Quantifying Impact of Pointing Errors on Secrecy Performance of UAV-Based Relay-Assisted FSO Links

This article presents accurate approximation expressions for the outage and secrecy outage probabilities of relay-assisted free-space optical (FSO) communication links utilizing unmanned aerial vehicles (UAVs). We consider the effects of weather attenuation and random fluctuations of UAVs' orientations and positions. The obtained expressions are applicable to different types of intensity modulation direct detection techniques. We utilized $L$ -ary multipulse pulse-position modulation $(L$ MPPM) scheme and optimized link performance by adjusting the $L$ MPPM settings using our derived expressions. We conduct numerical analyses to evaluate the impact of system settings, including beam divergence and modulation settings, on outage and secrecy outage probabilities. Our results demonstrate that adaptive $L$ MPPM can improve average spectral efficiency, particularly in time-varying UAV-based FSO channels, while maintaining the same outage and secrecy outage probabilities as ordinary multipulse pulse-position modulation. Moreover, we investigate the effect of an eavesdropper's location on the probability of a secure channel between legitimate users. We validate the accuracy of our expressions by comparing them with Monte Carlo (MC) simulation results under different channel conditions. Derived expressions provide efficient and accurate evaluation results to optimize the performance of UAV-based relay-assisted FSO communication systems.