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 random spreading sequences. Specifically, we let the number of users grow without bound, while the ratio of number of users to spreading sequence length is kept fixed to a value α. 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 different qualities of service are dealt with. As α → ∞, 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 α with the SUMF receiver, while with MMSE receiver the floor is present only for α ≥ 1. We also address the tradeoffs involved in the allocation of available bandwidth between spreading and coding.