  Relay-assisted free-space optical (FSO) communication systems are exploited
as a means to mitigate the limiting effects of the turbulence induced
atmospheric scintillation. However, conventional ground relays are stationary,
and their optimal placement is not always feasible. Due to their mobility and
flexibility, unmanned aerial vehicles (UAVs) provide new opportunities for FSO
relaying systems. In this paper, a hovering UAV-based serial FSO
decode-and-forward relaying system is investigated. In the channel modelling
for such a system, four types of impairments (i.e., atmospheric loss,
atmospheric turbulence, pointing error, and link interruption due to
angle-of-arrival fluctuation) are considered. Based on the proposed channel
model, a tractable expression for the probability density function of the total
channel gain is obtained. Closed-form expressions of the link outage
probability and end-to-end outage probability are derived. Asymptotic outage
performance bounds for each link and the overall system are also presented to
reveal insights into the impacts of different impairments. To improve system
performance, we optimize the beam width, field-of-view and UAVs' locations.
Numerical results show that the derived theoretical expressions are accurate to
evaluate the outage performance of the system. Moreover, the proposed
optimization schemes are efficient and can improve performance significantly.
