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Signal processing for through-the-Earth radio communication

Through-the-Earth (TE) propagation subjects a radio signal to considerable attenuation. The uplink signal must, however, be detected in the presence of full-strength atmospheric noise. Consequently, through-the-Earth communication systems are generally restricted to relatively low data rates. The electromagnetic characteristics of the TE signal differ from those of the surface-propagating noise. In addition, much of the energy in low-frequency noise is contained in discrete impulses. This paper describes an integrated combination of adaptive noise cancellation (ANC), maximum-likelihood detection (MLD), nonlinear processing (NLP), and decision-aided feedback (DAFB) that can be used to combat atmospheric noise in TEC systems. Tests with real VLF noise demonstrate a 10-to-24 dB improvement in SNR. The reduced effective noise level allows higher data rates, greater depth of operation, smaller transmitters, and/or smaller antennas.