Vortrag UNIX-Stammtisch 03/98

Vortrag UNIX-Stammtisch 03/98

Im Rahmen der Arbeiten wurde ein Algorithmus zur initialen Bestimmung und Verfolgung der Position von mobilen Netzwerkknoten erarbeitet und implementiert. Das Verfahren basiert auf Entfernungsinformationen zwischen mobilen und ortsfesten Knoten. Es wird ein zweistufiger Ansatz verfolgt. In einem ersten Schritt wird die Position des zu lokalisierendenKnotens durch mindestens drei (2D) bzw. vier (3D) Entfernungswerte zu Knotenmit bekannter Position, sogenannten Anker-Knoten, bestimmt. Auf Basis dieser Position erfolgt im zweiten Schritt die Verfolgung des mobilen Knotens mittels Kalmanfilter. Der Algorithmus wurdemit Hilfe eines Netzwerksimulators in einem realen Kommunikationsszenario verifiziert.

Vortrag innerhalb der Konferenz 'Feldtag: Landnetz trifft Feldschwarm' am 23. September 2021:Drahtlose Vernetzung als Voraussetzung für die Digitalisierung in der Landwirtschaft » Nomadisches Campusnetz » Netzplanung und Messung im Betrieb » Integration von Endgeräten » Zusammenfassung

Die Nutzung von Umgebungsenergie ermöglicht bei einer zunehmenden Anzahl von Anwendungen drahtloser Sensorknoten eine autarke Energieversorgung. Wartungseingriffe sowie unvorhergesehene Energieengpässe begrenzen den autarken Einsatz derartiger Sensorknoten oder erfordern eine deutliche Überdimensionierung der Energy Harvester und Energiespeicher. Anwendungen, wie der Einsatz in unzugänglichen Bereichen, Sensornetzwerken mit einer großen Anzahl von Knoten oder sehr kleinen Knoten / Smart Dust, können unwirtschaftlich werden, wenn kein wartungsfreier Betrieb gewährleistet werden kann. Erreicht ein drahtloser Sensorknoten einen energetisch niedrigen Zustand, ist es für einen erneuten Start erforderlich, dass zusätzlich zu einer ausreichenden Spannung genügend Energie für den Startvorgang zur Verfügung steht. Bei zu zeitiger Aktivierung wird der Startvorgang aufgrund der einbrechenden Spannung nicht abgeschlossen und verhindert die Aufnahme des Betriebs. Ein Voltage Supervisor wird benötigt, um den Startvorgang so lange zu verzögern, bis ausreichend Energie für diesen zur Verfügung steht. Im Rahmen der vorliegenden Arbeit wird ein Voltage Supervisor für das Energiemanagement umgebungsenergieversorgter Sensorknoten vorgeschlagen. Die Herausforderungen liegen in der Realisierung einer definierten Abschalt- und Startcharakteristik, auch bei langsamen oder variablen Spannungsanstiegen, in der Implementierung einer Kaltstartfähigkeit und im zuverlässigen Schaltverhalten unter widrigen Umgebungsbedingungen oder niedrigem Energieeintrag. Für eine hohe Effizienz wird bei jeder Spannung ein geringer Eigenleistungsverbrauch gefordert. Im Ergebnis wird ein Konzept vorgestellt, welches einen zuverlässigen wartungsfreien Betrieb ermöglicht. Funktionen, wie die Abschaltung des Mikrocontrollers nach Erledigung der Aufgabe, ermöglichen darüber hinaus, weniger Energie als im Schlafmodus zu verbrauchen. Mit dieser Methode kann ein intermittierender Betrieb in Abhängigkeit der verfügbaren Energie realisiert werden. Simulation, experimentelle Untersuchung und die Einbettung in einen umgebungsversorgten drahtlosen Sensorknoten validieren die Funktionsfähigkeit unter allen gestellten Anforderungen. Der mittlere Eigenleistungsverbrauch der vorgeschlagenen Schaltung liegt bei 5,58 µW. Bei niedrigen Spannungen von 0 V – 1,4 V werden lediglich 568 nW benötigt. / The use of ambient energy enables an autonomous energy supply for an increasing number of wireless sensor node applications. Maintenance interventions or unforeseen energy input drops limit the autonomous use of such sensor nodes or require significantly oversized energy harvesters and energy storages. Applications such as the use in inaccessible areas, sensor networks with a large number of nodes or very small nodes / Smart Dust can become uneconomical, if no maintenance-free operation can be guaranteed. If a wireless sensor node reaches an energetically low state, it is necessary for a restart, that sufficient voltage and in addition sufficient energy is available for the starting process. In the case of too early activation, the starting process will not be completed due to the voltage drop and will prevent the start of operation. A Voltage Supervisor is required to delay the start until sufficient energy is available. Within the scope of this thesis, a voltage supervisor for the energy management of environmental-powered wireless sensor nodes is proposed. The challenges are in the realization of a defined switch-off and start characteristic even with slow or variable voltage slopes, a cold start capability and in reliable behaviour under adverse environmental conditions or low energy input. For high efficiency, a low power consumption is required at every voltage. As result, a concept is presented which enables reliable maintenance-free operation. In addition, the microcontroller can switch itself off completely after completion of the task and thus consume even less energy than in sleep mode. With this method, intermittent operation depending on the available energy, can be realized as well. Simulation, experimental investigation and inclusion in an energy harvesting supplied wireless sensor node validate the functionality under all given conditions. The average power consumption of the proposed circuit is 5.58 µW. At low voltages in the range 0 V - 1.4 V only 568 nW are required.

