Underwater acoustic communication by passive-phase conjugation uses a channel probe signal transmitted prior to the data signal in order to estimate the channel response. At the receiver, the received probe signal can be truncated to correlate with the late arrived data signal. In a multipath channel, there are paths that undergo incoherent scattering by the sea surface, and they decrease the coherence between the estimated channel response and the channel response for data signal. For a linear frequency modulation probe signal, a novel processing method is proposed to select time delayed arrivals from the received probe, and this method can also suppress the noise level for the truncated probe signal. Applying this method to the data collected in a sea trial, the communication performance is improved.

A multiband OFDM transmitter and receiver are presented for underwater communications at low SNR. Compared with a single-band OFDM scheme, the multiband approach leads to a considerable reduction in the receiver complexity. The proposed system has been tested at sea with 16 subbands covering a total bandwidth of 3.6 kHz, at user data rates of 4.2 and 78 bit/s, and over ranges up to 52 km. At the lower rate, successful message recovery is achieved on a single hydrophone at SNRs down to -17 dB in a benign channel. In channels with a severe delay-Doppler spread the critical SNR rises by some 4 dB. At 78 bit/s the limits of the OFDM signaling scheme are clearly revealed, but at 4.2 bit/s the performance is limited by failure of signal detection and initial synchronization.

In-situ measurements of the scattering function are used to drive a channel simulator developed in the context of underwater acoustic telemetry. Two operation modes of the simulator are evaluated. A replay mode is accomplished by interpolation of measured impulse responses. A second, stochastic mode delivers multiple realizations of a given scattering function. The initial assumption of wide-sense stationary uncorrelated scattering is violated by strong phase correlations between taps. It is shown that time-varying Doppler shifts due to platform motion must be eliminated from measured scattering functions in order to provide the stochastic tap gains with the true Doppler spectrum of the channel. The simulator is validated through a comparison of acoustic data measured at sea, and emulated data, governed by the same scattering function. This comparison is based on scattering and coherence functions, multipath phase measurements, and application of a decision feedback equalizer. After the Doppler correction, the synthetic data are indistinguishable from the acoustic data in terms of delay-Doppler spread, temporal coherence, phase behavior, equalizer mean square error, and bit error ratio.