Assigning Hierarchy to Collaborative Mobile Charging in Sensor Networks

Wireless power transfer is used to fundamentally address energy management problems in Wireless Rechargeable Sensor Networks (WRSNs). In such networks, mobile entities traverse the network and wirelessly replenish the energy of sensor nodes. In recent research on collaborative mobile charging, the mobile entities are also allowed to charge each other. In this chapter, we enhance the collaborative feature by forming a hierarchical charging structure. We distinguish the Chargers in two groups, the hierarchically lower, called Mobile Chargers, that charge sensor nodes and the hierarchically higher, called Special Chargers, that charge Mobile Chargers. We define the Coordination Decision Problem and prove that it is NP-complete. Also, we propose a new protocol for 1-D networks which we compare with a state-of-the-art protocol. Motivated by the improvement in 1-D networks, we design four new collaborative charging protocols for 2-D networks to achieve efficient charging and improve important network properties. Our protocols are either centralized or distributed, and assume different levels of network knowledge. Extensive simulation findings demonstrate significant performance gains, with respect to non-collaborative state-of-the-art charging methods. In particular, our protocols improve several network properties and metrics, such as the network lifetime, routing robustness, coverage, and connectivity. A useful feature of our methods is that they can be suitably added on top of non-collaborative protocols to further enhance their performance.