UAV-Enabled Wireless Power Transfer: Trajectory Design and Energy Optimization

This paper studies a new unmanned aerial vehicle (UAV)-enabled wireless power transfer system, where a UAV-mounted mobile energy transmitter is dispatched to deliver wireless energy to a set of energy receivers (ERs) at known locations on the ground. We investigate how the UAV should optimally exploit its mobility via trajectory design to maximize the amount of energy transferred to all ERs during a finite charging period. First, we consider the maximization of the sum energy received by all ERs by optimizing the UAV's trajectory subject to its maximum speed constraint. Although this problem is non-convex, we obtain its optimal solution, which shows that the UAV should hover at one single fixed location during the whole charging period. However, the sum-energy maximization incurs a “near-far” fairness issue, where the received energy by the ERs varies significantly with their distances to the UAV's optimal hovering location. To overcome this issue, we consider a different problem to maximize the minimum received energy among all ERs, which, however, is more challenging to solve than the sum-energy maximization. To tackle this problem, we first consider an ideal case by ignoring the UAV's maximum speed constraint, and show that the relaxed problem can be optimally solved via the Lagrange dual method. The obtained trajectory solution implies that the UAV should hover over a set of fixed locations with optimal hovering time allocations among them. Then, for the general case with the UAV's maximum speed constraint considered, we propose a new successive hover-and-fly trajectory motivated by the optimal trajectory in the ideal case and obtain efficient trajectory designs by applying the successive convex programing optimization technique. Finally, numerical results are provided to evaluate the performance of the proposed designs under different setups, as compared with benchmark schemes.

I. Introduction

Radio frequency (RF) transmission enabled wireless power transfer (WPT) is a promising solution to provide perpetual and cost-effective energy supplies to low-power electronic devices, and it is anticipated to have abundant applications in future Internet-of-things (IoT) wireless networks (see, [3], [4] and the references therein). In conventional WPT systems, dedicated energy transmitters (ETs) are usually deployed at fixed locations to send RF signals to charge distributed energy receivers (ERs) [5] such as low-power sensors or IoT devices. However, due to the severe propagation loss of RF signals over long distance, the performance of practical WPT systems for wide coverage range is fundamentally constrained by the low end-to-end power transmission efficiency. As a consequence, in order to provide ubiquitous wireless energy accessibility for massive low-power ERs distributed in a large area, fixed-location ETs need to be deployed in an ultra-dense manner. This, however, would tremendously increase the cost, and thus hinder the large-scale implementation of future WPT systems. In the literature, various approaches have been proposed aiming to alleviate this issue by enhancing the WPT efficiency at the link level, including multi-antenna energy beamforming [6]–[12], energy scheduling [13], [14], and energy waveform optimization [15], [16]. Different from these prior studies, in this paper we tackle this problem from a fundamentally new perspective at the system level, i.e., we propose a radically novel architecture for WPT systems by utilizing unmanned aerial vehicles (UAVs) as mobile ETs.