Research On Underwater Acoustic Email Transmission

Sound is the only effective information carrier for medium or long distance wireless communication in the sea. Underwater acoustic (UWA) communication plays an important role in national defense and in the exploration of ocean resource. With the characteristics of high ambient noise level, very narrow bandwidth, low carrier frequency, enormous propagation latency and time-space-frequency variant multipath effect (MPE), the stochastic ocean channel is very complex and difficult to achieve reliable UWA communications. Among them, MPE is the biggest obstacle that results in signal fading and inter-symbol interference (ISI).Based on the researches on the UWA image transmission and UWA digital voice communication carried out by UWA Communication Group of Xiamen University, this dissertation focuses on the transmission techniques for UWA Email. By using digital communication mode, we have realized the transmission of underwater information, the connection of the wireless repeater. Further more, we have achieved the interconnection between Mobile Communication Network and Internet. An UWA Email sending and receiving system with its own independent intellectual property right is successfully developed in the dissertation. It lays a good foundation for the further research on UWA network in the future.A deterministic channel model is setup in this dissertation to simulate the communication system of the shallow acoustic channels. The nature of the MPE has decided the phenomenon that the longer the working distance, the worse the MPE. New methods should be studied and employed to combat the 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 UWA channel have been analyzed and then Frequency Hopping (FH) technology is chosen. Diversity method is the most effective way to reduce signal fading.Time-frequency diversity can provide inexplicit diversity without the penalty of large peak-to-average power ratio, so it is propitious to acoustic channels. How to detect synchronized signal quickly, accurately and effectively is one of the key techniques of FH communication in the shallow water acoustic channels。A synchronized scheme suited to acoustic channels is proposed in the thesis. The design is based on...