A cross-layer approach to mitigate fading on bidirectional free space optical communication links

In free-space optical (FSO) communications the transmitted signal is subject to different fading effects. These effects can cause short-term outages of a few milliseconds, caused by atmospheric turbulence-induced fading, and long-term outages up to a few seconds duration caused by line of sight obstructions or pointing errors. To mitigate these effects, several different approaches have been presented in the past. At the physical layer, forward-error correction (FEC), dynamic thresholding, and time-delayed diversity (TDD) have been shown to be reasonable solutions. At higher layers, FEC has also been demonstrated to be a possible solution, but it imposes a penalty on channel throughput. For bidirectional communications, automatic repeat request (ARQ) protocols are proposed to be a more efficient solution. In this paper we will investigate physical layer delay diversity as well as link layer FEC and ARQ.