Anti-strong Multi-channel Interference In Underwater Acoustic Digital Voice Communications

Since sound is the only effective information carrier for medium or long distance wireless communication in sea water, underwater acoustic (UWA) communication plays an important role in national defense and in the development of marine economy. With the characteristics of large ambient noise, very narrow bandwidth, low carrier frequency, great propagation latency and time-space-frequency variant multipath effect (MPE), the stochastic ocean channel has demonstrated the greatest complexity and difficulty for underwater acoustic wireless communications. Among them multipath effect (MPE) is the most difficult obstacle that results in signal fading and inter-symbol interference (ISI).Upon the request from some institutions that have the practical needs for UWA applications, the research is conducted to achieve the objective of developing a digital underwater acoustic voice communication system with longer working distance. The nature of the multipath effect (MPE) has decided that phenomenon that the longer the working distance is adopted, the worse the multipath effect (MPE) plays the role. The new methods should be studied and employed to eliminate or mitigate the negative resulted by multipath effect (MPE). It has been proven that the Spread Spectrum Technologies have good capabilities of overcoming ISI in electromagnetic wireless channel. The feasibilities of their applications to underwater acoustic channel have been analyzed, the Frequency Hopping technology is chosen. Instead of using time-frequency diversity, convolutional coding and Viterbi decoding are employed to combat signal fading, consequently the bit-rate and bit-error-rate performance of the underwater acoustic voice communication system can be balanced and controlled.For medium or long distance underwater acoustic communication, the narrow bandwidth and low carrier frequency only supports low bit-rate signal transmission. In order to achieve real time communication, Speech Recognition technology is used to keep the semantic information, meanwhile the greatest compression on the information is obtained. Consequently the data rate needed on the channel is reduced dramatically. When the semantic information is received, Speech Synthesis is used to generate highly clear voice in the receiver. Synchronization is the commander of every digital communication system, without it the system is unable to work orderly and correctly. A five-tuple of time-frequency diversity is adopted to guarantee that the synchronization signal be retrieved reliably in the receiver.An innovative and novel concept of Communication Sonar Equation (CSE) is introduced in the dissertation. According to the Communication Sonar Equation (CSE), all solutions for improving the working range of underwater communication system are fully discussed. Considering the restrictions in practical application, some methods are confirmed, others are discarded. During the R&D process of the system, a new type of High Q Bandpass Filter with self-calibration and integrate and dump is designed, which is cooperated with instantaneous frequency analysis to greatly improve the resolution and reliability of signal detection. That is another highlight of innovation in the study.The performance of the system has been investigated both in the laboratory's pool and on sea trials in real ocean environments at Xiamen bay coast. The results have indicated that the new generation of underwater acoustic voice communication system has the capability to overcome the severe multipath interference and has longer working range. It has been demonstrated that the objective of the research is achieved.