Increasing the spectral efficiency in microcell hotspots using M-ary CDMA: capacity analysis and simulation results

We consider a single "hotspot" microcell where many wireless CDMA users transmit data concurrently using power control. As opposed to traditionally considered binary CDMA transmission or conventional M-ary CDMA, we explore the potential benefits of using generalized M-ary spreading codes. We compute the symmetric sum capacity when single-user detection and decoding are used at the receiver (base station) and observe that the use of appropriate M-ary CDMA can lead to more than threefold increase in transmission rates. It turns out that even using one extra CDMA chip level (i.e., zero) can potentially improve the aggregate data throughput in a microcell by 20-50 percent. Using the insights from this analysis, we generate pseudo-random M-ary spreading codes and explore turbo coded architecture for corresponding M-ary CDMA transmission. Simulation results indicate that the proposed system can support up to 60 to 70 users at bit error rates of 10^-4, with a power control error of up to 1 dB at SNR\´s encountered in traditional microcells.