Refereed Articles and Book Chapters
|||Drahtlose Robuste Adaptive Industrielle Systeme , In ITG-Fachbericht-Mobilkommunikation VDE VERLAG GmbH, 2018. [bib] [abstract]|
Die industrielle Fertigung wandelt sich mit der Digitalisierung. Fertigungsstücke werden individuell durch Industrie 4.0-Technologien; sie sind jederzeit und überall erfassbar. Fertigungsprozesse werden digitalisiert und bilden Struktur und Verhalten einer Fertigungs- und Produktionsstrasse ab, womit sich wichtige Erkenntnisse zum Ablauf gewinnen lassen. Dies setzt eine Vernetzung der Fertigungsstücke voraus, welche die klassische Automatisierung ergänzt. Drahtlose Sensoren sind an Fertigungsstücke gekoppelt und bilden großflächige und dichte Sensornetze. Messgrößen der Sensoren sind beispielsweise Ort, Zeit, Temperatur, Beschleunigung, Geräusch oder Luftfeuchte, welche nach Anforderung der Datenanalyse ausgewählt und ausgewertet werden. Derartige Sensornetze stellen neue Herausforderungen und Forschungsfragen an Industrie 4.0-Anwendungen. Das Projekt DRAISE hat das Ziel, produktionsrelevante Daten in Industrieanlagen zuverlässig zu erfassen und entsprechend aufzubereiten. Hierfür werden Transportwagen einer Fertigung drahtlos vernetzt und Temperatur, Erschütterung, Position und Zeit erfasst. Zusätzlich interagieren die Mitarbeiter in der Produktion über eine definierte Mensch-Maschine-Schnittstelle mit dem System. Zur Minimierung der Auswirkungen durch andere, drahtlose Systeme werden Time Division Multiple Access (TDMA)-basierende Protokolle für die Single-Hop- und Multi-Hop-Kommunikation entwickelt. Die Kommunikation wird durch Lokalisierungskomponenten ergänzt, welche mit einem System auf Basis von Ultra-Wide-Band (UWB) implementiert ist. Für die angestrebte, zuverlässige und robuste Kommunikation werden zukünftig Frequenzsprungverfahren und Spectrum Sensing die entwickelten Protokolle erweitern. Ein großflächig und produktiv eingesetzter Demonstrator dient der ständigen Leistungsbewertung des Systems.
|||Entwicklung einer kompakten Sensorplattform für prototypischen Einsatz in der Medizintechnik , In ImpulsE (submitted), volume 20, 2015. [bib]|
|||Höhenbestimmung mittels Luftdrucksensoren und differentieller Messung für Indoor-Anwendungen , In ImpulsE, volume 17, 2013. [bib]|
|||Software Defined Radio - Flexible Plattform fuer Forschung und Lehre , In ImpulsE, volume 16, 2012. [bib] [pdf]|
Refereed Conference Papers
|||Listen and Talk in IEEE 802.15.4 with Dual Radio , In 2018 Advances in Wireless and Optical Communications (RTUWO) (RTUWO'18), 2018. [bib] [abstract]|
In wireless sensor networks, the license-free 2.4 GHz industrial medical and scientific band is increasingly popular. The popularity of this license-free band results in growing heterogeneous wireless networks in which the channel state detection becomes difficult. In this paper, we present a novel solution to enable Listen and Talk with an IEEE 802.15.4 dual radio transceiver sensor node, to provide a channel state detection. In contrast, to Listen before Talk or Clear Channel Assessment, our approach named Advanced Clear Channel Assessment talks and listens, which means checking the channel state during transmission. We show that Advanced Clear Channel Assessment increases the channel busy detection compared to Clear Channel Assessment by a factor of six. Furthermore, we provide a novel metric to compare detection algorithms for channel state based on threshold levels.
