  In this paper, we consider an unmanned aerial vehicle (UAV) aided mobile
relaying system under a buffer constraint. We propose a new relaying protocol
employing mixed free-space optical/radio frequency (FSO/RF) communication,
i.e., the source-relay and relay-destination links utilize FSO and RF links,
respectively, under the buffer constraint at the UAV relay node. Taking the
conditions of an imbalance in transmission rate between RF and FSO links into
consideration, we study the trajectory optimization problem of
buffer-constrained UAV relay node in order to maximize the end-to-end data
throughput. Especially, we classify two relaying transmission schemes according
to the delay requirements, i.e., i) delay-limited transmission and ii)
delay-tolerant transmission. We solve the locally optimal trajectory problem of
the UAV to maximize the throughput of ground user terminal. As a result, we
propose an iterative algorithm that efficiently finds a local optimum solution
for the throughput maximization problems. Through this algorithm, we present
the resulting trajectories over the the atmospheric condition, the buffer size,
and the delay requirement. Also, we show the optimum buffer size and the
throughput-delay tradeoff for a given system. Our numerical results validate
that the proposed buffer-aided mobile relaying scheme achieves 65.55%
throughput gains compared to conventional static relaying scheme.
