DF relayed FSO communication systems with time dispersion over Gamma Gamma turbulence and misalignment

Free Space Optical (FSO) communications have recently attracted a renewed commercial and research interest. However, the performance of such terrestrial wireless optical links is strongly degraded by the pointing errors effect between the trans-receivers terminals and by the atmospheric turbulence effect whose impact becomes even more detrimental by the propagation distance. In order to combat this combined distance-dependent turbulence-induced and misalignment-induced fading we often resort to place relays along the line-of-sight propagation path. Thus, intermediate shorter hops are created which can lead to significant performance improvements. Additionally, by adjusting as information bit carriers, negative chirped Gaussian pulses with narrow initial pulse width, the also distant-dependent time dispersion effect can further ameliorate the FSO performance for some initial, i.e. hop distance. In view of the above, a typical terrestrial FSO system is considered that may employ serial Decode-and-Forward (DF) relaying configurations over a wide turbulence range, modeled through the very accurate Gamma-Gamma distribution model, along with the presence of time dispersion effect and different amounts of pointing mismatch. Under these assumptions, the performance of the FSO system is evaluated by means of its probability of fade metric. Closed-form expressions for the probability of fade of such DF relay-assisted FSO systems are derived, which along with the numerical results, demonstrate the beneficial impact on the FSO performance due to DF relays employment and time dispersion effect, under specific link's characteristics.