Researches for oceans are limited to military purpose such as underwater sound detection and tracking system. Among the various ocean technologies, for the purpose of personal or military security, underwater acoustic communications with low-probability-of-interception covert characteristics were received much attention recently. Covert underwater acoustic communication system is designed for the transmission signal is not intercepted by the unintended another receivers. Covert communication system must operate at low signal-to-noise ratio. Therefore, typical covert communication systems use a spread spectrum communication technology.
Spread spectrum signals used to hide the transmitted signal by transmitting it at low power and making it difficult for an unintended listener to detect the signal in the presence of background noise.
The problem for spread spectrum communications in an underwater acoustic channel is the multipath arrivals, which create severe interchip and inter symbol interferences. In order to improve performance in multipath underwater channel, this thesis proposed turbo equalization techniques based on RAKE receiver for spread spectrum communications. RAKE receiver was applied threshold comparison and weighted coefficient. In this thesis, the performance of the proposed transceiver model is analyzed.
This thesis focused not only single user but also multiuser. Many earlier works focused on the single-user detection problem. They are often inspired by advances made in wireless radio communications and sometimes dedicated to the underwater acoustic communications. In a single user code acquisition algorithm was proposed. However, the growing interest in underwater communications led researchers to consider the multiuser detection and acquisition problem in an underwater communication network system.
By expanding single user communication concept, this thesis proposed a multiuser access communication system based on turbo equalized RAKE model. Thesis analyzed the performance of the proposed multiple access communication for covert communication by simulations. Also number of users are fixed on two, underwater experiment was conducted and demonstrate that proposed model is very effective for covert multiuser underwater communications.