  This paper studies a new unmanned aerial vehicle (UAV)-enabled wireless power
transfer (WPT) system, where a UAV-mounted energy transmitter (ET) broadcasts
wireless energy to charge distributed energy receivers (ERs) on the ground. In
particular, we consider a basic two-user scenario, and investigate how the UAV
can optimally exploit its mobility to maximize the amount of energy transferred
to the two ERs during a given charging period. We characterize the achievable
energy region of the two ERs, by optimizing the UAV's trajectory subject to a
maximum speed constraint. We show that when the distance between the two ERs is
smaller than a certain threshold, the boundary of the energy region is achieved
when the UAV hovers above a fixed location between them for all time; while
when their distance is larger than the threshold, to achieve the boundary of
the energy region, the UAV in general needs to hover and fly between two
different locations above the line connecting them. Numerical results show that
the optimized UAV trajectory can significantly improve the WPT efficiency and
fairness of the two ERs, especially when the UAV's maximum speed is large
and/or the charging duration is long.
