On Maximizing Min Flow Rates in Rechargeable Wireless Sensor Networks

In a rechargeable wireless sensor network (rWSN), the amount of data forwarded by source nodes to one or more sinks is bounded by the energy harvesting rate of sensor nodes. To improve sensing quality, we consider a novel approach whereby we place a finite number of auxiliary chargers (ACs) with wireless power transfer and energy harvesting ability to boost the energy harvesting rate of some sensor nodes. We formulate a mixed integer linear program (MILP) to determine the subset of nodes that if upgraded will maximize the minimum source rate. We also propose two heuristic algorithms to place ACs in large-scale rWSNs: greedy node deployment (GND), which checks every nonupgraded sensor node and places an AC next to the one yielding the highest increase in max-min rate; and one-unit energy deployment (OUED), which uses a relaxed version of the MILP to first share one unit of energy among sensor nodes. It then upgrades the sensor node with the highest one-unit share. Our results show that the max-min rate obtained by GND and OUED is, respectively, within 99.60% and 97.82% of the max-min rate derived by MILP in small networks with at most 90 nodes. In large networks with 200 nodes, the maximum gap between OUED and GND is only 0.191 kb/s. Lastly, OUED runs at least five times faster than GND.