Secrecy Analysis of H-ARQ Based Free-Space Optical Communication Systems

Nowadays, the hybrid automatic retransmission request (H-ARQ) protocols are prevalent in fifth-generation (5G) and beyond communications. The utilization of H-ARQ protocols is extended to free-space optical (FSO) communication systems to achieve a reliable link. In this paper, we investigate the secrecy performance of chase combining (CC) H-ARQ protocol in the presence of an eavesdropper. We present the closed-form expressions of the average secrecy capacity (ASC) and secrecy outage probability (SOP) of FSO systems over atmospheric turbulence-induced fading channels and different rounds of H-ARQ transmission. We also derive the conditional SOP and secrecy throughput of the considered FSO system. The expressions are valid for weak to strong atmospheric turbulence, intensity modulation direct detection, and different number of H-ARQ rounds. We have also validated our proposed results using Monte Carlo simulations.