Cross-layer throughput optimization using adaptive PHY/MAC layer techniques: A unified approach

This article develops a unified mathematical framework for maximization of the user throughput over generalized wireless channels using the symbol rate, packet length and constellation size of two distinct classes of digital modulation schemes (M-PSK and M-QAM) as optimization variables. First, we show that the throughput optimization problem for truncated selective-repeat automatic repeat request (SR-ARQ) is identical to that of no retransmission case ( N_R^MAX = 0 ) or the conventional SR-ARQ ( N_R^MAX = ∞ ) protocol that guarantees the end-to-end delivery of packet. Subsequently, we derive optimization equations for each of the above degrees of freedom in closed-form. A simple algorithm for triplet-parameter optimization is also presented. Numerical results reveal that it is sufficient to adapt symbol rate alone in the low average signal-to-noise (SNR) regime while joint optimization of both the packet length and the constellation size parameters is required in the high average SNR regime, to achieve maximum throughput in a myriad of wireless fading environments.