Wireless underwater-to-air communications via water surface modulation and radar detection

This paper explores a concept recently proposed for underwater-to-air communications using a combination of acoustic and electromagnetics modalities. The transmitter is an underwater acoustic projector that is used to vibrate the surface of the water whose vibrations are modulated by the communications message. The airborne sensor is a millimeter-wave radar hovering over the surface of the water which senses the water surface vibrations. The phase of the coherent radar signal is used to determine the displacement of the water surface to the sub-millimeter level. From this information, the communications message can be demodulated. Prior research and our calculations suggest that acoustic frequencies in the range 100–300 Hz are ideal in terms of throughput and signal-to-noise ratio. In our measurements, a loudspeaker covered in copper tape is used as a surrogate water surface responding to the acoustic stimulus. A W-band Doppler radar operating in the 94-GHz frequency range using analog in-phase/quadrature (I/Q) demodulation is used as the radar sensor. Early results are quite promising; while playing sinusoids at 100 and 300 Hz, the signal can clearly be observed in the radar output.