Performance analysis of adaptive M-QAM over a flat-fading Nakagami-m channel
Channel adaptive M-ary quadrature amplitude modulation (M-QAM) schemes have been developed to provide higher average link spectral efficiency by taking advantage of the time-varying nature of wireless fading channels. Much of the earlier work on such schemes uses the assumption that thresholds designed for additive white Gaussian noise (AWGN) channels can be directly applied to slowly varying block-fading channels. The thresholds are calculated with a commonly used approximation bit error rate (BER) expression in these schemes. The first aim of this paper was to investigate the accuracy of using this common BER expression in a fading channel. This was done by comparing the result of the average BER expression derived using the approximate expression with results of simulations over a Nakagami-m block-fading channel. The second aim was to show that the inaccuracy in the threshold values determined using the closed form approximation expression would lead to inappropriate operation of the adaptive M-QAM scheme in a fading channel. This was done by comparing expected theoretical values with the simulation results. Two alternative approximate BER expressions for M-QAM in AWGN were then presented and used to determine the average M-QAM of BER over a Nakagami-m fading channel. The comparison between the average BER expressions and the simulation showed a much closer match. More accurate thresholds for the adaptive M-QAM system were then determined using one of the two average BER expressions and the accuracy of these threshold points was then verified using simulation results.