A Wrapper-Based Approach to Sustained Time Synchronization in Wireless Sensor Networks

Time synchronization is an important service for wireless sensor network applications. Nodes in the network stay synchronized by exchanging periodic messages that carry local timestamps. Several algorithms have been proposed in the literature that are suited to different kinds of application scenarios. A common problem across these time synchronization algorithms is that the energy cost of message exchange is high. In fact, the cost of radio communication far outstrips the cost of performing local operations on the processor. If the message exchanges were stopped, nodes will fall out of sync, and may no longer be able to meet application requirements. This thesis presents a wrapper-based approach to sustained time synchronization for wireless sensor networks. As such, this solution Booster for Time Synchronization Protocol (BTSP) will act as a wrapper around a given time synchronization protocol, and will apply local corrector operations to extend the time duration between two message exchanges between nodes. The wrapper performs at least as good as the original protocol provided, reduces the number of message exchanges on average, and consequently the energy consumed, significantly. BTSP has been implemented for TinyOS and evaluated on XSM motes in conjunction with TPSN, a popular time synchronization protocol for sensor network

A Wrapper-Based Approach to Sustained Time Synchronization in Wireless Sensor Networks

Time synchronization is an important service for wireless sensor network applications. Nodes in the network stay synchronized by exchanging periodic messages that carry local timestamps. Several algorithms have been proposed in the literature that are suited to different kinds of application scenarios. A common problem across these time synchronization algorithms is that the energy cost of message exchange is high. In fact, the cost of radio communication far outstrips the cost of performing local operations on the processor. If the message exchanges were stopped, nodes will fall out of sync, and may no longer be able to meet application requirements. This thesis presents a wrapper-based approach to sustained time synchronization for wireless sensor networks. As such, this solution Booster for Time Synchronization Protocol (BTSP) will act as a wrapper around a given time synchronization protocol, and will apply local corrector operations to extend the time duration between two message exchanges between nodes. The wrapper performs at least as good as the original protocol provided, reduces the number of message exchanges on average, and consequently the energy consumed, significantly. BTSP has been implemented for TinyOS and evaluated on XSM motes in conjunction with TPSN, a popular time synchronization protocol for sensor network

Intelligent antenna sharing in cooperative diversity wireless networks

Thesis (Ph. D.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2005.Includes bibliographical references (p. 143-152).Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However, most of the proposed solutions require simultaneous relay transmissions at the same frequency bands, using distributed space-time coding algorithms. Careful design of distributed space-time coding for the relay channel is usually based on global knowledge of some network parameters or is usually left for future investigation, if there is more than one cooperative relay. We propose a novel scheme that eliminates the need for space-time coding and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of M available relays and then uses this "best" relay for cooperation between the source and the destination. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing gain tradeoff as achieved by more complex protocols, where coordination and distributed space-time coding for M relay nodes is required. Additionally, the proposed scheme increases the outage and ergodic capacity, compared to non-cooperative communication with increasing number of participating relays, at the low SNR regime and under a total transmission power constraint.(cont.) Coordination among the participating relays is based on a novel timing protocol that exploits local measurements of the instantaneous channel conditions. The method is distributed and allows for fast selection of the best relay as compared to the channel coherence time. In addition, a methodology to evaluate relay selection performance for any kind of wireless channel statistics is provided. Other methods of network coordination, inspired by natural phenomena of decentralized time synchronization, are analyzed in theory and implemented in practice. It was possible to implement the proposed, virtual antenna formation technique in a custom network of single antenna, half-duplex radios.by Aggelos Anastasiou Bletsas.Ph.D

Indoor wireless metering networks - A collection of algorithms enabling low power / low duty-cycle operations