|||TriClock – Clock Synchronization compensating Drift, Offset and Propagation Delay , In IEEE International Conference on Communications, 2018. [bib] [abstract]|
In wireless sensor networks (WSN) precise clock synchronization is still a challenge e.g. for synchronized medium access control (MAC). State of the art solutions require many messages or neglect clock drifts or propagation delay. In multi-hop networks synchronization errors increase with the number of hops because numerous messages increase latency. The latency and clock drift reduces synchronization accuracy. Finally, propagation delay introduces additional synchronization offsets. We introduce a novel synchronization protocol that requires a single message to compensate both clock offset and clock drift and one additional message to account for propagation delay. With this minimal amount of messages, an efficient multi-hop synchronization is practicable. We implement our approach on a DWM1000 hardware and evaluate the protocol in single-hop and multi-hop configuration. In our preliminary experiments, we achieved a synchronization accuracy of 0.46 ns in a single-hop configuration within 3.6 ms and 6 ns in a multi-hop configuration for 5 hops within 11 ms which is appropriate for MAC and time-division multiple access (TDMA) implementations.
|||A Model-based Approach for Self-healing IoT Systems , In Proceedings of the 7th International Conference on Sensor Networks (SENSORNETS) , 2018. [bib] [abstract]|
IoT systems become more and more important in our daily life. They will perform monitoring and control tasks which are often safety-critical. Therefore, it is obviously important that IoT systems work reliably, i.e., fulfill their specification. Even if something unexpected happens, it is required that the system moves back into a correct state which we name self-healing. In this paper, we present our idea for a model-based approach for self-healing IoT systems. Based on a formal specification of the system’s properties, we derive monitors which observe the system behavior and trigger healing actions when necessary. In IoT systems, the placement of such systems becomes important due to the increased unreliability of single devices. The paper outlines basic ideas where to place monitors and how to assign monitoring tasks to IoT devices.
|||Name-Centric Services for the Internet of Things , In 3rd International Conference on Networked Systems (NetSys), 2017. [bib] [abstract]|
Today, we see the development of the Internet of Things (IoT). Latest IoT devices are embedded, single-board computers (SBC) equipped with Ethernet and WiFi. Those SBCs are run an operating system and have network interfaces and use standard Internet protocols like IP, TCP, and so on. Compared to the envisioned devices for the future IoT these SBCs are still large and connected to a steady power supply. The next step in evolution towards the future IoT will be smaller, microcontroller driven devices which will operate for years on small batteries. Such small devices communicating via lowpower radios and also equipped with sensors are known as wireless sensor nodes. Ad-hoc networks formed by multiple wireless sensor nodes are so called wireless sensor networks (WSN).
|||DRAISE - Drahtlose, Roboste Adaptive, Industrielle Systeme , In Zukunft der Netze 2017, Postersession, 2017. [bib]|
|||Indoor Localization based on Bi-Phase Measurements for Wireless Sensor Networks , In 2015 IEEE Wireless Communications and Networking Conference (WCNC): - Track 3: Mobile and Wireless Networks (IEEE WCNC 2015 - Track 3- Mobile and Wireless Networks), 2015. [bib] [abstract]|
Indoor localization is important for medical and industrial application as well as for wireless emergency and security systems. For such applications an accuracy within a few meters is desired. Available radio based systems within that accuracy are neither cost effective nor easy to deploy. In this work, we suggest an approach called biphase measurement based on phase measurements with two frequencies to determine the location of a tag. We design and build a complete indoor positioning system based on bi-phase measurements with easy to deploy wireless sensor nodes. The wireless sensor nodes shape anchors and tags and communicate results to a location engine of the indoor positioning system. Our implementation comprises lowcost IEEE802.15.4 radio chips with built-in support for phase measurements unit for both, anchor and tags. We compute the position of the tag based on distance estimation retrieved with bi-phase measurements. We evaluate our indoor positioning system providing first measurement results for accuracy and precision and discuss trade-off between scalability, real-time and accuracy.
