Capacities of spectrally overlaid single-code and multicode CDMA systems

This paper is concerned with the capacities of spectrally overlaid narrow-band and wide-band systems using two code-division multiple-access (CDMA) techniques: a single-code and a multicode. The capacity is defined here as the maximally achievable total throughput in the narrow-band and wide-band 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 wide-band and narrow-band users according to the spreading bandwidth, spectral overlay ratio, and the number of spreading codes assigned to a user in single-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 does the multicode scheme in terms of the capacities of the overlay systems. Furthermore, the use of a multicode technique for wide-band subsystems and a single-code scheme for narrow-band subsystems is desirable for enhancing the performance of CDMA overlay systems.