Performance improvement of FSO communication systems using hybrid-ARQ protocols

In release 16 of the third-generation project partnership, it has been suggested that hybrid-automatic repeat request (H-ARQ) protocols can be used to ensure robust communications. In this paper, we incorporate H-ARQ in the free-space optical (FSO) communication systems to improve their performance and communication link reliability. We derive novel, to the best of our knowledge, closed-form expressions for the outage probability of the FSO systems under the combined impact of gamma-gamma distributed atmospheric turbulence and pointing errors (PEs) for heterodyne detection and intensity modulation and direct detection techniques. This is accomplished using various H-ARQ protocols, such as, at least once, chase combing, and incremental redundancy (INR). Further, using outage probability expressions, we calculate throughput for the FSO systems using H-ARQ protocols. Useful insights into the FSO system performance are obtained through the asymptotic analysis for the considered H-ARQ protocols and detection techniques. It is observed through the numerical results that H-ARQ with INR performs better than the other considered H-ARQ protocols. We also draw a comparison between the outage performance of an FSO system with boresight PE and without boresight PE.