Signal enhancement techniques for through-the-earth communication based on multiple references and beamforming

Through-the-Earth Communication (TEC) is very vulnerable to electromagnetic interference (EMI) and the conventional signal enhancement techniques employing local primary and remote reference antennas are unfeasible for downlink signal reception due to the limited space and complicated terrain on the receiving sites. In this paper, a novel magnetic field sensing system for downlink reception, in which multiple reference sensors are deployed locally and orthogonally with the primary receiving sensor, is introduced and the signal model is constructed from a perspective of array reception. A modified minimum variance distortionless response (MVDR) beamformer is designed by exploiting the TEC operating conditions for suppressing the EMIs and enhancing the signal. Real tests with power line harmonic interferences (PLHIs) from household wiring in laboratory and an additional wideband jamming demonstrated a great reduction of the EMIs and thus reliable downlink signal reception could be obtained.