Efficient Charging of Access Limited Wireless Underground Sensor Networks

Wireless underground sensor networks (WUSNs) present a variety of new research challenges. Magnetic induction (MI)-based transmission has been proposed to overcome the very harsh propagation conditions in underground communications in recent years. In this approach, induction coils are utilized as antennas in the sensor nodes. This solution achieves larger transmission ranges compared with the traditional electromagnetic wave-based approach. In the past, some efforts have been made to characterize the signal transmission in MI-WUSNs. Those investigations, however, mostly refer to the information transmission. One of the open issues that may constrain the system design in some of the applications is the powering of the individual sensor nodes. Due to the low accessibility of the nodes, a new method of wireless power transfer for MI-WUSNs is proposed in this paper. This method is mainly based on simultaneous signal transmissions from multiple sensor nodes with optimized signal constellations. Furthermore, the optimal scheduling for power transmission and reception is provided, which maximizes the energy efficiency of the network charging procedure. The proposed method is compared with the naive approaches and shows a significant improvement of the system performance in terms of energy efficiency.