Improving the Spectral Efficiency of Adaptive Modulation in Amplify-and-Forward Cooperative Relay Networks with an Adaptive ARQ Protocol

This paper investigates the effectiveness of a cross-layer design that combines adaptive modulation (AM) at the physical layer with an adaptive truncated automatic repeat request (ARQ) protocol at the data link layer to maximize the throughput of cooperative amplify-and-forward (CAF) relay networks under prescribed delay and/or error performance constraints. A by-product of our proposed analytical framework, that involves evaluation of the marginal moment generating function (MGF) of the end-to-end signal-to-noise ratio (SNR), for computing the average throughput and average packet error rate performance metrics is the unification and generalization of existing studies on cross-layer combining of AM and truncated ARQ for non-cooperative diversity systems to generalized fading channels. Our analytical methodology is motivated by the fact that the MGF of end-to-end SNR for multi-relay CAF networks is either available in the research literature (for certain fading environments) or it can be computed much more conveniently than its PDF, while the desired marginal MGF can be evaluated efficiently using a multi-precision Laplace inversion formula of an auxiliary MGF function. The efficacy of an adaptive R^max (maximum retransmission limit) strategy over the classical fixed R^max scheme is also examined.