  In this paper, we investigate the statistics of the free space optics (FSO)
communication channel between a hovering unmanned aerial vehicle (UAV) and a
central unit. Two unique characteristics make UAV-based FSO systems
significantly different from conventional FSO systems with static transceivers.
First, for UAV-based FSO systems, the incident laser beam is not always
orthogonal to the receiver lens plane. Second, both position and orientation of
the UAV fluctuate over time due to dynamic wind load, inherent random air
fluctuations in the atmosphere around the UAV, and internal vibrations of the
UAV. On the contrary, for conventional FSO systems, the laser beam is always
perpendicular to the receiver lens plane and the relative movement of the
transceivers is limited. In this paper, we develop a novel channel model for
UAV-based FSO systems by quantifying the corresponding geometric and
misalignment losses (GML), while taking into account the non-orthogonality of
the laser beam and the random fluctuations of the position and orientation of
the UAV. In particular, for diverse weather conditions, we propose different
fluctuation models for the position and orientation of the UAV and derive
corresponding statistical models for the GML. We further analyze the
performance of a UAV-based FSO link in terms of outage probability and ergodic
rate and simplify the resulting analytical expressions for the high
signal-to-noise ratio (SNR) regime. Finally, simulations validate the accuracy
of the presented analysis and provide important insights for system design. For
instance, we show that for a given variance of the fluctuations, the beam width
should be properly adjusted to minimize the outage probability.
