Wireless sensor networks are traditionally constrained by finite energy supply. Recent wireless charging technology enables possibility for timely replenishing consumed energy of dispersed sensors. This paper investigates on-demand wireless charging problem with multiple mobile chargers, which can sustain a large-scale wireless sensor network operation perpetually without knowing energy consumption information of sensors a priori. We evaluate the nearest-first and recent-rarestfirst strategies, which schedule multiple chargers for incoming charging requests. Also four synthesized occurrence processes of charging requests are presented and then used to compare scheduling strategies in terms of concurrent charging load and serving latency. Simulation results show that the nearest-first scheduling strategy suits for uniform charging request occurrence processes, while the recent-rarest-first scheduling strategy is better for localized charging request occurrence processes.
I. INTRODUCTIONWireless sensor networks have been intensively investigated and prototyped in the past decade. Conventionally, the sensors are powered by batteries or super-capacitors. As the sensors monitor the surrounding environment, relevant data would be generated. Afterwards, such data is forwarded in a hop-by-hop manner towards the sink that would make the application-specific processing. Energy constraint imposed by finite-capacity batteries and super-capacitors are increasingly hindering the practical deployment of sensor networks. It is desirable for wireless sensor networks to operate perpetually.Energy replenishment is a promising approach. For instance, harvesting energy from the environment, such as solar energy, wind, and vibration, has been studied in the past. Yet the harvested energy is almost always in a very low rate and normally cannot sustain continuous operation of the individual sensors. Furthermore, the harvested energy rate often varies considerably and unpredictably, which further limits the workload of individual sensors. Recently, wireless charging technology opens new opportunities for mitigating the energy constraint of sensors. In particular, there are three kinds of concrete implementation: inductive coupling, electromagnetic radiation, and magnetic resonant coupling. Inductive coupling can only function in several centimeters, and hence is not suitable for wireless charging individual sensors. Electromagnetic radiation and magnetic resonant coupling have a maximum charging range of 1-3 meters, and usually the wireless chargers are mounted on mobile vehicles in order to charge large-scale geographically distributed sensors practically.Nearly all mobile charger based sensor charging studies focus on offline programming, where the charger moves to