Adaptive Modulation for Multiuser Amplify-and-Forward Relay Networks with Feedback Delays

In this paper, the performance of adaptive modulation for multiuser amplify-and-forward relay networks over Nakagami-m fading is studied. Specifically, adaptive discrete rate five-mode M-ary quadrature amplitude modulation is employed with fixed switching thresholds. In particular, the detrimental effect of feedback delays in multiuser scheduling is quantified. To this end, an upper bound of the end-to-end signal-to-noise ratio (SNR) is obtained and used to derive the cumulative distribution function, probability density function and moment generating function. Moreover, lower bounds for the outage probability and average bit error rate, and upper bounds for the spectral efficiency and amount of fading are derived in closed-form. Our lower and upper bounds are asymptotically exact at high SNRs, and hence, provide accurate assessment of the system performance.