Analysis of an adaptive QAM scheme with non-zero delay for Rayleigh fading channels

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These techniques adapt signal transmission parameters, e.g. modulation constellation and transmit power, according to time-varying channel conditions, in order to better utilise channel capacity compared to fixed transmission schemes. Adaptive modulation schemes, which adapt modulation constellation, exploit time-varying channel conditions without increasing the level of co-channel interference, and have gained considerable favour for this reason. However, one drawback is the effect of estimation delay, whereby the transmitter updates modulation mode based on outdated channel state information from the receiver. In this paper, we propose 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, resulting in a closed form expression for the average number of bits per symbol (BPS) throughput.