UAV-Enabled Wireless Power Transfer A Tutorial Overview
Abstract : 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-andfly, 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), by integrating the emerging WPCN and MEC techniques, respectively. For both cases, we investigate the UAV trajectory design jointly with communication/computation resource allocations to optimize the system performance, subject to the energy availability constraints at GDs. Finally, open problems in UAV-enabled WPT and promising directions for its future research are discussed.
? In conventional WPT systems, dedicated energy transmitters (ETs) are usually deployed at fixed locations to send RF signals to charge distributed energy receivers (ERs)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.
? To our best knowledge, this work is the first that explores the UAV’s trajectory design for WPT performance optimization.
? The total flying time is minimized by finding the path with the shortest traveling distance to visit all of these hovering locations.
? It is worth noticing that the trajectory optimization problem (P1) for the single UAV-enabled WPT is different from that for the UAV-enabled multiuser wireless communications .
? In this regard, how to maintain the sustainable operation of these lowpower devices is becoming a more practically important as well as challenging problem to tackle.
? We first use a toy example with one single GD to show the benefit of trajectory design, and then present a generic utility maximization problem to maximize the energy amounts transferred to multiple GDs in a fair manner subject to practical UAV flight constraints.
? We can directly adopt time quantization to transform problem (P2.2) with continuous-time variables into an equivalent optimization problem with discrete-time variables.
• Various approaches have been proposed aiming to alleviate this issue by enhancing the WPT efficiency at the link level, including multi-antenna energy beamforming, energy scheduling, and energy waveform optimization.
• It is observed that the two proposed trajectory designs, namely the successive hover-and-fly and the SCP-based trajectory designs, outperform the singlelocation-hovering design, and achieve higher average max-min average power as T becomes large.
• When T = 15 s, the proposed successive hover-and-fly and the SCP-based trajectory designs also outperform the successive hover-and-fly trajectory over all the ERs.
• Motivated by UAV-assisted wireless communications, in this paper we propose a new UAVenabled WPT architecture .
? We investigate the UAV trajectory design jointly with communication/computation resource allocations to optimize the system performance, subject to the energy availability constraints at GDs.
? There have been a handful of prior works in the literature that investigated the trajectory design for enhancing the energy transfer performance for UAV-enabled WPT when there is only one single UAV.
? The utility/objective functions for communication versus WPT performance optimization are also different in general.
? It is observed that the SHF and time-quantization-based trajectory designs significantly outperform the static hovering scheme and the performance gain becomes more significant when T increases.
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