Generation and use of very wideband chip signals for coded underwater applications

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We present the generation and use of coded signals with very wideband chip signals for underwater applications. We performed measurements using a parametric array in a large water tank. We fabricated a transducer consisting of a 3 mm thick and 75 mm-by-75 mm square-shaped PZT ceramic plate, which is matched to water media at the radiating face and terminated by a very low impedance at the back. We used the square-root amplitude modulation and complementary Golay sequences to code the driving signals centered around 855 kHz primary frequency. We employed a matched filter at reception. Our measurement results showed that generating coded signals with varying code and chip signal lengths at 10–80 kHz difference frequency range is possible. We measured that the normalized matched filter output at 40 kHz for a 2-chip coded signal with the widest bandwidth chip signal length of 2-cycle/chip is 0.94, which indicates a strong correlation and is consistent with those measured at other difference frequencies. As a result, we obtained well-defined coded signals with a chip length of 2-cycle/chip at a 10–80 kHz frequency range, although we also observed coded signals with shorter chip signals.