Rake receiver detection of Adaptive modulation aided CDMA over frequency selective channels

A closed form Bit Error Ratio (BER) formula is derived for a fixed-mode Quadrature Amplitude Modulation (QAM) scheme employing Rake receivers and receiver antenna diversity. The analysis is extended to constant power Adaptive QAM (AQAM) expressing the average BER and the average throughput as a closed form function of the modulation mode switching levels. Then, the switching levels are optimised so that the average throughput is maximised, while maintaining a given target BER. This results in a constant-BER, variable-throughput arrangement. The results show that our constant-power AQAM scheme exhibits an SNR gain of about 5dB in comparison to fixed-mode QAM, when operating over a Wireless Asynchronous Transfer Mode (W-ATM) channel employing one antenna. However, the achievable throughput gain of the system over conventional fixed-mode modems is substantially reduced, as the diversity order of the receiver is increased.