Adaptive Flight Control of Fixed-Wing UAVs for Enhanced Optical Signal Line-of-Sight Maintenance

Uncrewed aerial vehicles (UAVs) based wireless free space optics communication (FSOC) is one of the strong candidate 6G technologies for 3rd Generation Partnership Project (3GPP) standardization. For reliable FSOC, maintenance of the optical signal’s line of sight (LoS) between UAVs and ground terminals (GTs) is important as it has a significant impact on the link quality and throughput. Previous studies have considered various factors that affect LoS in FSOC, including atmospheric attenuation, turbulence, and pointing error loss. However, practical aspects of the UAV physics and flight characteristics have not been properly considered. Among UAV types, fixed-wing UAVs have many advantages, which make them a strong candidate for FSOC. Low altitudes up to 1 km provide the best channel characteristics as UAVs fly closest to the GTs and are the easiest to deploy and operate, which is why fixed-wing UAVs that serve as low altitude platforms (LAPs) are the focus in this paper. The details of fixed-wing UAVs, including flight characteristics as well as physical limitations, are considered in the proposed LoS maintenance control scheme, which supports the FSOC link between the UAV and GT. The objective of the proposed scheme is to formulate an optimal control-based flight trajectory that minimizes the energy consumption while enhancing the communication throughput. This optimal control is designed based on the UAV’s three-dimensional (3D) position, attitude (i.e., roll, pitch, yaw), as well as its kinematic and dynamic physics. The UAV’s flight trajectory needs to consider these practical physical constraints to maintain LoS between the UAV and GT FSOC transceivers. The simulation experiment results show that the energy consumption remains similar, while the proposed scheme eliminates throughput degradation caused by LoS loss. This demonstrates that the proposed FSOC LoS maintenance (FLM) scheme can provide an improved performance compared to the convex area energy-efficient trajectory optimization (CA2ET) control scheme.