Underwater acoustic communication

Underwater communication is difficult due to factors such as multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges.

Adaptive equalization tries to model the highly reflective UAC channel and subtract the effects from the received signal.

The signal is impressed into the magnetic field x,y area by varying the sine and cosine inputs at a precise time.

Besides itss high spectral efficiency helps make optimal use of limited bandwidth underwater.

[14][15] The underwater telephone, also known as UQC, AN/WQC-2, or Gertrude, was used by the U.S. Navy in 1945 [16] after in Kiel, Germany, in 1935 different realizations at sea were demonstrated.

[18] The type designation "UQC" stands for General Utility (multi use), Sonar and Underwater Sound and Communications (Receiving/Transmitting, two way).

Voice or an audio tone (morse code) communicated through the UQC are heterodyned to a high pitch for acoustic transmission through water.

[19] In April 2017, NATO's Centre for Maritime Research and Experimentation announced[20] the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound (like modems and fax machines do over telephone lines).

[22][23] It is available for use with military and civilian, NATO and non-NATO devices; it was named after the Roman god of gateways, openings, etc.

[24] This enables multiple different applications such as Emergency location, Underwater AIS (Automatic Identification System), and Chat.

Commercial hardware products have been designed to enable two-way underwater messaging between scuba divers.

[30] The Android software app, AquaApp, from University of Washington uses the microphones and speakers on existing smartphones and smart watches to enable underwater acoustic communication.