|||Receiving more than Data - A Signal Model and Theory of a Cognitive IEEE 802.15.4 Receiver , In Proceedings of the 10th International Conference on Cognitive Radio Oriented Wireless Networks (CROWNCOM), 2015. [bib] [abstract]|
In standard medium access, transmitters perform spectrum sensing. Information about concurrent interferers is gained mainly during this sensing period.Especially during transmission respectively reception there is a blind gap where transmitter and receiver have limited capabilities to detect interferer. Standard radio receiver devices for IEEE 802.15.4 provide solely data output and no cognitive capabilities. Particularly mobile interferer create problems when moving gradually into reception range. First, they create small interference before actually causing collision later, when approaching. However, small interference is not yet detectable by today?s transceivers. As a solution, we provide a signal model and an architecture for an extended cognitive IEEE 802.15.4 receiver as a basis for advanced signal processing for interference detection. The results of our theoretical analysis verify that the received signal contains signal marks of the interferer and therefore holds more information than transmitted data. Our theory is evaluated by simulations and experiments with a pair of IEEE 802.15.4 transmitter and an extended cognitive receiver.
|||Interference Detection on Reception for IEEE 802.15.4 Transmissions , In Proceedings of the 21th European Wireless Conference, 2015. [bib] [abstract]|
With the increasing number of wireless devices and competing technologies in overlapping frequency bands an improved spectrum sensing is needed. Todays IEEE 802.15.4 transceiver perform spectrum sensing before transmission. They cannot detect and classify interference during transmission and reception reliably as there is a blind gap during transmission. Strong interference can be detected if bit errors occur. However, the transmission time is a long time interval compared to spectrum/carrier sensing before. In a crowded 2.4GHz ISM band with mobile interfering sources transceivers need continuous spectrum awareness to improve coexistence. In our approach we extend the physical layer (PHY) of a traditional receiver with additional signal processing. We perform signal analysis with Fourier transform of a demodulated signal to detect interferer during reception before bit errors or collisions occur. We implement our algorithm with a software defined radio as an extension of the physical layer (PHY) of an IEEE 802.15.4 transceiver. We evaluate our approach with measurements and detect mobile interferers in a real world environment. We discuss the design space of solutions as a trade-off between the quality of interference detection and the time for detection.
|||Wireless Medical Sensors - Context, Robustness and Safety , In 49th annual conference of the German Society for Biomedical Engineering (BMT 2015), 2015. [bib]|
|||Performance Evaluation of Cooperative Sensing via IEEE 802.15.4 Radio , In Wireless Communications and Networking Conference, IEEE (WCNC 2015), 2015. [bib] [abstract]|
Spectrum Sensing is one of the important tasks for the wireless devices but due to fading, shadowing and noise the performance of individual spectrum sensing devices is not ideal. Cooperative Sensing is seen as a way to improve the performance of individual spectrum sensing devices resultantly improving the efficient utilization of radio bandwidth and minimizing the interference among wireless devices. State of the art are extensive simulations and analysis on cooperative sensing although there are also number of performance evaluations of various fusion rules of cooperative sensing using software defined radios and FPGAs. The limitation of previous work is that they do not address the question how we can improve the overall performance of real systems with cooperative sensing. To the best of our knowledge, this is the first experimental work which presents cooperative sensing protocols with standard radios and evaluates the system performance using cooperative sensing. With IEEE 802.15.4 equipped radio devices we model primary, secondary and cooperating users. We implement cooperative sensing protocols, setup a scenario, perform measurements and compare system performance with and without cooperative sensing. All the experiments are automated with the wisebed testbed software. The evaluation results of cooperative sensing protocols indicate new challenges for optimization and provide awareness to the problem of improving the overall system performance
|||Accurate Radio Distance Estimation by Phase Measurements with Multiple Frequencies , In The Fifth International Conference on Indoor Positioning and Indoor Navigation 2014 (IPIN 2014), 2014. [bib] [abstract]|
Indoor localization is beneficial for logistics, industrial applications and for several consumer applications. In the area of logistics, e.g. warehouses, localization accuracy within a few meters is desired. Available radio based systems within that accuracy are neither cost effective nor easy to deploy. Distance estimations are one possible method for localization. In this work, we propose phase measurements between two wireless sensor nodes for distance estimation. We introduce a mathematical model to estimate distances from phase measurements with multiple frequencies and provide a systematic analysis of possible sources of errors. Additionally, we derive requirements, e.g. resolution and speed for a phase measurement unit to reach certain accuracy. To proof our theoretical results, we present evaluation results based on our implementation. Our implementation comprises a low cost IEEE 802.15.4 hardware with a built-in phase measurement unit. We implement the developed algorithm for distance estimation in our wireless sensor network and use two wireless sensor nodes to perform a phase measurement. The contribution of the paper comprises a new model for phase measurements to estimate distances and a preliminary evaluation with our hardware.
