Blind Estimation of the Ocean Acoustic Channel by Time-Frequency Processing

A blind estimator of the ocean acoustic channel impulse response envelope is presented. The signal model is characterized by a deterministic multipath channel excited by a highly non-stationary deterministic source signal. The time-frequency representation of the received signal allows for the separation between the channel and the source signal. The proposed estimator proceeds in two steps: first, the unstable initial arrivals allow for the estimation of the source signal instantaneous frequency, by maximization of the radially Gaussian kernel distribution; then, the Wigner-Ville distribution is sequentially windowed and integrated, where the window is defined by the previously estimated instantaneous frequency. The integral gives the channel impulse response envelope, which turns to be an approximation to the blind conventional matched-filter. The blind channel estimator is applicable upon the following conditions: that the multipath channel contains at least one dominant arrival well separated from the others, and that the instantaneous frequency of the source signal is an one-to-one function. Results obtained on real data from the INTIMATE ’96 underwater experiment, where the acoustic channel was driven by an LFM signal, show that the channel’s envelope detailed structure could be accurately and consistently recovered, with the correlation of the estimates ranging from 0.796 to 0.973, as compared to the matched-filter result.