Research Of Underwater Acoustic Communication Technology Based On Parametric Acoustic Array

Underwater acoustic communication is currently the most commonly used communication method that can effectively transmit information in the ocean environment without cable connection.At present,the most important problem of the underwater acoustic communication field is frequency selective fading and intersymbol interference caused by the multipath effect of the underwater acoustic channel.One of the most important contents in the field of underwater acoustic communication is improving the anti-multipath interference capability and the concealment of the communication system.Acoustic parametric array is a small-sized acoustic emission device that uses the nonlinear effect of sound waves in the medium to obtain low-frequency,wide-band,high-directivity difference frequency waves.These characteristics provide an ideal choice for long-distance and low-frequency communication in shallow sea environment.Its high directivity has important application value for combating multipath effects and achieving directional covert communication.In this paper,the Orthogonal Frequency Division Multiplexing(OFDM)signal parametric transmission method based on Single-sideband(SSB)parameter pre-modulation and the Pattern time shift delay coding(PDS)underwater acoustic communication method based on the time-domain equalization processing of the primary frequency signal are respectively proposed.The underwater acoustic communication system based on parametric array is designed and built,and its feasibility and effectiveness are verified.Firstly,the phase characteristics of difference frequency in seawater are studied.The theory of parametric array sound field is analyzed and derived,and it is proved that the phase of parametric array difference frequency sound wave is a function of the phase of the primary frequency signal.Therefore,the propagation law of parametric array difference frequency sound wave phase depends on the medium parameters at the primary frequency.It can be considered that the change of the seawater medium has a certain influence on the stability of the phase of the difference frequency sound wave.A delay forwarding measurement method was used to measure the phase difference between the parametric array frequency difference wave and the small amplitude wave of the same frequency in the water medium with and without bubbles in the pool.An experiment scheme is designed,and the fluctuation of the phase of the difference frequency wave in the propagation process is measured through the wharf test and the sea test.The test results show that the phase of the parametric array difference frequency signal is relatively stable during the signal propagation,and the difference frequency wave can maintain a relatively stable phase relationship with the small amplitude wave of the same frequency for a long time.This conclusion provides a certain theoretical basis for the parametric array frequency difference wave as the carrier of information in phase modulation underwater acoustic communication system.Secondly,in the face of the demand for high speed,robustness and concealment of shallow water acoustic communication,the OFDM underwater acoustic communication technology based on parametric array is studied.Aiming at the problem of OFDM signal transmitting by parametric array,a parametric emission method of OFDM signal based on single sideband parametric pre-modulation is proposed.Based on "Berktay far-field solution",when the preset communication signals are single frequency signal and chirp signal,the AM pre-modulation algorithm,DSB pre-modulation algorithm and SSB pre-modulation algorithm are theoretically derived and simulated respectively.The harmonics caused by the parametric array self-demodulation effect can be effectively reduced by using reasonable preprocessing technology.Simulation analysis shows that when SSB algorithm is used to pre-modulate single-frequency signal or chirp signal,the system power can be saved without harmonic interference.The feasibility of parametric OFDM underwater acoustic communication is deduced and analyzed,and an OFDM signal parametric transmission method based on SSB parametric pre-modulation is proposed,which realizes the effective combination of OFDM coding and parametric transmission.The algorithm performance is verified by theoretical derivation and the sea experimental results.Finally,aiming at the 12 d B/oct changing problem of LFM signal amplitude,a time-domain equalization method of primary frequency signal is proposed.Based on parametric array wideband signal theory,the signal processing algorithms of LFM signals in time domain and frequency domain are studied.Amplitude compensation is obtained through this algorithm,and it improves the mutual correlation of difference frequency LFM signals,and improves the anti-noise performance of the parametric PDS communication system.The performance of the algorithm is verified by theoretical derivation and sea experiment results.