|||A Solution for the Naming Problem for Name-Centric Services , In 12th International Conference on Wired & Wireless Internet Communications (WWIC 2014), 2014. [bib] [abstract]|
In recent past name-centric or content-centric networking (CCN) has gained substantial attention in the networking community. In a further development step name-centric service architecture enables the flexible placement and distribution of services in the network especially in a heterogeneous environment of wired and wireless (sensor) networks. However, the problem of structuring and creating hierarchies for names in name-centric networks is not solved yet. E.g. there is no configuration of service names in name-centric service WSN, no concept of unsolicited names or link-local names in CCN. In IP networks, DHCP or IPv6 auto- configuration is available, but no equivalent technique exists for CCN. We analyze the naming problem in the software development life cycle for name-centric services in WSN and propose a structure, hierarchy, and configuration mechanism for names. The paper introduces the overall concept and preliminary steps of implementation.
|||Experimental Evaluation & Optimization of a UWB Localization System for Medical Applications , In 48th annual conference of the German Society for Biomedical Engineering (BMT 2014), 2014. [bib]|
|||Evaluation of Radio Based, Optical and Barometric Localization for Indoor Altitude Estimation in Medical Applications , In The Fifth International Conference on Indoor Positioning and Indoor Navigation, 2014. [bib] [abstract]|
The advances of electronics provide options for improved monitoring of patients in clinical environment.Medical applications like blood pressure monitoring require precise and wireless altitude measurement in indoor environment. An error of only a few centimeters may lead to mistreatment of patients.Furthermore, user requirements like small form factor, usability and robust operation are important in the medical field.Existing evaluations of indoor localization systems focus on accuracy analysis of x- and y-coordinates and not on the z-coordinate (altitude). In this paper, we define evaluation criteria for altitude estimation in medical applications. We compare an Ultra-Wide-Band indoor localization system, an optical Microsoft Kinect camera system and our own development of a wireless barometric sensor against these criteria. We present a comparative measurement setup, results and a final evaluation of the three systems in an indoor environment.
|||CCN-WSN - a lightweight, flexible Content-Centric Networking Protocol for Wireless Sensor Networks , In 2013 IEEE Eighth International Conference on Intelligent Sensors, Sensor Networks and Information Processing (IEEE ISSNIP 2013), 2013. [bib] [abstract]|
In future Internet research, content centric networking (CCN) is a new promising approach. CCNx has been introduced recently as an open source protocol suite for CCN and implementation base for practical research. In wireless sensor networks (WSNs) research, data or content centric approaches like in-network processing and data aggregation are important. While the principle of CCN is a suitable approach in WSNs, the CCNx protocol suite designed for PCs is not applicable to resource-constrained WSNs. Additionally, gateways necessary between CCNx and WSN are difficult to implement. Therefore, we design, implement and evaluate a lightweight variant of a CCN protocol specifically for WSNs called CCN-WSN. Key concepts of CCNx protocol are integrated but a variety of aspects are revised to meet the memory and computational constraints of sensor nodes and communication patterns in WSNs. E.g. the message format is simplified and some fields are omitted completely. Instead, we propose a flexible naming strategy which extends the functionality of content names to add small amount of data in interest messages. For performance evaluation a challenging time-synchronization application was implemented with CCN-WSN to demonstrate the flexibility of the approach and a comparison with a reference protocol for data dissemination called AutoCast is presented.
