How fading affects CDMA : an asymptotic analysis with linear receivers

Using asymptotic analysis, we study the effect of frequency-flat fading on 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 _ . 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. We show in particular that, with fading, as _ ! 1 the system throughput tends to the same limit of 1.44 bit/symbol as for the non-fading channel. On the contrary, the outage probability exibits a floor for all _ values when a single-user matched filter is employed, while with a linear minimum-mean-square-error receiver the floor is present only for _ _ 1 . We also address the tradeoffs involved in the allocation of available bandwidth between spreading and coding.