Research On Underwater Acoustic Communication By Mimicking Dolphin Sounds

With the increase of sea-exploitation and request of underwater acoustic communication from national defense construction, underwater acoustic communication experiences a rapid development. Underwater acoustic communication technology often employs the fixed carrier-wave modulation-demodulation method. For the sake of correctly decoding, emission source level is generally significantly higher than that of background noise. The high sound source level signal is easily received by hydrophones in the communication process and exposes the information and underwater platform. Therefore, for the sake of national defense safety, covert underwater acoustic communication technology is required. The traditional underwater secure covert acoustic communications often reduces the sound power of communication signals at the transmitting end to conceal signals in oceanic ambient noise.However, this approach will undoubtedly limit the communication distance. It is difficult to solve the problem of underwater covert secure acoustic communications completely.To overcome the problems of traditional covert communication, in this thesis, a novel approach is proposed for providing ways to conceal communication by adopting real or mimicked sound of the dolphins. The approach is based on bionics and bionic camouflage.The communication signals proposed could be detected but would be excluded in the process,as they are associated with recognized waveforms of the dolphins. Through excluding the signals, underwater communication would be clandestine and the problem of underwater covert secure acoustic communication would be completely solved. The works done as follows:1?The noise sources in the ocean and their time-spectral characteristics are analyzed.According to the time-spectral characteristics of dolphin whistles, image processing for noise reduction of dolphin whistles is used. The time-spectral contour of the dolphin whistles is extracted. The algorithms are validated by computer simulation.2?According to the time-spectral contour, which is extracted form dolphin whistles, the modeling and synthesis of whistle signals is studied. The communication methods of longitudinal movement modulation, stretching modulation and MSK modulation of time-spectral contour of whistle signalsare proposed. The algorithms are validated by pool experiments.3?Combining time delay shift coding with M-ary spread spectrum coding, the signal emitted by dolphin sounds adopted has good properties of auto-correlation or cross-correlation,the information is modulated between different clicks,M-ary technique is used to improve the communication rate. An ultra-short duration, wide bandwidth pulse that imitates high frequency clicks of dolphin is adopted as the information carrier. We use multi-pulse modulation technique to improve anti-jamming capability is used. The algorithms are validated by computer simulation.In conclusion, the thesis provided method of noise reduction and extraction of dolphin whistle and two schemes to achieve bionic camouflage underwater acoustic communication.Two carriers are dolphin whistle and dolphin click. We have verified the correctness and effectiveness of these schemes by doing simulations or actual experiments.