Effect of Bandwidth and Spreading Codes on Spectrum Utilization of Spectrally Overlaid CDMA Systems

In this paper, we analyze capacity and spectrum utilization of spectrally overlaid systems using single-code CDMA (SC-CDMA) and multi-code CDMA (MC-CDMA) techniques according to varying system parameters. The spectrum utili zation defined as maximally achievable total throughput in the overlaid systems is compared with that of the un-overlaid systems. Analytical results show that the dominant parameter affecting the performance of the overlaid systems is the spectrum overlay ratio. Namely, multiuser interference is a dominant factor to affect the system performance comparing with mutual interference among multiple spreading codes assigned to users. Hence the use of SCCDMA technique is more desirable than MC-CDMA scheme for a higher capacity and spectrum utilization in spectrally overlaid CDMA systems.