VCEC: Velocity Control of EnergyConstrained RFBased Wireless Charger in Sensor Networks with MultiDepots Deployment RF energy transfer, as the main farfield wireless energy transfer technology in wireless sensor networks, allows the relatively long charging distance from wireless charger to sensor nodes. Existing charging schemes based on a mobile RF energy charger neglect the energy consumption of the charger and its limited battery capacity. Motivated by this, we consider the practical charging scenario where energyconstrained mobile charger (MC) travels along a constrained long trajectory in the network area to wirelessly power the sensors, with multiple depots (for the energy provision of MC) deployed on the trajectory to achieve high energy efficiency. In this paper, we introduce VCEC, a VelocityControl scheme of Energyconstrained mobile Charger to maximize the minimum charged energy in nodes after MC passes through the whole trajectory. Specifically, we first simplify the initial velocitycontrol problem to a tractable one by discretizing the trajectory into segments and propose a distributed algorithm to solve it. Then, we present a segment merging algorithm for the realworld applications. Our VCEC scheme achieves an approximation ratio of (1xcex98)(1 e)1. Simulations and testbed experiments are conducted to show that VCEC promotes the bottleneck nodeu0027s charged energy by at least 20% as compared to the baseline scheme where MC moves at a constant speed.

Optimal Charger Placement for Wireless Power Transfer Abstract   As a promising technology to achieve perpetual operation of batterypowered wireless sensor devices, wireless power transfer has attracted much attention recently. In wireless power transfer, the charger enables the energy to be wirelessly transmitted to the rechargeable sensor devices that are hungry for energy. Previous works mainly focus on maximizing the charging utility or minimizing the charging delay. This paper concerns two more practical issues of placing wireless chargers, the first one aims at minimizing the deployment cost of chargers while satisfying the overall requirement for charging levels, and the second one aims at maximizing the total charging levels subject to a deployment cost budget constraint. We investigate the above two optimal charger placement problems under two typical scenarios in which omni chargers and directional chargers are used, respectively. To resolve these two problems under the two charging models, we first prove their NPhardness and then propose four approximation algorithms with proven performance guarantees. Finally, we conduct extensive simulation experiments to validate our designs, and the experimental results demonstrate that the proposed algorithms significantly outperform the baselines.