Die Bezeichnung Wireless Metering Network (WMN) identifiziert eine spezifische Klasse von drahtlosen Sensornetzwerken. Solche Netze bestehen aus einer Vielzahl von kleinen, kostengünstigen batteriebetriebenen Knoten und bieten eine mögliche Lösung für unterschiedliche Aufgaben in der Gebäudeautomatisierung, vorausgesetzt dass die erwartete Lebensdauer des Netzes mindestens 10 Jahre betragt, um die Netzwerkwartung im selben Raster mit den Gebäudewartungsarbeiten planen zu können. Die starken Energieeinschränkungen erfordern die Einführung von Energiesparmaßnahmen und insbesondere die Auswahl einer durch einen extrem geringen Arbeitszyklus charakterisierten Aktivierungsstrategie. Schlüsselelemente für den Erfolg eines WMN-Projektes sind die Existenz eines energieeffizienten MAC-Protokolls, der Einsatz eines robusten Zeitsynchronisationsmechanismus und die Implementierung von effizienten Strategien für die Netzwerkinitialisierung und die Netzwerkwartung. Hauptziel dieser Arbeit war die Entwicklung von Algorithmen und Protokollen, mit denen der energieeffiziente Betrieb einer spezifischen Familie von WSN ermöglicht wird. Die Entwicklung und die Validierung eines Ausbreitungsmodells für den Indoor-Funkkanal war ein erforderlicher Schritt, um die Untersuchung der entwickelten Verfahren zu ermöglichen. Das erste im Rahmen des Projektes entstandene Ergebnis war ein heuristischer, robuster verteilter Algorithmus, der eine energieeffiziente Integration aller Sensorknoten und die Bildung einer robusten baumförmigen Routingstruktur ermöglicht. Derselbe Algorithmus ermöglicht eine begrenzte Anpassung der Netzstruktur an die wechselnden Charakteristiken des Funkkanals. Einfache Erweiterungen des ursprünglichen Algorithmus wurden hinzugefügt, um die Selbstheilungsfähigkeiten des Netzes zu verbessern. Ein auf einer neuen Formulierung des Synchronisationsproblems basierendes Verfahren wurde entwickelt. Es gewährleistet eine energieeffiziente und robuste Zeitsynchronisation zwischen Nachbarnknoten und, indirekt, die Synchronisation aller Netzelemente. Obwohl die vorgeschlagenen Lösungen für eine spezifische Netzkategorie entwickelt wurden, ist der Autor überzeugt, dass sich die Lösungsansätze auf ein weites Spektrum von Problemen anwenden lassen.Wireless Metering Networks (WMN), a special class of Wireless Sensor Networks (WSN), consisting of a large number of tiny inexpensive sensor nodes are a viable solution for many problems in the field of building automation especially if the expected lifetime of the network permits to synchronize the network maintenance with the schedule for routine maintenance of the building. In order to meet the resulting energy constraints, the nodes have to operate according to an extremely low duty cycle schedule. The existence of an energy efficient MAC Layer protocol, the adoption of a robust time synchronization mechanism and the implementation of effective network discovery and maintenance strategies are key elements for the success of a WMN project. The main goal of this work was the development of a set of algorithms and protocols which enable the low energy / low power operation in the considered family of WMNs. The development and validation of a propagation model reproducing the characteristics of the indoor radio environment was a necessary step in order to obtain appropriate instruments for the evaluation of the quality of the proposed solutions. The author suggests a simple localized heuristic algorithm which permits the integration of all sensor nodes into a tree-like failure tolerant routing structure and also provides some basic continuous adaptation capabilities of the network structure.\\ A subsequent extension of the basic algorithm makes the network able of self healing. An innovative approach to the solution of the synchronization problem based on a reformulation of the original problem into an estimation problem permitted the development of an efficient time synchronization mechanism. This mechanism, which makes an opportunistic usage of the beacon signals generated by the MAC layer protocol, permits an effective reduction of the synchronization error between directly communicating nodes and, indirectly, introduces a global synchronization among all nodes. All the proposed solutions have been developed for a specific network class. However, since the presence of a low duty cycle scheduling, the adoption of a beacon enabled MAC protocol and the presence of limited hardware resources are quite general assumptions, the author feels confident about the applicability of the proposed solution to a much wider spectrum of problems