Line-of-sight stability in unmanned aerial vehicle relays for hybrid free-space optical and visible light communication links under atmospheric effects

Optical links supported by an unmanned aerial vehicle (UAV) must sustain gigabit-class throughput despite atmospheric attenuation, turbulence, and platform-induced pointing errors. Single-technology designs based on free-space optical (FSO) or visible light communication (VLC) often lack robustness under changing altitude and weather conditions. This paper proposes a hybrid FSO/VLC UAV relay and a unified analytical model that combines Beer-Lambert path loss, turbulence-induced scintillation, and pointing jitter, coupling an FSO backhaul to a Lambertian V1LC access channel with a finite receiver field of view (FOV). MATLAB-based results show that, under light desert dust (extinction [Formula: see text]), the FSO branch retains [Formula: see text] of the transmitted power at 1 km and [Formula: see text] at 2 km, compared to [Formula: see text] and [Formula: see text] in clear visibility ([Formula: see text]) and [Formula: see text] and [Formula: see text] in haze ([Formula: see text]). The VLC branch maintains a signal-to-noise ratio (SNR) of at least 30 dB when [Formula: see text] for altitudes below 150 m. For line-of-sight stability, with a receiver capture half-angle of [Formula: see text], the alignment probability is [Formula: see text] at a root-mean-square (RMS) pointing jitter of [Formula: see text] (stabilized) versus [Formula: see text] at [Formula: see text] (weak/no-stabilization control). These results provide practical thresholds for robust hybrid relays, including visibility-aware switching between FSO and VLC and sub-[Formula: see text] stabilization to maintain high alignment probability for smart-city and emergency scenarios.