  This letter studies an unmanned aerial vehicle (UAV) aided multicasting (MC)
system, which is enabled by simultaneous free space optics (FSO) backhaul and
power transfer. The UAV applies the power-splitting technique to harvest
wireless power and decode backhaul information simultaneously over the FSO
link, while at the same time using the harvested power to multicast the
backhauled information over the radio frequency (RF) links to multiple ground
users (GUs). We derive the UAV's achievable MC rate under the Poisson point
process (PPP) based GU distribution. By jointly designing the FSO and RF links
and the UAV altitude, we maximize the system-level energy efficiency (EE),
which can be equivalently expressed as the ratio of the UAV's MC rate over the
optics base station (OBS) transmit power, subject to the UAV's sustainable
operation and reliable backhauling constraints. Due to the non-convexity of
this problem, we propose suboptimal solutions with low complexity. Numerical
results show the close-to-optimal EE performance by properly balancing the
power-rate tradeoff between the FSO power and the MC data transmissions.
