  A major hurdle in widespread deployment of UAVs (unmanned aerial vehicle) in
existing communications infrastructure is the limited UAV onboard energy.
Therefore, this study considers solar energy harvesting UAVs for wireless
communications. In this context, we consider three dimensional position
optimization of a solar-powered UAV relay that connects a distant sensor field
to an optical ground station (OGS) for data processing. The integrated
sensor-UAV-OGS network utilizes radio frequency band for sensor-to-UAV links
and the optical band for the UAV-to-OGS feeder link. Since atmospheric
conditions affect both the harvested solar energy as well as the optical
wireless signal, this study tackles UAV position optimization problems under
various channel conditions such as clouds, atmospheric turbulence and dirt.
From this study, we discover that the optimum position of the UAV -- that
maximizes the end-to-end channel capacity -- is heavily dependent on the
atmospheric channel conditions.
