How fading affects CDMA: An asymptotic analysis with linear receivers

Using asymptotic analysis, we study the effect of frequency-flat fading on code division multiple access (CDMA) systems with linear receivers and rand om spreading sequences. Specifically, we let the number of users grow witho ut bound, while the ratio of number of users to spreading sequence length i s kept fixed to a value alpha. We treat separately the cases of slow fading (nonergodic channel) and of fast fading (ergodic channel), For the former channel, we derive the outage probability, while for the latter, we compute the channel capacity. In both cases, multiple classes of users with differ ent qualities of service are dealt,vith, As alpha --> infinity, the system throughput tends to the same limit of 1.44 bit/symbol as for the nonfading channel with both single-user matched filter (SUMF) and linear minimum mean -square-error (MMSE) receivers. The outage probability exhibits a floor for all cr with the SUMF receiver, while with MMSE receiver the floor is prese nt only for alpha greater than or equal to 1. We also address the tradeoffs involved in the allocation of available bandwidth between spreading and co ding.