We study underwater acoustic communications using time reversal. The multiple-input/single-output communication system is active because the receiver first sends a pilot signal. Each element in transmitting array records this pilot signal. Then each element encodes a binary phase shift keyed bit stream onto the time reversed signal it recorded. Because of spatial compression in time reversal, only the receiver obtains a clear signal. Because of temporal compression in time reversal, the receiver records a waveform with zero intersymbol interference. Hence, the receiver
needs no equalization to recover the transmitted bitstream.
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