Outage analysis of Hybrid-ARQ protocols in coherent free-space optical communications

In this paper, we investigate the performance of automatic retransmission request (ARQ) protocols in coherent free-space optical (FSO) communications. Specifically, we study the outage performance of three different Hybrid-ARQ (H-ARQ) protocols and characterize their performance at large values of signal-to-noise ratio (SNR) through diversity and coding gains. Our results demonstrate that significant performance gains can be achieved by the deployment of H-ARQ in coherent FSO communications especially in strong turbulence regime. Our asymptotic outage analysis further demonstrates that all protocols provide the same diversity order (which is the product of the maximum number of ARQ rounds and the minimum of the channel parameters) while they provide different coding gains.