Echoes from the deep - Communication scheduling, localization and time-synchronization in underwater acoustic sensor networks

Wireless Sensor Networks (WSNs) caused a shift in the way things are monitored. While traditional monitoring was coarse-grained and offline, using WSNs allows fine-grained and real-time monitoring. While radio-based WSNs are growing out of the stage of research to commercialization and widespread adoption, commercial underwater monitoring is still in the stage of coarse-grained and offline monitoring and research on Underwater Acoustic Sensor Networks (UASNs) is in the early stage. Existing WSN research can only partially be applied to underwater communication and realization of large-scale mesh networks of underwater nodes requires rethinking of communication and networking protocols. Acoustic communication is the most widely used type of communication for underwater networks. This is because acoustic communication is the only form of communication which allows long-range communication in underwater environments. Acoustic communication, however, poses its own set of challenges for the design of networking and communication protocols. The slow acoustic propagation speed of about 1500 m/s, limited available bandwidth, high transmission energy costs and variations in channel propagation are some of the challenges to overcome. Existing Medium Access Control (MAC) protocols for underwater communication consider data communication only, however there is a need for reliable network protocols which provide not only data communication but also localization and timesynchronization. We will show that an integrated approach has significant advantages over three separate solutions. We have developed a collision-free MAC protocol that provides both time-synchronization and localization in an energy-efficient and scalable way and with high throughput. In this thesis we introduce a communication scheduling algorithm which we call Simplified Scheduling. A distributed scheduling approach reduces the computational and communication complexity of this scheduling algorithm to allow scheduling of large-scale networks. We introduce a combined Time-of-Flight (ToF) and Direction-of-Arrival (DoA) localization and time-synchronization approach for non-cooperative networks, and introduce a cooperative combined localization and time-synchronization algorithm called aLS-Coop-Loc for cooperative networks. By combining localization and timesynchronization the communication overhead is reduced compared to separate solutions. We show two examples of MAC protocols which combine the introduced scheduling and localization and time-synchronization techniques. In future work we will use these algorithms to design other efficient underwater MAC protocols which combine communication, localization and time-synchronization.