Nowadays, wireless sensor networks can be found in many new application areas. In these sensor networks there may exit a part of the network which are difficult to access or lie in a wide area, far apart. A change in the software (e.g., function update or bug fix) can entail reprogramming of all sensor nodes. This is very time consuming and labour intensive, if the patching has to be done manually for each individual sensor nodes. In the area of mobile phones, the over the air (OTA) update function has been established very well with good reliability. In embedded systems such as sensor nodes, where resources are severely restricted, an update cannot be stored but must be programmed directly with the transfer. For this to be possible, a lot of basic functionality is needed to be established to correct errors or to be able to resume a failed programming. Within the framework of this thesis a concept for the transmission and distribution of the firmware and programming the sensor node is established. Focus here is to optimize the use of resources and to provide basic functionality within the programming mode.

Die vorliegende Arbeit befasst sich mit dem Entwurf eines monolithisch integrierten 60-GHz-Empfängerschaltkreises in einer modernen Silizium-Germanium-Halbleitertechnologie mit 190 GHz maximaler Transitfrequenz. Drei für die Entwicklung von MMIC-Empfängerschaltkreisen äußerst wichtige Prinzipien liegen dem Entwurf zugrunde: die Optimierung von Rauschverhalten und Bandbreite sowie die Betrachtung der maximal erreichbaren Ausgangsleistung. Diese Prinzipien werden detailliert untersucht und typische Schaltungen dahingehend analysiert. Insbesondere wird eine Methode vorgestellt, die es erlaubt, die maximale Ausgangsleistung für die häufig verwendete Kaskodestufe vorherzusagen. Dabei handelt es sich um eine Erweiterung der Methode der Lastkurve nach Cripps. Weiterhin werden Ansätze zur Modellierung von Leitungen vorgestellt und ihre Verwendbarkeit für die unterschiedlichen Simulationsarten diskutiert. Der Hauptteil der Arbeit behandelt den Entwurf des Empfängerschaltkreises, welcher aus einem breitbandigen Eingangsverstärker mit niedrigem Rauschen und einstellbarer Verstärkung, einem Leistungsteiler, einem direkten Quadratur-Abwärtsmischer, einem Basisbandverstärker, einem Treiberverstärker für das Lokaloszillatorsignal sowie einem 90°-Phasenschieber besteht. Zusätzlich sind verschiedene Referenzstrom- und -spannungsquellen im Schaltkreis integriert. Die gefertigte Schaltung wurde messtechnisch vollständig charakterisiert, und alle Ergebnisse sind wiedergegeben. Der gemessene Mischgewinn beträgt bis zu 40 dB bei einer Bandbreite von mehr als 15 GHz. Die Zweiseitenbandrauschzahl liegt bei moderaten 7,5 dB. Die gemessene Phasen- und Amplitudenabweichung sind geringer als 5° und geringer als 0,15 dB. Die Gesamtschaltung nimmt 360 mW Leistung aus einer 2,2-V-Spannungsquelle auf. Insbesondere die Bandbreite des Empfängerschaltkreises stellt eine Verbesserung des aktuellen Standes der Technik dar. / The present work studies the development of a monolithic 60 GHz receiver IC in a modern 190 GHz-fT silicon-germanium semiconductor technology. The design is based on three fundamental principles, which are of great importance for MMIC receiver design: noise optimisation, bandwidth enhancement and output power considerations. Those principles are discussed in detail, and typical circuit examples are comprehensively analysed. Specifically, a method is presented that allows the prediction of output power for the frequently-used cascode stage. This method is an extension of Cripps’ load line theory. Furthermore, modelling approaches for transmission lines and their suitability for various types of simulations are discussed. The main part focuses on the design process of the receiver IC, which consists of a broadband low noise amplifier with variable gain, a power divider, a zero-IF quadrature mixer, a baseband amplifier, an LO driver amplifier and a 90°-phase shifter. Additionally, several reference current and voltage sources are implemented in the IC. The manufactured circuit is characterised in detail, and all measurement results are presented. Over a bandwidth of more than 15 GHz, the measured conversion gain is up to 40 dB with a moderate double sideband noise figure of 7.5 dB. An I/Q imbalance measurement reveals a phase accuracy of better than 5° and an amplitude error of less than 0.15 dB. The total power consumption is 360 mW from a 2.2 V-source. Particularly in terms of bandwidth, the circuit performance exceeds the current state of the art.

