Long-Range Double-Differentially Coded Spread-Spectrum Acoustic Communications With a Towed Array

This paper presents a novel double-differentially coded spread-spectrum (DD-SS) system design for long-range acoustic communications (LRAC) between a moving source and a towed horizontal line array (HLA). The proposed design relies on two techniques that have been historically developed for radio-frequency terrestrial wireless communications, namely, direct sequence spread spectrum and double-differential coding. In DD-SS, direct sequence spread spectrum is employed to: 1) increase the signal-to-noise ratio; 2) suppress multipath interference; and 3) support data multiplexing, while double-differential coding makes reliable symbol recovery possible without explicit phase and Doppler tracking/correction. Thanks to the two techniques together with traditional beamforming, the DD-SS system is shown capable of effectively dealing with many challenging issues posed by LRAC with a towed HLA, without using complicated receiver processing. During the 2010 Long-Range Acoustic Communication Experiment (LRAC10) in deep waters, the DD-SS system was tested by using a moving source at a speed of 2-3 kn at 75-m depth and a 64-element HLA towed at 3.5 kn at a depth of 200 m. Excellent uncoded error performance (less than 4% bit error rate) is demonstrated at a data rate of 6.4 b/s for a bandwidth of 200 Hz and at a range of 550 km.