  This paper studies an unmanned aerial vehicle (UAV)-enabled wireless powered
communication network (WPCN), in which a UAV is dispatched as a mobile access
point (AP) to serve a set of ground users periodically. The UAV employs the
radio frequency (RF) wireless power transfer (WPT) to charge the users in the
downlink, and the users use the harvested RF energy to send independent
information to the UAV in the uplink. Unlike the conventional WPCN with fixed
APs, the UAV-enabled WPCN can exploit the mobility of the UAV via trajectory
design, jointly with the wireless resource allocation optimization, to maximize
the system throughput. In particular, we aim to maximize the uplink common
(minimum) throughput among all ground users over a finite UAV's flight period,
subject to its maximum speed constraint and the users' energy neutrality
constraints. The resulted problem is non-convex and thus difficult to be solved
optimally. To tackle this challenge, we first consider an ideal case without
the UAV's maximum speed constraint, and obtain the optimal solution to the
relaxed problem. The optimal solution shows that the UAV should successively
hover above a finite number of ground locations for downlink WPT, as well as
above each of the ground users for uplink communication. Next, we consider the
general problem with the UAV's maximum speed constraint. Based on the above
multi-location-hovering solution, we first propose an efficient successive
hover-and-fly trajectory design, jointly with the downlink and uplink wireless
resource allocation, and then propose a locally optimal solution by applying
the techniques of alternating optimization and successive convex programming
(SCP). Numerical results show that the proposed UAV-enabled WPCN achieves
significant throughput gains over the conventional WPCN with fixed-location AP.