Handling mobility in wireless sensor networks can pose formidable challenges in protocol design, especially, at the link layer. Since most of the proposed Medium Access Control (MAC) protocols do not accommodate mobility, a node has two options to deal with a deteriorating link: (a) to continue data transmission until the link breaks and then establishes a new link with a new relay node; or (b) to seamlessly transfer the communication to a better link parallel to the data transmission over the existing link. Different from option (a) where a node can only search for a new link after the original link disrupts, option (b) enables a node to perceive the change in the quality of a link in advance. The link quality prediction is implemented by an adaptive handover mechanism. Both approaches will inevitably introduce latency. This thesis aims to quantify and compare such latency. Specifically, it investigates the latency of packet transmission in a mobile wireless sensor network with and without the support of a handover mechanism. To start with, the thesis elaborates the effect of mobility on the performance of the existing MAC protocols, and the need to maintain an unbroken link during data transmission. To implement the handover, a target MAC protocol is required to be selected first. Since the Receiver-Initiated MAC protocol (RI-MAC) uses only short beacon and data packets during communication that substantially reduce overhearing, collision probability and data recovery cost, it is chosen as the carrier for the latency evaluation. Even though RI-MAC performs well in many aspects, it has several demerits. To address the monotonous increment in the backoff window size and to reduce the occurrence frequency of the dwell time, a burst data transmission pattern is adopted to optimize RI-MAC. With the optimization, the protocol reduces the long idle listening time that a node has to wait before data transmission, and thus, works well in a static scenario. However, due to the high probability of link disconnection, the burst data transmission does not perform well in case of mobility. For the sake of accommodating mobility, an adaptive handover mechanism is developed on top of the optimized RI-MAC. Once a node evaluates that the data packets cannot be completely transmitted before the link terminates, it will search for a new relay node while keeping communicating with the original collaborator. It is implemented by embedding a neighbor discovery request in a data packet that will be transmitted in a broadcast channel. Neighbors of the node will participate in the handover process as long as they are in an active state and their distance to the transmitter does not exceed a pre-defined threshold. As a proof-of-concept for the handover mechanism, a mathematical model is established. The transmission rate, the moving speed of human beings, the duty cycle and the network density are all taken into consideration. The analytical result shows that the communication latency decreases with an increment in the network density and the duty cycle when the handover mechanism is used, whereas the latency exhibits a reverse trend when the handover mechanism is not used. To validate the mathematics-based evaluation, the NS2 network simulator is employed. The simulation result is found to perform in accordance with the analytical result. It is asserted that the latency of packet transmission with the handover support is much less than that without the handover support. The communication latency can be saved by at least 0.28s when the handover mechanism is applied. This figure can even grow as the duty cycle and the network density increase. From this perspective, the handover mechanism is verified to improve the latency of packet transmission as far as mobility is concerned.

