CDMA system design through asymptotic analysis

We use results from the asymptotic analysis of code-division multiple access 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 the following: (1) the optimal tradeoff between coding rate and spreading gain and (2) the comparison of different multirate schemes. Our analysis shows that, for the sake of system spectral efficiency maximization, there exists a threshold E_b/N_o 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 that multicode and variable-spreading with SUMF detection are equivalent, while the former is uniformly better than the latter with linear minimum-mean-square error 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 E_b/N_o ratio.