Minimizing the number of mobile chargers in a large-scale wireless rechargeable sensor network

Wireless Rechargeable Sensor Networks (WRSNs) have attracted wide attention in recent years due to their potential to eliminate energy and lifetime bottlenecks. In a typical WRSN, a Mobile Charger (MC) is employed to conduct charging on rechargeable sensor nodes, so that they can work continuously. However, since charging power, moving speed and total energy of one MC are limited, scalability becomes a major problem in such WRSNs. Therefore, multiple MCs are required in serving large-scale WRSNs. This paper tackles the problem of deciding the minimum number of MCs to keep every sensor node working continuously. We solve the problem in two steps. Firstly, we propose a greedy method for a Tour Construction Problem to construct a set of tours to 1-cover the WRSN. Energy of the sensor nodes in each of these tours can be timely replenished by one MC according to a decision condition, which is derived from a Greedy Charging Scheme (GCS). Secondly, we develop a heuristic algorithm for a Tour Assignment Problem to assign these tours to minimum number of MCs. The applied heuristic rules minimize the conflict ratio of each MC, so that it can charge along maximum number of tours. Simulation results show that, on average, the ratio of our results achieved by the two-step solution over a derived lower bound is less than 1.1.