Die wechselhafte Energiebereitstellung für drahtlose Sensorknoten durch Solarzellen stellt das Energiemanagement dieser Systeme vor große Herausforderungen. Bedingt durch saisonale und kurzfristige Effekte treten kontinuierlich Schwankungen in der Eingangsleistung auf, gleichzeitig soll jedoch eine zuverlässige und konstante Systemfunktion realisiert werden. Um dies miteinander zu vereinbaren, wird ein Modell zur Beschreibung der erwarteten Eingangsleistung aufgestellt, mit welchem der planmäßige Energieverlauf bestimmt werden kann. Dieser kann wiederum mit der realen Eingangsleistung verglichen werden, um den tatsächlichen energetischen Zustand des Sensorknotens zu bestimmen. Daraus lassen sich beispielsweise Entscheidungskriterien für die Steuerung der Energieverteilung oder Betriebszustände ableiten. Im Rahmen der Arbeit werden die physikalischen Hintergründe zur Modellierung der eingehenden Sonnenenergie beschrieben, der Stand der Technik zur Modellierung aufgezeigt und ein Modell als Basis für die weiteren Untersuchungen ausgewählt. Dieses wird auf die stark limitierte Hardware von drahtlosen Sensorknoten angepasst. Die Herausforderungen liegen dabei hauptsächlich in der geringen verfügbaren Rechenleistung, wenig Datenspeicher im System und dem Ziel, möglichst wenig Energie für die Berechnung zu verbrauchen. Im Ergebnis zeigt sich, dass ein angepasstes Modell auf drahtlosen Sensorsystemen umgesetzt werden kann und trotz der starken Limitierungen lauffähig ist. Es wird eine deutliche Verbesserung in der Verteilung der Energie über den Tag ermöglicht, wodurch sich trotz wechselhafter Quelle eine konstante Systemfunktion ergibt. Nebenher wird die Zuverlässigkeit und Ausfallsicherheit erhöht und Überdimensionierungen in Energiespeicher und Solarzelle können verringert werden. Das modellbasierte Energiemanagement stellt somit einen wichtigen Baustein für eine gesicherte Energieversorgung drahtloser Sensorsysteme dar. / The volatile energy supply by solar cells for wireless sensor nodes causes vast challenges for the energy management of such systems. Conditioned by seasonal and short time effects, the incoming power continuously varies. Simultaneously a reliable and constant function of the system has to be realized. To reconcile this, a model for the expected incoming solar power has been derived, which enables the estimation of the planned energy curve. This curve can be compared with the real progression of incoming power measured in parallel, to determine the current state of energy of a sensor node. This comparison is used to derive decision criteria for the control of the energy distribution or operating conditions. Within this work, the physical backgrounds for the modelling of the incoming solar energy and the state of the art of modelling solar power are described. A model is chosen as basis for further investigations and adapted to the limited hardware of wireless sensor nodes. The main challenges are the reduced processing power, few data memory in the system and the objective to consume as few energy as possible for the calculation. The results show that an adapted model can be implemented on wireless sensor systems and that it is executable despite the heavy limitations. This enables a distinct improvement of the distribution of energy across the day, which results in a constant systems function, despite the varying incoming power. At the same time the reliability and failure safety are being improved and the oversizing of the solar cell and the storage elements can be reduced. Therefore the model based energy management is an important component for a stable power supply of wireless sensor systems.

