9131-85 28919, Email: [email protected] Abstra t | In this paper we investigate the synchronous transmission over time–variant multipath Rayleigh–fading channels employing Direct Sequence 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 efficiency achievable by application of various nonlinear receivers can be given. Here, we consider the application 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 propagation paths.