Research On Key Technologies For Parametric Array Directional Acoustic Communication

The most important problems for the acoustic communication system are frequency selective fading and intersymbol interference over multi-path channel. So improving the performance on anti-multi path interference and point to point confidential communication are hot topics in the acoustic communication field. Based on the acoustic parametric array theory, a parametric array directional acoustic communication system is proposed and constructed in combination with the modern digital communication technology, nonlinear acoustic signal processing method and beam forming technology.In this dissertation, the structure of a parametric array directional acoustic communication system is proposed. In this structure, the parametric array is used as signal source to produce the communication carrier signals which are formed by conventional digital carrier modulation method. Comparing to the conventional acoustic communication system, this system can realize the directional and secret communication with a small aperture physical sound source, and also has a good performance on the anti-multipath interference.Based on the Berktay`s far-field solution, the feasibility is proved that the parametric array difference frequency wave may be used as the communication carrier signal and the omni-directional sound source may be replaced by the parametric array source. DSB and SSB signal processing methods of the wideband parametric array are respectively employed to explain the generation mechanism of the communication carrier and synchronizing signal in MFSK, MPSK and OFDM modulation method. The theoretical derivation and the simulation analysis indicate that the fundamental signals in the generated carrier have the similar expression with the conventional ones. The fundamental signals can be used as the new communication carrier signals for the system. Besides, the second harmonic signals have the similar characteristics of communication carrier signal. Thus it can be utilized as a standby communication channel from theoretical point of view.Based on Westervelt`s equation, the beam-forming method of communication carrier wave is discussed. With the sound source array and the primary wave parametric design, an electronic control beam-steering method over the digital phased array technology for carrier wave is proposed. The simulation analysis and experimental test are implemented at different beam-steering angles. Then the Doyle-chebyshev amplitude weighted shaped-beam method is proposed to realize the beam shape control of the carrier wave. Finally, a multi-beam forming method is proposed and verified experimentally to realize the multi-point directional communication.According to the above theoretical analysis, a parametric array directional sound communication testing system working in the air is constructed. The system consists of FPGA signal processing platform, communication carrier generator and piezoelectric ultrasonic transducer array et al. Directional communication experiments are carried out by adopting the DSB and SSB methods respectively, and it successfully realized the image transmission at the conditions of 8FSK and 8PSK modulation. The results indicate that the multi-path delay of carrier signals is significantly reduced when the acoustic parametric array is employed to emit communication carrier. The experimental results also show that the SSB method restrains the generation of the second harmonic signals and the system obtains lower error rate and lower distortion than the DSB method.