Cooperative Wireless Charging Vehicle Scheduling

Recent breakthroughs in wireless energy transfer-based rechargeable batteries enable a promising application of Wireless Charging Vehicles (WCVs) in Wireless Rechargeable Sensor Networks (WRSNs). This paper studies cooperative WCV schedules in WRSNs to optimize sensor recharging. The objective is to minimize the number of WCVs under the constraint that all sensors must be periodically recharged before running out of energy (i.e., before lifetime). Our problem is NP-hard and is very challenging due to the complexity of WCV route schedules. WCVs can be used to recharge sensors in turn. Our problem is thoroughly explored in line, cycle, and metric spaces (such as a three-dimensional Euclidean space). In terms of line and cycle spaces, greedy algorithms with ratios of 2 and 4, respectively, are proposed. By exploring two WCV schedule patterns, the optimal algorithm is found for the cycle space when sensor lifetimes are identical. For the metric space with an identical sensor lifetime, an algorithm with a ratio of 2.5 is proposed through constructing the minimum distance forest among sensors. It is also extended to the metric space with non-identical sensor lifetimes by grouping sensors according to their lifetimes. Finally, real data-driven experiments demonstrate the efficiency and effectiveness of the proposed approximation algorithms.