Spatiotemporal Coordination in Wireless Sensor Networks
Large-scale networks of low-power, wireless devices integrated with sensors and actuators have emerged as a new platform that promises to seamlessly integrate computational devices with our environment. Information about various phenomena occurring both in nature and human created environments can be gathered at a fine scale, unobtrusively, remotely, and in a cost-effective manner. Such information can be used to control and manage production facilities, predict and asses natural disasters, or obtain better understanding of animal species or natural processes. The notion of time and space is fundamental in the context of these wireless sensor networks (WSNs): data is typically gathered at different physical locations and at a different time, thus the interpretation of the data is contingent upon the existence of inter-node synchronization of time and location coordinates. In this thesis, we address both temporal and spatial coordination of WSNs. We argue that structuring time synchronization protocols into layers and standardizing interfaces of these layers improve adaptability, reusability, and portability of the protocols and help to decrease the complexity and increase the efficiency of their implementation. In our approach, time synchronization protocols are structured in three layers which communicate through well defined application programming interfaces (APIs). Further, we provide implementation, performance analysis, and quantitative comparison of a number of time synchronization services which will help developers to identify the time synchronization needs of their WSN applications. Despite the considerable research effort invested in the area of node localization, a robust sensor localization is still an open problem today when applied to real world problems. Existing techniques have limited range and accuracy, require extensive calibration, or need extra hardware that adds to the cost and size of the platform. We propose a novel radio interferometric ranging technique that utilizes two transmitters emitting radio signals at almost the same frequencies. The relative distances between the nodes are estimated by measuring the relative phase offset of the generated interference signal at two receivers. We implement interferometric ranging on a low-cost low-power off-the-shelf hardware and demonstrate that both high accuracy and large range can be achieved simultaneously. Furthermore, we present the results of localization and real-time tracking experiments deployed in rural and urban environments, demonstrating that our techniques allow for economical deployments and outperform existing localization services.