A simple and effective noise whitening method for underwater acoustic OFDM

Underwater acoustic OFDM enables simple frequency domain equalization, but its performance is often limited by intercarrier interference (ICI) that is induced by channel variation, in addition to the ambient noise. As the signal itself, the variance of the ICI is frequency dependent as i) the transmitter often has a non-ideal transmit voltage response (TVR), and ii) underwater acoustic propagation introduces frequency dependent attenuation. In this paper, we propose a simple method to account for the frequency-dependent spectrum of the ICI plus noise. Specifically, we approximate the power spectrum of the ICI plus noise by a straight line in the log-domain, by fitting the measurements on the null subcarriers embedded in each OFDM block. Prewhitening is then applied to each OFDM block before channel estimation and data demodulation. We test the proposed method using experimental data collected from the SPACE08 experiment, where we compare signals with and without transmitter precompensation side by side. The proposed method effectively improves the performance of the uncompensated transmission, making it comparable to that of the compensated transmission at short distances. At a longer distance, the performance of both transmissions is improved by noise-whitening, with an impressive gain on the uncompensated signals.