Outage performance of DF relay-assisted FSO communications using time diversity

Outage probability as a performance measure for a decode-and-forward (DF) relaying scheme based on repetition coding with the relay is presented in the context of free-space optical (FSO) communication systems with intensity modulation and direct detection over atmospheric turbulence and misalignment fading channels. Novel closed-form asymptotic expressions for the outage probability are derived for a three-way FSO communication setup when the irradiance of the transmitted optical beam is susceptible to moderate-to-strong turbulence conditions, following a gamma-gamma distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size, and jitter variance is considered. Fully exploiting the potential time-diversity available in the relay turbulent channel, a greater robustness to the relay location and pointing errors is achieved, maintaining a high diversity gain regardless of the source-destination link distance and remarkably superior to that of the two-transmitter case as well as the conventional DF scheme. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.