Performance of an adaptive QAM scheme over correlated Rayleigh fading with non-zero delay

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These schemes aim to utilise channel capacity better than fixed transmission schemes, by adapting signal transmission parameters, such as modulation constellation and transmit power. Adaptive modulation schemes, which adapt the modulation constellation, have gained considerable favour for exploiting time-varying channel conditions without increasing the level of cochannel interference. However, the traditional adaptive modulation schemes are designed assuming zero delay between the channel estimation and the modulation mode adaptation. In the case of non-zero delay, the transmitter would update the modulation mode based on outdated channel state information from the receiver. We investigate an adaptive quadrature amplitude modulation (AQAM) scheme, which, by way of retransmissions, allows a targeted reliability level to be met, even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated. Closed form expressions for the average number of bits per symbol (BPS) throughput and the average bit error rate (BER) of the proposed scheme are derived.