Nonlinear Doppler Warp Correction for Acoustic OFDM

Abstract

The Underwater Acoustic (UWA) channel has been an area of interest for many researchers and engineers, but also a very challenging area. Compared to the over-the-air radio frequency (RF) channel, the underwater acoustic channel causes multiple distortions – due to multipath, frequency selectivity, noise, and Doppler – some of which are more severe. The increased distortion causes many techniques and assumptions made for typical RF communication systems to break down. One of the assumptions that breaks down is that the Doppler effect that the signal undergoes can be modelled with a simple shift in frequency, since the signals used to communicate in a UWA channel are effectively wideband. In this work the Doppler Effect that a signal undergoes is modelled as a nonlinear time warp. A procedure is developed to estimate the parameters of the time warp from the observed signal. These time warp parameters are then used to reverse the effect of the time warp. Two different methods for estimating the time warp parameters and correcting the Doppler are compared. The first technique uses sinusoids placed at the beginning and end of the signal to estimate the parameters of the warp that the signal undergoes. The second technique uses sinusoids that are present during the signal to estimate and correct for the warp. The frequencies of the sinusoids are outside of the frequency range used for the transmitted data signal, so there is no interference with the information that is being sent. The transmitted data signal uses Orthogonal Frequency Division Multiplexing (OFDM) to encode the data symbols, but the Doppler Correction technique will in principle work for other kinds of wideband signals as well. The results, which include MATLAB based simulations and over-the-air experiments, show that performance improvements can be realized using the time warp correction model.