Resolving Collisions Via Incremental Redundancy: ARQ Diversity

A cross-layer approach is adopted for the design of finite-user symmetric random access wireless systems. Instead of the traditional collision model, a more realistic physical layer model is adopted. An incremental redundancy automatic repeat request (IR-ARQ) scheme, tailored to jointly combat the effects of user collisions, multi-path fading, and channel noise, is proposed. The diversity-multiplexing-delay tradeoff of the proposed scheme is analyzed for fully-loaded queues, and compared with that of the Gallager tree algorithm for collision resolution and the network-assisted diversity multiple access (NDMA) protocol of Tsatsanis et at.. The fully-loaded queue model is then replaced by one with random arrivals, where the three protocols are compared in terms of the stability region and average delay. Overall, our analytical and numerical results establish the superiority of the proposed IR-ARQ scheme and reveal some important insights. For example, it turns out that the performance is optimized, for a given total throughput, by maximizing the probability that a certain user will send a new packet and minimizing the transmission rate employed by each user.