Research On The Key Technologies Of MFSK Underwater Acoustic Communication Based On Parametric Array

The MFSK underwater acoustic communication based on parametric array is a method of underwater acoustic communication technology which transports signal by high directional acoustic signal. This method modulates the MFSK signal into the parametric array ultrasonic signal, the original MFSK signal will be demodulated during the transmission by the medium’s nonlinear effects, and have narrow beam, high directivity, low frequency and long transmission distance. This way of underwater communication can inhibit the Multipath Interference and the Inter Symbol Interference(ISI), and improve the Signal-Multipath Ratio, and provide a new solution for the High-speed, Stable, Long-distance underwater acoustic communication system.This thesis analyzes the Westervelt equation and “Berktay far-field solution” briefly, simulates the basic parametric array signal by Matlab, and find the Single-Sideband(SSB) modulation for the parametric array can effectively inhibit the generation of harmonic signal and improve the frequency utilization, and more suitable for the parametric array underwater acoustic communication compared with Double-Sideband(DSB) modulation. It’s find by the simulation that adjusting the SSB modulation parameters can inhibit the harmonic distortion and intermodulation distortion.In order to analyze the quality of the underwater acoustic channel, this thesis analyze and simulate the shallow sea environment, confirm the environment factors that affect the quality of underwater acoustic communication channel, and point out the formulas for those factors. Thesis builds a Deterministic model for the shallow sea underwater acoustic multipath channel, confirms the Signal-multipath Ratio(SMR) be defined as the metrics for measuring the interference of underwater acoustic multipath signal, and analyzes the SMR with the normal acoustic source by the Matlab simulation. It’s found that the SMR of short distance transmission is far from well, mainly due to the multipath signals have some energy in the short distance, and with the increase of distance, more reflection of the multipath signal brings more energy lose. At long distance, the shallow sea environment noise turns to be the mainly factors of interference for SMR. The difference frequency of underwater acoustic based on parametric array not only has high direction, but also has important characteristic of tapered-distribution for sound energy field during the transmission. The most energy of sound concentrate in the axial direction, greatly inhibit the underwater acoustic channel’s multipath interference and improve the quality of underwater acoustic communication channel.Finally, according to theory analysis and simulation, a hardware platform is made for the underwater acoustic communication based on parametric array to take experiment and test. The result of experiment and test shows that this method of underwater acoustic communication can effectively improve the Signal-multipath Ratio and the quality of underwater acoustic channel with limited bandwidth resources, which provides a new method for the High-speed, Long-distance and reliable underwater acoustic communication system.