In multiple sequences spread-spectrum (MSSS) system, multiple cyclic shifted spread sequences are superimposed to increase data rate, in which the first spread sequence is used as pilot for channel estimation and the remained spread sequences for carrying information bits. In this paper, a cyclic index modulation aided multiple sequences direct spread-spectrum system for underwater acoustic communication is proposed. In the proposed scheme, a part of spread sequences of the maximum number of available superimposed spread sequences, whose indexes are determined by extra information bits, are selected to spread phase shift keying (PSK) symbols. By comparing error performance of the proposed system and multiple sequences spread-spectrum system, results show that the proposed system can achieve obvious performance gains over time-frequency doubly selective fading channels and underwater acoustic channels.
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