On the design of automatic repeat request protocols for turbulent free-space optical links

The effective communications range of a free space optical link can be dramatically reduced by weather-dependent atmospheric absorption and other losses. Even in clear weather, air turbulence over a long-distance FSO link can be a limiting factor, resulting in signal fades that may be deep and persistent. High data rate communications over intermittent FSO links has proven to be a significant challenge. Techniques such as adaptive automatic gain control and forward error correction can be used to improve the channel but may not be sufficient to provide a reliable communications service. In this paper, we describe a practical automatic repeat request design that guarantees reliable delivery of packets. We investigate the performance of the proposed design through simulation studies, using models of the fading channel representative of low, moderate, and high turbulence conditions. In particular, we quantify the buffer sizes and latency required to ensure with high probability that the intermittency of the FSO channel does not impact the clients at either end of the link. We show that these requirements can be met in systems of practical interest to enable high data rate communications over poor quality FSO links.