Existing coherent underwater acoustic communication systems rely on single carrier transmission and adaptive decision feedback equalization to deal with time-varying and highly dispersive underwater acoustic (UWA) channels. Equalization complexity prevents any substantial rate improvement with the existing single-carrier approach, as the channel frequency selectivity increases considerably when the symbol rate increases. Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM), on the other hand, converts a frequency selective channel into a set of parallel frequency-flat subchannels, thus greatly simplifying receiver equalization. Motivated by the success of OFDM in radio channels, we investigate its use for underwater acoustic channels. In this paper, we develop a pilot-tone based receiver design for zero-padded OFDM transmissions, and test it in a real underwater acoustic channel. Our proposed receiver performs carrier frequency offset compensation, channel estimation, and data demodulation on the basis of individual OFDM block. This approach is appealing to applications with short data bursts, or fast varying channels, as it does not rely on channel dependence across OFDM blocks.
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