Effective Capacity for Multi-Rate Relay Channels with Delay Constraint Exploiting Adaptive Cooperative Diversity

The effective capacity (EC) for a wireless system expresses the maximum arrival source rate that the system can transmit over the wireless channel by fulfilling certain probabilistic delay constraints. This paper presents an EC analysis for a relay-assisted cooperative protocol exploiting adaptive transmission at physical and data-link layers. In the considered model, a source employs adaptive modulation and coding (AMC) in conjunction with a cooperative ARQ protocol, to transmit delay-constrained traffic to the destination node. To achieve adaptive cooperative diversity, a relay node, also equipped with AMC, performs cooperation when it decodes the source packet correctly and the destination requested a packet retransmission. We derive a tight closed-form upper-bound expression for the EC of the queue service process at the source node when wireless links are subject to time-correlated fading. To this end, a Markov model for the relay channel is used. We also present a cross-layer approach to delay-aware AMC design for the relay channel which provides the maximum system throughput subject to a probabilistic delay constraint. The numerical and simulation results show that the proposed adaptive cooperative protocol can dramatically improve the EC and the system throughput over time-correlated fading channels when compared to a direct transmission system.