###### Abstract  
In a multiple-sequences-spread-spectrum (MSSS) communication system, the transmitted signal is made up of superimposed cyclic shift sequences, each sequence carrying a symbol, so the data rate is increased. In this context, we propose an in-phase/quadrature index modulation-aided MSSS system (MSSS-I/Q-IM), which achieves higher data rate and better bit-error-rate (BER) performance. The new system maps the additional bits to the cyclic shift length of the superimposed sequences of I/Q branches. The sequences in two branches carry the real and imaginary parts of the \(M\)-ary phase-shift-keying (MPSK) symbols, respectively. We derive the theoretical BER of the MSSS-I/Q-IM system over an additive white Gaussian noise (AWGN) channel. Then, the simulation experiments of the proposed system and MSSS system over an underwater acoustic channel are analyzed and compared. Finally, field experiments are carried out in the pool environment. The experiment results show that the proposed scheme can achieve higher data rate or better BER performance via adjusting the IM parameter. This is important for the time-variant underwater acoustic transmission environments. cyclic shift; index modulation; multiple sequences spread spectrum; underwater acoustic communication +
Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/)]([https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/))). chip sequence to increase the signal bandwidth and reduce the power spectral density, thus providing good multiple access performance, anti-multipath interference ability, and low detection probability [3; 4]. However, the traditional spread-spectrum underwater acoustic communication system has been unable to meet the requirements of high data rate and reliability. With the rise of other technologies, how to combine the spread spectrum with different technologies effectively has become a new hot topic. Cyclic shift keying spread spectrum [5] is an \(M\)-ary direct-sequence spread-spectrum (DSSS) technique for low probability of intercept communications, which upgrades the spectral efficiency of DSSS systems. A cyclic shift keying spread-spectrum system has been proposed for underwater acoustic communications [6], and it uses the cyclic shift characteristic of the spreading sequences to map information bits, which can improve the data rate exponentially. However, the multipath delay spread of the channel will cause serious inter-symbol interference, and the phase offset caused by the Doppler effect will result in a decrease in spreading gain. Therefore, a method of applying time reversal technology to the cyclic shift keying spread-spectrum system is proposed to suppress the influence of the multipath effect [7]. For the \(M\)-ary spread-spectrum system in [8; 9], a spreading sequence is selected according to the input information bits from a local sequence group that contains \(M\) sequences, and the data rate improvement depends on the size of \(M\). Coincidentally, a parallel combined spread spectrum is similar to an \(M\)-ary spread spectrum, the difference is that the parallel combined spread spectrum selects several sequences from the local sequence group for spreading [10], which can be regarded as the generalization of an \(M\)-ary spread spectrum and can achieve a higher communication rate. In the multiple sequences spread-spectrum (MSSS) system, a plurality of spreading sequences with different cyclic shift lengths are superimposed, and each shifted sequence spreads an information symbol, which can also increase the data rate [11].