Adaptive estimation of an underwater acoustic channel using real test data

The authors discuss a section of a larger research program concerned with the investigation of methods of high-data-rate through-water acoustic signaling, for the purposes of TV and other wide-bandwidth transmission between a tetherless ROV (remotely operated vehicle) and a support vessel or fixed receiver. Research relating to high-data-rate underwater acoustic transmission in a high multipath noise environment has emphasized the need for further investigation into the impulse response characteristics of the shallow water channel. The method used to estimate the channel characteristics by adaptive modeling is described, and field results are presented for typical transmission conditions. The acoustic channel has been characterized under short-range shallow water conditions using 55-kHz omnidirectional transducers. The first surface echo is seen to be the most troublesome source of interference on the channel, with considerable phase rotation being encountered under normal conditions. The total energy content of the multipath interference is seen to increase substantially with distance, though most returns are confined to within 10 ms of the direct signal. An echo canceller with this time span in the filter coefficients would therefore be adequate to cancel most of the interference. The phase fluctuation encountered on the channel places a severe burden on the tracking capabilities of the echo canceller.