Capacities of spectrally overlaid single-code and multicode CDMA systems

This paper is concerned with the capacities of spectrally overlaid narrowband and wideband systems using two code-division multiple-access (CDMA) techniques - single-code and multicode. The capacity is defined here as the maximum achievable total throughput in the narrowband and wideband subsystems. User signal power in the overlay systems affects the performance of the systems. The signal power is characterized as the relative power level of wideband and narrowband users according to the spreading bandwidth, spectral overlay ratio, and the number of spreading codes assigned to a user in single-cell and multiple-cell environments. The capacities of the overlay systems are then analyzed. The results of the analysis show that the single-code technique yields better performance than the multicode scheme in terms of the capacities of the overlay systems. Furthermore, the use of a multicode technique for wideband subsystems and a single-code scheme for narrowband subsystems is desirable for enhancing the performance of CDMA overlay systems.