Virtual Time-Reversal Mirror M-ary Spread-Spectrum Method for Underwater Acoustic Communications

The complexity of the underwater environment including long time-delay, large background noise, and excessive transmission loss, leads to two main challenges for the current mid-distance and long-distance underwater acoustic (UWA) communications: limited transmission rate and system reliability. To improve the performance of mid-range and long-range underwater acoustic communication, this paper proposes an M-ary multi-sequence superimposed spread spectrum(M-MSSS) UWA communication scheme based on a virtual time-reversal mirror (VTRM). Multiple sequences with periodic correlation characteristics of the original spread spectrum waveform are superimposed, and M-ary multi-sequence modulation is performed in each data block at the transmitter. A VTRM matched filter is designed for the receiver. The VTRM technology uses the physical characteristics of the typical coherent multipath of the underwater acoustic wave channel to reduce the signal amplitude through its strong spatial focusing ability. The inter-symbol interference is suppressed by reconstructing multipath signals and improved receiver. Simulations show that the scheme not only significantly improves the communication rate, but also ensures the system's better robustness in terms of bit error rate.