|||Precise Indoor Altitude Estimation based on differential barometric Sensing for wireless Medical Applications , In Body Sensor Networks Conference 2013 (BSN2013), 2013. [bib] [abstract]|
Some medical applications require precise information of position and orientation of a patient as changes affect pressure condition inside the body. In this paper we focus on altitude estimation, where altitude is a distance, in vertical direction, between a reference and a point of a human body. We suggest equipping wireless sensor nodes with high resolution pressuresensors to calculate the altitude with the barometric formula. We implement a body sensor network based on IEEE 802.15.4 and synchronization mechanism with a reference. Pressure variations due to environmental effects are compensated by cancellation with this differential measurement setup. We demonstrate the need for differential measurements and show with a series of measurements that environmental pressure variations have no significant effect on the proposed altitude estimation. Compared to existing systems, our solution is cost effective, easy to deploy and provides a flexible tradeoff between precision and location lag by adjusting a filter constant.
|||Performance Evaluation Metric for Cooperative Sensing in Heterogeneous Radio Environments , In European Wireless Conference, IEEE, 2013. [bib] [abstract]|
Spectrum sensing is a major task for wireless devices in order to improve coexistence among them in heterogeneous radio environments. Wireless communication includes at least two partners: transmitter and receiver. Therefore, cooperation between partners can improve the performance of spectrum sensing by reducing effort, improving sensing result or a combination of both. An optimal cooperative sensing scheme is a first step to achieve complete awareness of the radio environment for wireless devices. To the best of our knowledge, this is the first theoretical work performed in order to understand the problem of developing optimal cooperative sensing schemes for heterogeneous radio environments for multiple users and single channel. We analyze the problem and perform analytical work which results in a cooperative sensing model. The model comprises sensing schedule, data fusion rules, PU's traffic pattern, and detection performance of the sensing device. A new performance evaluation metric is introduced for optimum spectrum sensing in heterogeneous radio environments. An evaluation of available exemplary cooperative sensing schemes shows that none provides optimality in all scenarios.
|||Non-Invasive Cognitive Radio for Firm Real Time Sensor Applications in heterogeneous Radio Environments , In Proceedings of the Sixth International Conference on Sensor Technologies and Applications (SENSORCOMM), 2012. [bib] [pdf] [abstract]|
Some applications in Sensor Networks need firm real-time support in order to work properly. The difference to hard real-time systems is that this type of application can withstand minor violations of the maximum delay and minimum throughput if these violations are limited. Many standards like IEEE 802.15.4 provide standardized means to ensure delay and bandwidth constraints which work well when there are no interferers in the same frequency band. However, in a heterogeneous environment today these approaches fail when the interference is not aware of the IEEE 802.15.4 traffic. Switching the channel is one option to avoid this kind of interference. We suggest a new non-invasive cognitive radio protocol approach where all participants follow simple rules to enable firm real-time conditions in decentralized design. As a demonstrator we use a three-fold pendulum with firm real-time signal delay constraints of 5ms. The contributions of the paper comprise evaluation results by real measurements with the demonstrator system.
