Characterization of an Acoustic Communication Channel with Pseudorandom Binary Sequences

The joint European project "UUV Covert Acoustic Communications" explores methods for underwater communication at low signal-to-noise ratios. The first phase of the project focuses on characterization of the communication channel. Sea trials were conducted in two littoral environments in September 2006. The first site is near the island of Bornholm, in the Baltic Sea, and the second site close to the Norwegian coast in the vicinity of the city of Bergen. Part of the trials time was devoted to pseudorandom binary sequences, broadcast in the frequency band between 2.1 and 5.6 kHz. This type of waveform combines time delay resolution and Doppler resolution in a versatile probe signal. The present paper examines the evolution of the channel impulse response, together with instantaneous and time-averaged scattering functions. Motion of the transmitter, which is towed by a surface vessel, is found to be the dominant cause of Doppler spreading. Finally, a direct indication of the quality of the communication channel is obtained by demodulating the binary sequences with a communications receive