This paper presents a comparative study of discrete cosine transform (DCT) spread orthogonal frequency division multiplexing (OFDM) with discrete Fourier transform (DFT) spread OFDM, applied to an underwater acoustic (UWA) channel. The UWA channel is characterized as a frequency selective multipath fading channel with long delay spread. Energy compaction property of DCT works well in reducing the peak to average power ratio (PAPR) in UWA channel. It is also shown that DCT spread OFDM has lower computational complexity compared to DFT spread OFDM. The bit error rate (BER) performance has been evaluated for both the schemes. Simulation results show that DCT spread OFDM has a better performance than DFT spread OFDM in UWA channel. A novel scheme of using the null subcarriers to implement complex field repetitive coding in case of DCT spread OFDM is proposed to further reduce the BER.
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