Energy Consumption and Completion Time Tradeoff in Rotary-Wing UAV Enabled WPCN.

In this paper, we consider an unmanned aerial vehicle (UAV)-enabled wireless-powered communication network (WPCN), where a rotary-wing UAV is employed as a hybrid access point (AP) to serve multiple ground users (GUs). Specifically, the GUs harvest radio frequency (RF) energy from the signal sent by the UAV, which is then used by the GUs to power their uplink information transmission to the UAV. In practice, the mission completion time and energy consumption are two important indexes to evaluate the performance of UAV-enabled communication. To complete the mission as soon as possible, the UAV should fly above the ground users it serves at maximum speed, but this leads to more propulsion energy being consumed. Our objective is to reveal the energy-time tradeoff, characterized by the boundary of the so-called "Energy-Time" region. Wefirst derive the mathematical form of the tradeoff, the UAV trajectory, user scheduling and mission completion time, as well as the time allocation, all of which need to be jointly optimized. To this end, we propose two communication protocols: (i)fly-hover-communicate and (ii) path discretization. For each protocol, we first find the two extremes, where minimum energy consumption and minimum mission completion time are achieved. We then complete the boundary for minimizing the energy consumption for given mission completion time. Moreover, because of the nonconvexity of the problem, we propose an algorithm to obtain a locally optimal solution based on the successive convex approximation (SCA) technique for both designs. Finally, the simulation results are provided to validate the effectiveness of our study.