CDMA system design through asymptotic analysis

We use results from the asymptotic analysis of CDMA with random spreading as a tool for gaining insight and deriving design guidelines on practical system issues, inspired by the current UMTS/IMT2000 standardization process. In particular, we consider a simple synchronous single-cell system with perfect power control and linear detection and we examine: i) the optimal trade-off between coding rate and spreading gain; ii) the comparison of diferent multirate schemes. Our analysis shows that, for the sake of system spectral eÆciency maximization, there exists a threshold Eb =N0 below which the single-user matched filter (SUMF) is optimal (within the limits of our system model). As far as multirate schemes are concerned, we show thatmulticode and variable-spreading with SUMF detection are equivalent, while the former is uniformly better than the latter with linear minimum-mean-square error (LMMSE) detection. Variable-spreading can perform very close to multicode if high-rate users are detected by observing the whole \low-rate" symbol interval. Finally, we compare the capacity regions of the multimodulation and multicode schemes versus the Eb =N0 ratio.