  Unmanned aerial vehicle (UAV)-enabled wireless power transfer (WPT) has
recently emerged as a promising technique to provide sustainable energy supply
for widely distributed low-power ground devices (GDs) in large-scale wireless
networks. Compared with the energy transmitters (ETs) in conventional WPT
systems which are deployed at fixed locations, UAV-mounted aerial ETs can fly
flexibly in the three-dimensional (3D) space to charge nearby GDs more
efficiently. This paper provides a tutorial overview on UAV-enabled WPT and its
appealing applications, in particular focusing on how to exploit UAVs'
controllable mobility via their 3D trajectory design to maximize the amounts of
energy transferred to all GDs in a wireless network with fairness. First, we
consider the single-UAV-enabled WPT scenario with one UAV wirelessly charging
multiple GDs at known locations. To solve the energy maximization problem in
this case, we present a general trajectory design framework consisting of three
innovative approaches to optimize the UAV trajectory, which are multi-location
hovering, successive-hover-and-fly, and time-quantization-based optimization,
respectively. Next, we consider the multi-UAV-enabled WPT scenario where
multiple UAVs cooperatively charge many GDs in a large area. Building upon the
single-UAV trajectory design, we propose two efficient schemes to jointly
optimize multiple UAVs' trajectories, based on the principles of UAV swarming
and GD clustering, respectively. Furthermore, we consider two important
extensions of UAV-enabled WPT, namely UAV-enabled wireless powered
communication networks (WPCN) and UAV-enabled wireless powered mobile edge
computing (MEC).
