A Block Index Modulation Aided Multiple Sequences Spread-Spectrum System for Underwater Acoustic Communication

Multiple sequences spread-spectrum (MSSS) systems have the characteristic of transmitting pilot sequence and data sequences simultaneously, making them more channel adaptable than traditional spread-spectrum communication systems. In this paper, we propose a new L-based block index modulation system based on the MSSS system (MSSS-B-IM), which significantly boosts the data rate. This scheme divides all possible index ranges of the spreading sequences into <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J$</tex> groups, each group including at least (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L+1$</tex>) indexes. In each group, the information bits are mapped to the index to select one circularly shifted spreading sequence in one of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L$</tex> indexes to complete code index modulation. The pilot signal is also spread by the original spreading sequence <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{c}$</tex> and then superimposed with the information symbols in the time domain at the transmitter. By analyzing and comparing the data rate of the MSSS, MSSS-IM and MSSS-B-IM systems, the proposed system shows significant improvement in the data rate under the same resource restrictions. In addition, the performance of the proposed scheme is evaluated through Monte Carlo simulation and sea trial, the results verify the superiority of the proposed scheme.