A real-time M-ary differential phase-shift keying (MDPSK) communication system utilising parametric transduction has been constructed. The system employs a 50 kHz primary frequency and a 5 kHz difference frequency. It has been tested in the Gulf of Lion and at Cap Ferrat in the Mediterranean Sea. Experimental results indicate that the system can be used in shallow water to realise real-time acoustic communications at ranges of tens of kilometres and can achieve data rates of 1, 2, and 3 kb s/sup -1/ for 2-, 4-, and 8-DPSK respectively.
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