Weak Signal Underwater Communications in the Ultra Low Frequency Band

Abstract

Sub-surface activity sensors, unmanned undersea vehicles and airplane underwater locator beacons all use underwater acoustic communications. We investigate the problem of underwater communications with weak received signal strength in the ultra low frequency band. It is a band where attenuation is relatively lower, but it has a thin half-power bandwidth. We leverage the work of Taylor and Walker (2017) on Weak Signal Propagation Reporter. It comprises narrow band multiple frequency-shift keying modulation, long synchronization bit sequences intertwined with data bits and high constraint convolutional forward error correction with probabilistic decoding. In contrast to the original work of Taylor and Walker, in our system frames can be sent anytime. No synchronization in reference to a universal clock is required. We review the mathematical foundations, software design and preliminary tests of our approach for underwater communications under weak received signal strength conditions.