Wireless underground sensor networks (WUSNs) can enable many important applications such as intelligent agriculture, pipeline fault diagnosis, mine disaster rescue, concealed border patrol, crude oil exploration, among others. The key challenge to realize WUSNs is the wireless communication in underground environments. Most existing wireless communication systems utilize the dipole antenna to transmit and receive propagating electromagnetic (EM) waves, which do not work well in underground environments due to the very high material absorption loss. The Magnetic Induction (MI) technique provides a promising alternative solution that could address the current problem in underground. Although the MI-based underground communication has been intensively investigated theoretically, to date, seldom effort has been made in developing a testbed for the MI-based underground communication that can validate the theoretical results. In this paper, a testbed of MI-based communication system is designed and implemented in an in-lab underground environment. The testbed realizes and tests not only the original MI mechanism that utilizes single coil but also recent developed techniques that use the MI waveguide and the 3-directional (3D) MI coils. The experiments are conducted in an in-lab underground environment with reconfigurable environmental parameters such as soil composition and water content. This paper provides the principles and guidelines for developing the MI underground communications testbed, which is very complicated and time-consuming due to the new communication mechanism and the new wireless transmission medium.
Despite the active theoretical research, the implementations and experimental results on MI underground communications are seldom developed. A few implementation of MI techniques has been done in the areas other than communications. In @cite_5 , MI techniques are used to create a charger along the railway that keeps charging the trains. In [17,24-28], two coupled MI coils are used to transfer the electric power between portable wireless devices. In [17,26], the MI waveguide with strong coupled neighbor coils is tested. None of the above implementation and experiments are used to evaluate the MI-based communications. Moreover, those experiments on MI techniques are taken in the air medium. No experiments have been taken to see the MI communication performance in the complicated underground medium.
@cite_5
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In this paper, we will report some recent progress on wireless power transfer (WPT) based on resonant coupling. Two major technologies will be discussed: the use of metamaterials and array of coupled resonators. With a slab of metamaterial, the near-field coupling between two resonant coils can be enhanced; the power transfer efficiency between coils is boosted by the metamaterial. The principle of enhanced coupling with metamaterial will be discussed; the design of metamaterial slabs for near-field wireless power transfer will be shown; recent experimental results on wireless power transfer efficiency improvement with metamaterial will also be presented. By using an array of resonators, the range of efficient power transfer can be greatly extended. More importantly, this new technology can provide wireless power to both static and mobile devices dynamically. The principle of this technology will be explained; analytical and numerical models will be used to evaluate the performance of a WPT system with an array of resonators; recent experimental developments will also be presented.