On Optimal Scheduling of Multiple Mobile Chargers in Wireless Sensor Networks

The limited battery capacities of sensor nodes have become the biggest impediment to the applications of wireless sen-sor networks (WSNs) over the years. Recent breakthroughs in wireless energy transfer-based rechargeable batteries pro-vide a promising application of mobile vehicles in WSNs. These mobile vehicles act as mobile chargers to transfer en-ergy wirelessly to static sensors in an efficient way. In this paper, we study the mobile charger coverage problem of sen-sor nodes distributed on a 1-dimensional line and ring. Each sensor needs to be recharged at a given frequency. A mobile charger can charge a sensor after it moves to the location of the sensor. We assume that the mobile charger has an unlimited charging capability, moves at a speed subject to a given limit, and that the charging time is negligible. An optimization problem is then presented on a time-space cov-erage of sensors so that none of them will run out of energy: (1) What is the minimum number of mobile chargers need-ed? (2) Given the minimum number of mobile chargers, how should these mobile chargers be scheduled in terms of trajectory planning? Given homogeneous sensors with the same recharging frequency, we provide an optimal solution with a linear complexity in finding the minimum number of charges, as well as the actual schedule. We then examine an extension to heterogeneous sensors and provide a greedy ap-proach that has a constant ratio of 2 to the optimal solutions for a line and ring. Extensive simulations are conducted to verify the competitive performance of the proposed scheme