|||CSOR - Carrier Sensing On Reception , In Proceedings of the 4th International Conference on Cognitive Radio and Advanced Spectrum Management (CogART), 2011. [bib] [pdf] [abstract]|
Since the 1990s the number of wireless devices increases and new areas of applications evolved. Therefore, frequency spectrum has become a scarce resource with no free frequencies left all over the world and interference between transmissions sharing the same frequency band is started to become one of the major problems in wireless transmission. The ISM bands become crowded with various standards sharing the same frequency band. One solution to the problem is to use frequency bands that are rarely used by the licensed users like TV channels, where in some region specific channels are not used at all. As a result, we need to develop adaptive systems that search for unused spectrum, use it as long as the band is free and shift to other frequency bands if there is a risk to interfere with a primary user. Such systems being aware of their radio environment are called Cognitive Radios. To search for unused frequencies and detect primary users approaches that listen before talk or efficient carrier/spectrum sensing algorithms have been presented in the past. These mechanisms are incomplete and one of the drawbacks of today's wireless transmissions is that communication partners do not detect interference reliable during an ongoing transmission. In this paper we suggest a cross layer approach Carrier Sense on Reception (CSOR) that extends the functionality of the physical layer of a transceiver to be able to detect interference while receiving data. We introduce the idea, describe the concept and give first evaluation results as a proof of concept based on real measurements.
Refereed Workshop Papers
|||Comparison of wired and wireless synchronization with clock drift compensation suited for U-TDoA localization , In 13th Workshop on Positioning, Navigation and Communication, 2016. [bib] [abstract]|
Indoor localization with Uplink Time Difference of Arrival (U-TDoA) provides good scalability, high updates rates and high accuracy. However, clock errors lead to localization errors and synchronization is important. In this paper, we design and implement wired and wireless synchronization and provide a comparison between them. We design and implement a wireless synchronization with clock drift compensation. For wired and wireless synchronization, we discuss reasons for clock deviation that lead to localization errors. We evaluate both approaches in a U-TDoA measurement setup. Finally, we provide recommendations for wired and wireless synchronization.
|||Integrated Low-Power SDR enabling Cognitive IEEE 802.15.4 Sensor Nodes , In Proceedings of the 8th Karlsruhe Workshop on Software Radios, 2014. [bib] [abstract]|
The number of mobile devices with radio transceivers is increasing. However, standard wireless sensor nodes have limited spectrum awareness in order to avoid col- lisions with other concurrent transmissions in dense spectrum. E.g. IEEE 802.15.4 standard performs carrier sensing before start of the transmission. Spectrum sensing or awareness during the transmission is not provided. These low power devices have only limited capabilities in order to detect and forecast upcoming collisions. In this work, wireless sensor nodes are equipped with additional piggybacked hardware and supplementary signal processing capabilities. An additional RF-frontend and small-size SDR hardware enable sensor nodes to perform cognitive radio functionality. Although the sensor nodes transmit data fully com- pliant to IEEE 802.15.4 the supplementary hardware enhances spectrum awareness significantly even during transmission. A previously published cognitive radio scheme was implemented to demonstrate the signal processing capabilities of the SDR hardware. Additionally, power consumption and battery lifetime were evaluate and calculated.
|||Software Defined Transceiver for Underwater Communication , In Proceedings of the European Workshop on Testbed based Wireless Research, 2014. [bib] [abstract]|
New applications for autonomous underwater vehicles require underwater wireless communication with data rates up to several megabit per second and a transmission range up to several meter. Regular radio transceivers achieve a poor performance in seawater. Therefore, our project evaluates and implements new approaches for optical, acoustic and electric underwater communication. The goal is to implement a flexible software defined transceiver consisting of a FPGA with exchangeable communication modules.
