Basin-scale acoustic communication: a feasibility study using tomography m-sequences

Tomography transmissions made over a 3250 km path in the North Pacific as part of the Acoustic Thermometry of Ocean Climate (ATOC) program are analyzed as data communications signals in order to estimate the rate and reliability of very long range undersea telemetry. The ATOC tomography signal is a phase-encoded maximal-length shift-register sequence transmitted at 37.5 symbols per second using a 75 Hz carrier. It may be interpreted as a BPSK data signal at 37.5 bits per second, or as a direct sequence spread spectrum signal with a spreading rate 1023 (the m-sequence period) and a resulting data rate of 1 bit every 27 seconds. The multipath arrivals observed at the receiver span nearly 8 seconds, with the majority of the energy occupying the last two seconds. The data are processed using an adaptive multi-channel decision feedback equalizer with integrated phase tracking and Doppler compensation. Equalization of the signal on one hydrophone is sufficient to extract the low-rate spread spectrum modulation, while joint use of all twenty hydrophone channels provides near symbol-rate communications. The excellent results may be attributed to the short term stability (several minutes) of the deep-ocean sound channel at low frequencies.