Performance Characterization of Rotary Wing UAV-Mounted FSO Links in the Presence of Pointing Errors

Non-terrestrial networks (NTNs) involve the use of unmanned aerial vehicles (UAVs), high-altitude platform stations (HAPSs), and low-earth orbit satellites (LEOs) and have emerged as a powerful tool to enable global connectivity. With their flexible deployment, rotary-wing UAVs are particularly useful to deliver versatile airborne wireless access and backhaul in areas where there is limited terrestrial infrastructure. In this paper, we consider a scenario where free space optical (FSO) terminals are mounted on rotary wing UAV to provide high capacity wireless backhaul for ground base stations. Due to the narrow divergence angle of laser tranmsitters, FSO systems are prone to pointing errors. In our study, we classify and analyze two primary sources of pointing errors in rotary-wing UAV-based links: horizontal movement and rotational stability. The former stems from the semi-fixed hovering nature of UAVs, introducing uncertainties in horizontal displacement that influence the positioning of the received beam along the x and y directions. Additionally, rotational stability issues in rotary-wing UAVs, including rolling, yawing, and pitching, lead to shifts in the center of the received beam. We classify these pointing errors as altitude-dependent and altitude-independent and analyze their distinct impacts on the bit error rate (BER) of UAV-based FSO links. Our findings contribute to a better understanding of pointing errors in UAV-based FSO communication, paving the way for enhanced airborne connectivity solutions.