|||A Reusable and Extendable Testbed for Implementation and Evaluation of Cooperative Sensing , In The 8th ACM International Workshop on Performance Monitoring, Measurement and Evaluation of Heterogeneous Wireless and Wired Networks PM2HW2N'13, 2013. [bib] [abstract]|
Cooperative sensing has been identi?ed as a potential improvement for cognitive radios to perceive their radio environment. In the past, algorithms have been developed by analysis and simulations exclusively. With cheaper hardware experimental platforms have been used for evaluation purpose recently. Simulations lack realistic propagation models for radio transmission but are reproducible compared to experimental evaluation done by hand. The effects of reduced detection probability and false alarms are not realistic in these simulations. In this paper, we suggest a reusable and extendable automated testbed software and instructions for deployment of own testbeds. Primary users as well as secondary users with cooperating cognitive radios can be flexibly deployed in the testbed within seconds. The advantage is that a series of even long lasting measurements including automatic logging of results can be easily repeated. Results can be assessed on the fly during the ongoing evaluation by accessing debug output remotely. The testbed supports stationary, portable, and in the future mobile radio devices for flexible scenarios as well as monitoring devices for debugging. The testbed and the radio devices are validated by deploying primary and secondary user in a small scenario whose outcome was analyzed beforehand. The results are as predicted and show the usefulness of this approach.
|||Non-Invasive Cognition driven Spectrum Access in Medical Application via Baseband Processing , In Proceedings of the 7th Karlsruhe Workshop on Software Radios, 2012. [bib] [abstract]|
In the field of medical applications there are special regulations and requirements as many devices are sensitive to interference by other equipment. Although wireless links are susceptible to interference they are potential technologies enabling mobile and wireless applications substituting cables in all medical areas like operating room, intensive care unit or ambient assistant living. With continuous increase of these devices equipped with radio interfaces a non-homogenous radio environment with dense occupancy will form. We propose an approach with high sensitivity for non-invasive cognition of radio environment by fusion of two baseband processing blocks. First, we detect and identify interference by concurrent radio links non-invasively during reception. Second, we minimize invasive channel switching by continually probing and classification of available channels based on adapted probabilities. We use GNU Radio a Software Defined Radio platform to implement this system to provide a non-invasive continuous cognition of the radio environment and minimize the invasive utilization of occupied channels. In this paper we present preliminary studies, the overall idea, the approach and first implementation results.
|||Using and Operating Wireless Sensor Network Testbeds with WISEBED , In Proceedings of the 10th IEEE IFIP Annual Mediterranean Ad Hoc Networking Workshop, 2011. [bib] [pdf] [abstract]|
Current surveys and forecast predict that the number of wireless devices is going to increase tremendously. These wireless devices can be computers of all kinds, notebooks, netbooks, Smartphones and sensor nodes that evolve into realworld scenarios forming a "Real-World-Internet" in the future. In our work we focus on the Future Internet with small battery driven devices forming the "Internet of Things". In recent networking research, testbeds gain more and more attention, especially in the context of Future Internet and wireless sensor networks (WSNs). This development stems from the fact that simulations and even emulations are not considered sufficient for the deployment of new technologies as they often lack realism. Experimental research on testbeds is a promising alternative that can help to close the gap. The deployment of testbeds is challenging and user and operator requirements need to be considered carefully. Therefore, the goal is to design an architecture that allows operators of WSN testbeds to offer numerous users access to their testbeds in a standardized flexible way that matches these requirements. In this paper we first identify some of the requirements, then introduce the architecture and general concepts of our WISEBED approach and show how this architecture meets the requirements of both groups. We give an overview of existing WISEBED compatible WSN testbeds that can be used for experimentation today. Main focus in this paper compared to previous work is to address the perspective of both users and operators on how to experiment or respectively operate a WSN testbed based on WISEBED technology.
