A simple and effective noise whitening method for underwater acoustic orthogonal frequency division multiplexing

Underwater acoustic orthogonal frequency division multiplexing (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 nonideal transmit voltage response (TVR) and (ii) underwater acoustic propagation introduces frequency dependent attenuation. In this paper, a simple method to account for the frequency dependent spectrum of the ICI plus noise is proposed. Specifically, the power spectrum of the ICI plus noise is approximated using a low-order polynomial in the log domain, by fitting the measurements on the null subcarriers embedded in each OFDM symbol. Prewhitening is then applied to each OFDM symbol before channel estimation and data demodulation. The proposed method is tested using experimental data collected from the SPACE08 and RACE08 experiments, where signals with and without transmitter precompensation are compared side by side in the former. Impressive performance gains are found whenever the signal is significantly colored. This is the case when either the TVR is not compensated or the transmission distance and bandwidth are large.