CDMA design through asymptotic analysis : fading channels

Using asymptotic analysis, we show how flat fading affects CDMA with linear re ceivers. 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 case of flat, slow fading with lognormal shadowing (nonergodic channel) and of flat, fast Rayleigh fading (ergodic channel). For the former channel we derive the outage probability, while for the latter we compute the channel capac-ity. We show in particular that, with fading, as _ !1 the system capacity 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 .