|||SAFH - Smooth Adaptive Frequency Hopping , In International Workshop on Cross-Layer Design (IWCLD 2011), 2011. [bib] [pdf] [abstract]|
Wireless systems based on WLAN (802.11), ZigBee (802.15.4) and Bluetooth (802.15.1) are continuously deployed in new applications covering consumer, industry or medical fields. Especially, Bluetooth is recommended by the Health-Care-Organization for medical applications as frequency hopping is considered as a robust scheme. However dealing with frequency-dynamic sources of interference in the 2.4GHz ISM band is important due to the increase of wireless devices. Adaptive frequency hopping (AFH) suggested by the Bluetooth standard and implemented in many of todays products identifies and avoids using bad channels. It is a good and established coexistence mechanism in the presence of frequency-static sources of interference such as WLANs when the 2.4GHz band is not crowded. However, AFH is facing problems in a crowded 2.4GHz band, especially when the interference is dynamic. We developed a cross-layer algorithm SAFH (Smooth Adaptive Frequency Hopping) that is inspired by entropy maximization and the conventional Bluetooth AFH. SAFH assigns usage probabilities to all channels based on an exponential smoothing filter for frame error rates to estimate and predict the channel conditions. The application layer can adapt SAFH by parameter settings in a cross-layer approach. SAFH achieves low average frame error rate and responds fast to changing channel conditions if required from the application. Simulative Evaluation in the presence of different types of interference (802.11b, 802.15.4 and 802.15.1) shows that our algorithm outperforms conventional frequency hopping and AFH. Additionally, SAFH works smoothly and stable exploiting frequency diversity compared to previous approaches like entropy-maximization based adaptive frequency hopping and Utility Based Adaptive Frequency Hopping (UBAFH).
|||Limitations of Frequency Hopping in 2.4 GHz ISM-Band for Medical Applications Due to Interference , In Proceedings of the Workshop Consumer eHealth Platforms, Services and Applications (CeHPSA) IEEE CCNC 2011 Conference Proceedings, 2011. [bib] [pdf] [abstract]|
In the past a number of wireless standards evolved in industry and the consumer market that operate in the unlicensed ISM (Industrial, Scientific, Medical) Bands especially in the 2.4GHz range. Today these products are well engineered and standardized, so that more and more medical applications consider wireless transmission based on these standards. Especially in the clinical environment radio links will substitute cables and further enable new fields of applications. Also personal healthcare and ambient assisted living are emerging fields for the future. However, in contrast to many consumer applications medical applications need to guarantee interoperability, ensure coexistence with other applications and have very high requirements for robustness, safety and security. There is an upcoming need for investigation if existing radio standards are prepared for the usage in medical applications regarding reliability and interoperability. In this work we will focus on Frequency Hopping and Bluetooth as one of the technologies that has been designed for robustness and coexistence and evaluate the underlying principles by a comprehensive analysis, simulation and measurements. To the best of our knowledge this comprehensive investigation for robustness of wireless transmission based on Frequency Hopping in a heterogeneous mix of interference from standards like WLAN, IEEE 802.15.4, Bluetooth and proprietary systems in health care environment has not been performed yet. We will conclude the paper by an outlook for further improvement of wireless transmission based on Frequency Hopping suited to the needs of medical applications.
|||Lessons learned: Indoor Ultra-Wideband localization systems for an industrial IoT application , Technical report, Technische Universität Braunschweig, 2018. [bib] [pdf] [abstract]|
Since ultra-wideband (UWB) transceivers are available for wireless sensor networks, the usage in research and industry increased. Research efforts resulted in methods, measurement results, and solutions under laboratory conditions for a variety of indoor localization problems provided to the community. In this paper, we present an indoor positioning system (IPS) that is installed in a $mathbf1500 m^2$ real world production facility. In this real-world application, we faced some challenges that research has not addressed yet. For instance, challenges are receiving UWB signals from other floors in a multistory building through windows and multipath effects at walls like reflexions. We present solutions to increase the availability of such large-scale IPS, give a performance evaluation and recommendation for a modified NMEA sentence named iNMEA for IPS receivers.
|||Evaluation of Bluetooth Positioning for Medical Device Tracking , GRIN (T. M. Buzug et. al., ed.), 2017. [bib]|
|||Comparison and Performance Evaluation of Indoor Localization Algorithms based on an Error Model for an Optical System , GRIN (T. M. Buzug et. al., ed.), 2015. [bib]|
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