SIGNAL-PROCESSING FOR THROUGH-THE-EARTH RADIO-COMMUNICATION

Through-the-earth (TE) propagation subjects a radio signal to consider able 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 lo w data rates. The electromagnetic characteristics of the TE signal dif fer from those of the surface-propagating noise. In addition, much of the energy in low-frequency noise is contained in discrete impulses. T his paper describes an integrated combination of adaptive noise cancel lation (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 demonstrat e a 10-to-24 dB improvement in SNR. The reduced effective noise level allows higher data rates, greater depth of operation, smaller transmit ters, and/or smaller antennas.