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

## TL;DR

This paper proposes a hybrid free-space optical and visible light communication system using a drone to maintain stable high-speed communication despite weather and movement challenges.

## Contribution

The paper introduces a hybrid FSO/VLC UAV relay and a unified analytical model for stability under atmospheric effects.

## Key findings

- The FSO branch retains significant power under desert dust and haze conditions.
- The VLC branch maintains high SNR at low altitudes with limited field of view.
- Stabilization below 10 μrad is critical for high alignment probability in hybrid relays.

## Abstract

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 κ≈0.35 km−1), the FSO branch retains ≈70.5% of the transmitted power at 1 km and ≈49.7% at 2 km, compared to ≈81.9% and ≈67.0% in clear visibility (κ=0.2 km−1) and ≈49.7% and ≈24.7% in haze (κ=0.7 km−1). The VLC branch maintains a signal-to-noise ratio (SNR) of at least 30 dB when FOV≤20∘ for altitudes below 150 m. For line-of-sight stability, with a receiver capture half-angle of 20 μrad, the alignment probability is ≈99.97% at a root-mean-square (RMS) pointing jitter of 5 μrad (stabilized) versus ≈58.9% at 15 μrad (weak/no-stabilization control). These results provide practical thresholds for robust hybrid relays, including visibility-aware switching between FSO and VLC and sub-10 μrad stabilization to maintain high alignment probability for smart-city and emergency scenarios.