Communications and Networking in Underwater Acoustic Networked Systems

The Earth is mostly a water planet, with two thirds of its surface covered by water. Exploration of the mysterious water world has never ceased in human history, yet at the time being, less than one percent of this environment has been explored, since it cannot be probed through satellites nor visited by humans for a long time. Driven by the unprecedented development of wireless communications and networking in terrestrial radio applications, underwater wireless networked systems, especially underwater acoustic (UWA) networked systems, are envisioned to revolutionize underwater exploration through providing long-term, continuous and real-time unmanned data acquisition. Nevertheless, a plethora of research issues associated with the UWA networked system have to be identified and addressed before meeting its great potential. Out of a myriad of challenges, UWA communications and networking are the most important components that underpin the system architecture. This thesis aims to identify and address challenges in practical acoustic networked systems. Tailored to the orthogonal frequency-division multiplexing (OFDM) modulation, three research directions are pursued: Communication techniques for UWA channels with widely separated multipath clusters: This type of channel exists in many scenarios, such as the deep-sea horizontal communications and underwater broadcasting networks. Due to the extremely large delay spread and time variation of UWA channels, both interblock and intercarrier interferences are present in the received signal. Advanced receiver processing algorithms are investigated to address the above interferences and recover the transmitted information. External interference cancellation in UWA OFDM: Despite rich interference in UWA environments, few studies are available for interference mitigation in UWA communications and networking. In this vein, we propose a parameterized interference cancellation approach to mitigate an external interference from OFDM transmissions, which is shown applicable to other kinds of interferences in UWA networked systems. Asynchronous multiuser OFDM reception: Multiuser communication is an effective methodology to increase spectral efficiency. Due to the large signal propagation delay in water, signals from multiple users could be severely misaligned at receivers. By introducing the concepts of overlapped truncation and interference aggregation, we convert the asynchronous multiuser problem to a quasi-synchronous multiuser problem with interference contamination, which therefore can be solved through a traditional quasi-synchronous multiuser receiver equipped with interference cancellation. Proposed solutions in the above research directions are validated using both simulated and field experimental data sets