Code Index Modulation CSK Spread Spectrum Underwater Acoustic Communications

Limited bandwidth restricts the data rate of spread spectrum underwater acoustic communication systems. In this paper, we propose a novel high-rate spread spectrum underwater acoustic communication approach, which utilizes spread spectrum code index modulation (CIM-SS), cyclic shift keying (CSK) and maximum ratio combined frequency domain equalization (MRC-FDE). In this approach, information bits are jointly conveyed by CIM, CSK and the M-ary PSK modulated symbols. PeCAN polyphase sequences with excellent correlation characteristics are used as spreading codes, and spreading code index modulation is to select spreading code from predefined spreading code set. CSK uses the cyclic shifted version of the selected spreading codes to obtain higher transmission efficiency than direct sequence spread spectrum (DSSS) system. A low complexity detector is designed at the receiver to detect the code index and cyclic shift value of the selected spreading code, so as to despread and demodulate the symbols. Simulation and lake experiment show that the proposed method can improve the data rate and spectrum utilization, and resist the underwater acoustic multipath effect.