The remarkable growth in the applications of low power wireless networks (LPWNs) in various disciplines such as health-care, wildlife monitoring, unmanned vehicles and the emerging Internet of Things (IoT) brings along various challenges. Such applications demand the transfer of large amounts of data in short durations. Unlike conventional medium access control protocols, which force each competing node to contend for each packet it transmits, bulk data transmission enables a node to exclusively use a channel for transferring a large amount of data in succession. Bulk data transmission is a technique in which a sender node is granted exclusive access of the channel in order to transmit all the packets accumulated in its buffer. However, there are two problems with this strategy: (1) For how long should bulk data transfer last if there are multiple contending nodes? (2) How should this strategy deal with the significant fluctuation in the quality of a low-power wireless link? Understanding link quality fluctuations in a wireless sensor network is useful for various reasons. For example, nodes can determine when and for how long they should transmit packets, so that they can reduce the packet loss rate and the cost of retransmission (delay as well as power consumption). However, the quality of a link depends on many factors, which cannot be known except in a probabilistic sense. In this dissertation, I propose an efficient burst transmission scheme that measures and models the dynamic link quality fluctuations. Introducing a large empirical study at the beginning of this dissertation leads to a good understanding of the effect of external factors such as the environment (indoor,outdoor), Cross Technology Interference (CTI) and mobility of a sender node causing link quality to fluctuate. The analysis and observations of the empirical study establishes the basis on which the model for link quality estimation is built and designed. Here I propose three approaches to deal with different aspects of link quality fluctuation. (i) Offline approach- long-term characteristics: The offline approach models the link quality fluctuations by taking into account a large set of data. To obtain such a data set, experiments were performed on the site under study for several weeks. It was observed that the link quality fluctuates considerably even in static deployment. Understanding the stable durations, good and bad alike contribute to the efficient transmission of packets. I propose two offline approaches: (i) The first uses the conditional probability distribution function of signal-to-noise (SNR) fluctuation to estimate the expected reliable and unreliable period. (ii) The second uses k-mean clustering to characterise the link quality fluctuations into different states where the relationship between the states is defined by transitional probabilities. The advantages of employing an offline approach is (i) availability of sufficient memory, (ii) low computational cost, and (iii) possible use of a complex algorithm. However, these approaches can not deal with short-term link quality fluctuation. (ii) Online approach- short-term characteristics: Unlike the offline approaches, an online approach models the link quality in real time and deals with short-term link quality fluctuation. However, this approach has some limitations, such as (i) limited memory space to store data, (ii) high computational cost, (iii) and employment of a simple algorithm to estimate the burst size. My proposed online approach uses adaptive history array to estimate the duration of good and bad states from the statistics of incoming acknowledgement packets. (iii) Hybrid approach- long-to-short-term characteristics: A hybrid approach combines both offline and online methods. I also take advantage of both offline and online models in my proposed hybrid approach. My aim is to characterise the long-term link quality fluctuation with statistics that are obtained offline and to employ the statistics of received acknowledgement packets in real-time to deal with short-term link quality fluctuations. The online statistics are used to fine-tune and calibrate the offline model. To evaluate the performance of my proposed approaches, I implement them in TinyOS and deploy them on TelosB sensor nodes. Furthermore, the proposed approaches in this thesis are compared with the state-of-the-art approaches. The thesis concludes by showing that my approaches efficiently model the link quality fluctuation and propose correct burst size to achieve high throughput, reduce transmission delay, and power consumption under different channel conditions.