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Ref.: Journal of Acoustical Society of America Express Letters, 148(3), September 2020 (doi).
This paper proposes a superimposed training method for low probability of detection underwater acoustic communications. A long pilot sequence was superimposed to the message for equalization and synchronization purposes. A fast Hadamard transform (FHT) estimated the channel impulse response and compressed the pilot energy. A Wiener filter performed equalization. The interference signal was removed using "Hyperslice Cancellation by Coordinate Zeroing". An inverse FHT decompressed the remaining sequence energy and the message was retrieved. Results from a shallow water experiment presented bit error rates < 0.01 for signal-to-noise ratios < -8 dB.