Research On Underground Near-field Magneto-Inductive Propagation Characteristics

Recently, due to the urge requirement of human accuracy localization and survival signal propagation after earthquake and mine disaster, underground communication and localization technology are currently a fairly active and promising research field. However, owing to the complexity of underground propagation media consist of soil, rock, and water at varying ratios, the original electromagnetic technology does not operate well because of the limitation on traditional algorithm and antenna size. Magneto-induction (MI) technique is an excellent candidate to establish underground wireless links. Compared with electromagnetic (EM) communication technology, the near field low frequency MI communication technology is more suitable for through-the-earth communication and localization due to the advantages of no multipath effect, relatively slow attenuation, no large delay and small size antenna. However, the complexity of the communication medium, the limitations of the communication range and the precision of the through-the-earth localization are still difficult problems. On the above problems, this paper proposes the propagation model and application of near field MI communication under different deployment conditions based on the electric small antenna:through the research on point-to-point model, the basic propagation characteristics are showed; through the research on two- and three- dimensional antenna array communication model based on magnetic relay technology, long distance communication strategy are proposed; through the research on localization based on three-dimensional antenna array model, the important application—high precision localization solution is proposed. The major innovations are as follows:Propose a channel model and study its propagation characteristics by comprehensive considering geological and environmental factors. Traditional near-field MI communication model is built by mutual inductance between antennas under ideal environment without taking geological and environmental factors into account. Through considering the effect of moisture content on soil electrical conductivity and the influence of ambient noise and thermal noise on the received voltage intensity, we build underground near-field MI point-to-point communication model. Meanwhile, design and verify the communication device.By use of this device, we do many experiments in different sizes of coils, different moisture, different angles and different distances to study the propagation characteristics of magnetic field strength, receiving power and path loss, and so on.Propose a two-dimensional antenna array and study its propagation characteristics by use of magnetic relay technology. Large size of coil could get long distance communication, but small coil is more suitable for burying. In this paper, the received power of linear antenna array is studied under the condition of strong and weak coupling, and the optimal communication relay distance and frequency are given under the condition of strong coupling and weak coupling, respectively. On this basis, the hexagonal antenna array is put forward. Five kinds of propagation characteristics in the dispersion equation, bandwidth, group velocity, energy density and path loss are compared with traditional quadrilateral antenna array. The results show that the hexagonal antenna array proposed in this paper is superior to the conventional quadrilateral array in the five propagation characteristics. Furthermore, the bandwidth of the new array increased 15.4% and the energy density of the new array increased 3 times.Propose a three-dimensional (3D) antenna array and study its propagation characteristics; by use of this model, propose a 3D through-the-earth localization algorithm based on pattern recognition and classification. The propagation characteristics of three-dimensional hexagon antenna array are greatly improved in bandwidth, group velocity and energy density. Especially, the bandwidth of the new array increased 15% and the energy density of the new array increased 3.6 times. Based on this array, we generalize two deployment scenarios and propose a generic approach combining support vector machine (SVM) and classification by comprehensive consideration of five random factors, including the frequency fluctuation of the transceiver, the fluctuation of the channel noise intensity, the fluctuation of soil water content, and the uncertainty of the angle of display, to overcome the disadvantage of traditional localization method in harsh environments with abundant sources of random effects. Our results not only show that the proposed system provides a localization accuracy of over 95% when uncertainties and random effects are present, but also suggest that the applicability of magnetic-induction communication can be extended to harsh environments by machine learning algorithms.