  In this work, we address the trajectory optimization of a fixed-wing unmanned
aerial vehicle (UAV) using free space optical communication (FSOC). Here, we
focus on maximizing the flight time of the UAV by considering practical
constraints for wireless UAV communication, including limited propulsion energy
and required data rates. We find optimized trajectories in various atmospheric
environments (e.g., moderate-fog and heavy-fog conditions), while also
considering the channel characteristics of FSOC. In addition to maximizing the
flight time, we consider the energy efficiency maximization and operation-time
minimization problem to find the suboptimal solutions required to meet those
constraints. Furthermore, we introduce a low-complexity approach to the
proposed framework. In order to address the optimization problem, we conduct a
bisection method and sequential programming and introduce a new feasibility
check algorithm. Although our design considers suboptimal solutions owing to
the nonconvexity of the problems, our simulations indicate that the proposed
scheme exhibits a gain of approximately 44.12\% in terms of service time when
compared to the conventional scheme.
