High SNR outage analysis for power efficient multi-relay cooperative networks with ARQ assistance

In this paper we consider a multi relay Decode and Forward (DF) relay network assisted by Hybrid automatic repeat request (HARQ) in a Rayleigh block fading environment. The system uses an incremental relaying strategy in which the best relay is selected to cooperate with source. A Space-Time Block coding (STB) is used for cooperation between source and selected relay to transmit the data to the destination. Relays and destination use maximum ratio combining (MRC) technique to decode the sent message using previously received versions. We derive a closed-form expression of asymptotic outage probability at high Signal-to-Noise Ratio (SNR) regime. We also derive the average delay and throughput in a delay constrained application. The derivation method is based on the Laplace transform, which proves to be a very effective analytical tool when combined with the properties of the special functions. In addition, we investigate the optimum power allocation strategy to minimize the outage probability and consequently maximize the average throughput perceived by the end user. The simulation results show that the obtained analytical bound for the outage probability is very tight at high SNR regime. In addition the proposed power allocation scheme clearly outperforms the equal power allocation.