Assessment of the limits of applicability and methods of modulation of near-field magnetic coupling

The development of near-field magnetic systems and means of transmitting messages through media that significantly absorb the electromagnetic field is one of the topical areas of research in the field of wireless communication. These lossy media include water, soil, buildings. The attenuation of the magnetic field in conducting media increases with increasing frequency. To organize communication channels through a conductive medium such as sea water electromagnetic radiation of extremely low frequencies and ultra-low frequencies from 3 Hz to 300 Hz is used. Traditional communication methods due to electromagnetic radiation in that frequency ranges require large sizes of transmitting and receiving antennas. The near-field communication method makes possible significant reduction both the dimensions of receiving and emitting antennas and the transmitter power consumption. A significant limitation of near-field long-wave communication is the low bandwidth and small, up to tens of meters, communication range. The operating principle of the proposed communication system is based on the use of the magnetic component of an electromagnetic field. Transmitting element in proposed system is a solenoid with a magnetic core. Receiving magnetic field sensor is a magnet fixed on a torsion suspension. The magnet is combined with a mirror reflecting the laser beam. Rotation of the magnet under the action of an external magnetic field leads to a change in the angle of reflection of the laser beam from the mirror surface of the magnet. The reflected signal is recorded by a linear photodetector. The attenuation of the magnetic field during the transmission of radiation from a dielectric to a conducting medium was evaluated with the solution of Maxwell's equations. A three-position binary phase shift keying and a modified three-position binary phase shift keying are developed and substantiated. The proposed solutions provide the opposite arrangement of signal symbols, high message information density, localization of the emitted signal energy in low-frequency region and an increase in communication range. Experiments had shown that usage of modified keying type shown an increase the communication range by 10 % with the same reliability of message delivery in comparison with three-position binary phase keying. The estimates of the weakening and attenuation of the magnetic field during propagation in layered media obtained from the simulation are confirmed by experimental measurements. The results of research could be used in solving problems of local deployment of secure near-field communication systems through media that absorb an electromagnetic field.