A method to estimate the path gains and propagation delays of underwater acoustic channels using the arrival phase information of the multipath components

We propose a method for the estimation of the channel parameters of underwater acoustic channels (UACs) by using a pseudo-noise (PN) sequence. For the channel sounding, the PN sequence is coherently modulated on a carrier frequency by using a binary phase-shift keying (BPSK) modulation scheme. For the demodulation, the arrival phases of the received BPSK signals must be known at the receiver. In practice, however, the arrival phases of the received signal are unknown. We propose to demodulate the received BPSK signal by using an hypothetical arrival phase, which will be increased from 0 to 2π. The demodulated signal obtained by using the hypothetical arrival phase is then correlated with the PN sequence. It is shown that if the hypothetical phase coincides with the arrival phase of a multipath component, then the cross-correlation function (CCF) of the demodulated signal and the transmitted PN sequence reaches its maximum value. Owing to this property, the arrival phases of all channel paths are detected. With the information of the arrival phases of all channel paths, the local maxima of the CCF allow us to estimate the path gains as well as the corresponding propagation delays, which determine the channel impulse response (CIR). To analyse the performance of the proposed channel sounder under real underwater transmission conditions, we have implemented this method to measure shallow UACs in Halong Bay and Hotay lake, Vietnam. Our measurement results show, that the estimated multipath components of the UACs do not significantly vary if the transceivers are moored. In other cases, where the transceivers are moving, the CIR behaves like a fast time-varying process.