  Unmanned aerial vehicle (UAV)-based communications is a promising new
technology that can add a wide range of new capabilities to the current network
infrastructure. Given the flexibility, cost-efficiency, and convenient use of
UAVs, they can be deployed as temporary base stations (BSs) for on-demand
situations like BS overloading or natural disasters. In this work, a UAV-based
communication system with radio frequency (RF) access links to the mobile users
(MUs) and a free-space optical (FSO) backhaul link to the ground station (GS)
is considered. In particular, the RF and FSO channels in this network depend on
the UAV's positioning and (in)stability. The relative position of the UAV with
respect to the MUs impacts the likelihood of a line-of-sight (LOS) connection
in the RF link and the instability of the hovering UAV affects the quality of
the FSO channel. Thus, taking these effects into account, we analyze the
end-to-end system performance of networks employing UAVs as buffer-aided (BA)
and non-buffer-aided (non-BA) relays in terms of the ergodic sum rate.
Simulation results validate the accuracy of the proposed analytical derivations
and reveal the benefits of buffering for compensation of the random
fluctuations caused by the UAV's instability. Our simulations also show that
the ergodic sum rate of both BA and non-BA UAV-based relays can be enhanced
considerably by optimizing the positioning of the UAV. We further study the
impact of the MU density and the weather conditions on the end-to-end system
performance.
