OFDM Transmission Without Guard Interval in Fast-Varying Underwater Acoustic Channels

In this paper, we consider orthogonal frequency-division multiplexing (OFDM) transmission in fast-varying underwater acoustic channels. We demonstrate on experimental data that reliable communications can be achieved without any guard interval (such as cyclic prefix or zero padding) and with a superimposed pilot. Such OFDM transmission possesses a high spectral efficiency, but incurs severe intersymbol and intercarrier interference, and interference from the superimposed pilot. We propose a receiver that can efficiently deal with the interference and has a relatively low complexity as most of its operations are based on fast Fourier transform and local spline interpolation. The receiver is verified in an experiment with a transducer towed by a surface vessel moving at a high speed; a complicated trajectory of the transducer resulted in a severe Doppler distortion of the signal received on a single hydrophone. The performance of the proposed receiver is investigated for different parameter settings and compared with an ideal receiver with perfect channel knowledge, operating in interference-free scenarios, and mimicking the signal-to-noise ratio (SNR) of the experiment. The proposed receiver has provided error-free detection of encoded data at data rates of 0.5 b/s/Hz at a distance of 40 km and 0.33 b/s/Hz at a distance of 80 km, approaching the performance of the ideal receiver with a less than 3-dB loss in SNR.