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The rapid development of wireless power transfer technology brings forth innovative vehicle energy solutions and breakthroughs utilizing wireless sensor networks (WSNs). In most existing schemes, wireless rechargeable sensor networks (WRSNs) are generally equipped with one or more wireless charging vehicles (vehicles) to serve sensor nodes (SNs). These schemes solve the energy issue to some extent; however, due to off-road and speed limitations of vehicles, some SNs still cannot be charged in time, negatively affecting the lifetime of the networks. Our work proposes a new WRSN model equipped with one wireless charging drone (drone) with a constrained flight distance coupled with several wireless charging pads (pads) deployed to charge the drone when it cannot reach the subsequent stop. Our model solves this charging issues effectively and overcomes energy capacity limitations of the drone. Thus, a wireless charging pad deployment problem is formulated, which aims to apply the minimum number of pads so that at least one feasible routing path can be established for the drone to reach every SN in a given WRSN. Four feasible heuristics, three based on graph theory and one on geometry, are proposed for this problem. In addition, a novel drone scheduling algorithm, the shortest multi-hop path algorithm, is developed for the drone to serve charging requests with the assistance of pads. We examine the proposed schemes through extensive simulations. The results compare and demonstrate the effectiveness of the proposed schemes in terms of network density, region size and maximum flight distance. INDEX TERMS Wireless power transfer, wireless charging drone, wireless charging pad, sensor node, wireless rechargeable sensor networks. WEN OUYANG was born in Hsinchu, Taiwan. She received the Ph.D. degree in computer science from The University of Texas at Dallas, in 1993. From then on, she worked as a Member of Scientific Staff and a Senior Member of Scientific Staff with Bell Northern Research, later became Nortel Networks, where she was involved in large-scale telecommunication systems research and development.

As a special kind of attack, node capture attack in wireless sensor networks destroys the confidentiality and integrity of network traffic by physically capturing sensor nodes and extracting the cryptographic keys for eavesdropping the communications. However, previous researches pay little attention to the attacking efficiency, which leads to big attacking times and long execution time. In this paper, we propose a graph-based method for modeling the node capture attack on random key pre-distribution scheme. A full graph is constructed to specialize the compromising relationships between nodes and paths, and then the way of evaluating the destructiveness is introduced. The full graph attack (FGA) is designed to ensure the maximum destructiveness of each captured node so as to reduce the attacking times and shorten the execution time, simultaneously. To promote the performance of FGA in attacking efficiency, two optimal attacking algorithms, namely opti-graph attack (OGA) and path covering attack (PCA), are developed. Simulation results show that FGA, OGA and PCA reduce 38, 10 and 51 % attacking times, and enhance 39, 11 and 40 % attack-B Chi Lin 123 C. Lin et al.ing efficiency than traditional node capture attack methods. Moreover, they save 39, 11 and 50 % energy in compromising the networks.

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