PERFORMANCE ANALYSIS OF NEAR-FIELD MAGNETIC INDUCTION COMMUNICATION IN EXTREME ENVIRONMENTS
Progress In Electromagnetics Research Letters
Ultra-reliable and low-power wireless communications are desirable for wireless networking in extreme environments such as underground tunnels, underwater, and soil. Existing wireless technologies using electromagnetic (EM) waves suffer from unpredictable multipath fading and blockage. The recent development of magnetic induction (MI) communication provides a low-power and reliable solution, which demonstrates negligible multipath fading, high penetration efficiency, and low attenuation loss in
... attenuation loss in lossy media. However, existing works neglect the fact that MI communication only demonstrates such advantages in the near-field, beyond which the MI communication converges to electromagnetic wave-based communication and all the aforementioned advantages disappear. This letter develops a magnetic field propagation model to show MI communication's different performances in the near-field and the far-field. We develop rigorous models to capture the multipath fading, the penetration efficiency through inhomogeneous media, and the attenuation loss in lossy media. The results show that although MI communication can provide reasonable signals in the far-field, it only demonstrates negligible multipath fading, high penetration efficiency, and low attenuation loss in the near-field. INTRODUCTION Magnetic Induction (MI) communication is a reliable low-power solution for extreme environment wireless networking [4, 7, 11] . Thanks to its reliable wireless channel, it requires simple signal processing algorithms [6, 8] , which consume negligible power. Also, its low propagation loss in extreme environments demands small transmission power to successfully send data packets to a receiver. MI communication has been extensively adopted in underground and underwater environments [8, 9, 11] . The advantages that distinguish it from electromagnetic (EM) wave-based communication are its negligible multipath fading, high penetration efficiency through inhomogeneous media, and low attenuation loss in lossy media. Therefore, MI communication is much more reliable and power-efficient than that of EM wave-based communication in extreme environments. It is well known that in the vicinity of an antenna the electric fields and magnetic fields are decoupled, i.e., quasi-stationary [1, p. 241]. The information in MI communication is carried by magnetic fields instead of EM waves. By using small magnetic coils and relatively low carrier frequency, the transceivers are coupled by magnetic induction, through which wireless information can be delivered. Since most of the materials in nature have the same permeability, using magnetic fields for communication has more significant advantages than using EM waves. However, as the distance from the antenna increases, the magnetic fields and the electric fields are coupled together and become EM waves, which demonstrate RF signal behaviors. Originally, the magnetic induction communication only considers the near-field  , which has a very limited communication range since the field strength fall-offs in the speed of d 3 , where d is the distance between the transmitter and receiver. Later on, researchers noticed that this model is not comprehensive since it neglects the far-field of a coil, which . communication range d is much smaller than λ, the signals do not experience attenuation loss in a lossy medium. Therefore, the attenuation loss can be as low as zero for MI communication in the nearfield, whereas it is −10 log(e −2kcd ) dB in the far-field. Since k c is the imaginary part of the propagation constant, it is a function of the conductivity, which can be considered as an indicator of the absorption loss. In view of Eq. (7), MI communication experiences low attenuation loss in the near-field. However, in the far-field, MI communication suffers from a similar attenuation loss to EM wave-based wireless communication. Discussions The results in preceding subsections show that MI communication in the near-field is reliable since it experiences negligible multipath fading, demonstrates high penetration efficiency, and suffers from small attenuation loss. Therefore, in extreme environments such as underground and underwater, MI communication is a promising solution to provide reliable wireless connections. However, as the communication range increases to the far-field, MI communication becomes complicated. It behaves like EM wave-based communication, which suffers from multipath fading, low penetration efficiency, and high attenuation loss. Although we can still receive MI communication signals in the far-field due to the slow fall-off speed, the signals may be weak and suffer from inter-symbol interference. In such a case, we have to increase the complexity of MI receivers to tackle these changes and, thus, the cost and power consumption of MI radios increase. In general, MI communication is a reliable and low-power technology, which has great potentials in wireless applications in extreme environments. However, we should be aware that its advantages mainly lie in the near-field. CONCLUSION Magnetic induction (MI) communication is an important solution for wireless applications in extreme environments such as soil, tunnel, cave, underwater, and in-body. MI communication enjoys negligible reflection, high-penetration efficiency, and small absorption loss in a lossy medium. However, these advantages lie in the near-field since as distance increases the electric fields and magnetic fields are coupled and the communication relies on electromagnetic waves, which suffers from multipath fading Progress In Electromagnetics Research Letters, Vol. 90, 2020 83 and absorption loss. In this letter, we derived analytical models to evaluate the strength of MI communication and compare its performance in the near-field and the far-field. The results show that MI communication in the far-field behaves like electromagnetic wave-based communication, which does not demonstrate the aforementioned advantages. In MI communication system design, if the required communication range is within the near field, we can design very simple and reliable wireless systems, whereas if the required communication range is in the far field, we may need to carefully choose between MI communication and the electromagnetic wave-based communication.