Environmental equalizer for underwater communications

This paper presents an environmental-based equalization algorithm for underwater communications. This algorithm is based on the passive time-reversal (pTR) and waveguide invariant properties of ocean channels. Passive time-reversal allows for the implementation of a simple communications system, but it loses performance in the presence of geometric mismatch between the probe-signal and the actual data symbols transmission. The waveguide invariant properties state that geometric mismatches, both in depth and range, can be partially compensated by applying an appropriate frequency shift in the passive time-reversal operator. Results with binary PSK data at a carrier frequency of 3.6 kHz, collected during the MREA´04 sea trial, show that the Mean Square Error (MSE) between the transmitted and the received data symbols remains stable at least to a range mismatch of about 37.5 m in the presence of source depth and an array depth oscillations of approximately 0.7 m. In such conditions, when comparing the proposed pTR equalizer with plain pTR, an overall gain of approximately 4.11 dB in output MSE is achieved.