Spread-spectrum Coherent Acoustic Communication between a Submarine and a Surface Ship

This paper describes an underwater acoustic communication experiment executed in the North Sea near Norway. A submarine broadcast direct-sequence spread-spectrum signals with data rates of 167 and 33 bit/s. The receiver station was a surface ship with a towed hydrophone array. As the platforms sailed away from each other the SNR at the receiver progressively dropped, with a continuously changing channel impulse response. It is shown that these complicating factors are largely overcome by beamforming of the hydrophone signals in combination with chip-level channel equalization. After the spread-spectrum gain, applied at a late stage in the reception chain, the transmissions are virtually free of errors up to a distance of 12 km.