A Cross-Layer Design to Improve Spectral Efficiency in Wireless Networks

A cross-layer design is developed which combines the Adaptive Modulation and Coding (AMC) at the physical layer and truncated Automatic Repeat Request (ARQ) at the data link layer to maximize the spectral efficiency under prescribed error performance constraints. Depending on the error correcting capability of the truncated ARQ, which depends on the maximum allowable number of retransmissions, we design AMC transmission modes, with each mode consisting of a specific modulation and FEC code pair as in IEEE 802.16 standard, that guarantee the required performance. Numerical results reveal that retransmissions at the data link layer relieve stringent error control requirements at the physical layer and thereby enable considerable spectral efficiency gain. This gain is provided by the maximum number of retransmissions per packet which equals the diversity order. The improvement on spectral efficiency decreases when the number of retransmissions increases, suggest that a small number of retransmissions offers a desirable delay-throughput tradeoff, in practice. The performance analysis is done in terms of outage probability also.