Albert Krohn

Mutual interference in OFDM-based spectrum pooling systems

The public mobile radio spectrum has become a scarce resource while wide spectral ranges are only rarely used. Here, the new strategy called spectrum pooling is considered. It aims at enabling public access to these spectral ranges without sacrificing the transmission quality of the actual license owners. Unfortunately, using OFDM modulation in a spectrum pooling system has some drawbacks. There is an interaction between the licensed system and the OFDM based rental system due to the non-orthogonality of their respective transmit signals. This interaction is described mathematically, providing a quantitative evaluation of the mutual interference that leads to an SNR loss in both systems. However, this interference can be mitigated by windowing the OFDM signal in the time domain or by the adaptive deactivation of adjacent subcarriers providing flexible guard bands between licensed and rental system. It is obvious that both approaches sacrifice bandwidth of the rental system. A quantitative comparison of both approaches is given as a tradeoff between interference reduction and throughput in the rental system.

A relative positioning system for co-located mobile devices

If a mobile computing device knows how it is positioned and oriented in relation to other devices nearby, then it can provide enhanced support for multi-device and multi-user interactions. Existing systems that provide position information to mobile computers are reliant on externally deployed infrastructure, such as beacons or sensors in the environment. We introduce the Relate system, which provides fine-grained relative position information to co-located devices on the basis of peer-to-peer sensing, thus overcoming dependence on any external infrastructure. The system is realised as a hardware/software plug-in, using ultrasound for peer-to-peer sensing, USB to interface with standard mobile devices, and data abstraction and inferencing to map sensor data to a spatial model that maintains both quantitative and qualitative relationships. We present a set of services and applications to demonstrate the utility of the system. We report experimental results on the accuracy of the relative position and orientation estimates, and other aspects of system performance.

The particle computer system

This paper presents a sensor-based, networked embedded system, referred to as the particle computer system. It is comprised of tiny wireless sensor nodes, capable of communication with each other, as well as connectivity with backend, PC-based systems, thereby facilitating software development and data analysis in an integrated systems package. The core design principles of the sensor nodes enable operation in very mobile settings and truly ad-hoc, peer-to-peer interoperation without the intervention of a master or explicit middleware layer. The two main system properties highlighted in this paper are: 1) information distribution to all components within the system and 2) the usage of a common communication language in all system components. This language has been proprietarily developed for the particle system and is known as ConCom. As a result of these system properties, we have found the particle system to be very extensible and applicable in many everyday scenarios. The paper presents insights to the implementation of the particle computer system, including software development and data analysis capabilities, and the overall system integration.

AwareCon: Situation Aware Context Communication

Ubicomp environments impose tough constraints on networks, including immediate communication, low energy consumption, minimal maintenance and administration. With the AwareCon network, we address these challenges by prescribing an integrated architecture that differs from classical networking, as it features an awareness of the surrounding situation and context. In various settings, where AwareCon was implemented on tiny battery driven devices, we show that applications and usability of devices benefit from this approach.

Collaborative sensing in a retail store using synchronous distributed jam signalling

The retail store environment is a challenging application area for Pervasive Computing technologies. It has demanding base conditions due to the number and complexity of the interdependent processes involved. We present first results of an ongoing study with dm-drogerie markt, a large chemists retailer, that indicate that supporting product monitoring tasks with novel pervasive technology is useful but still needs technical advances. Based on this study, we uncover problems that occur when using identification technology (such as RFID) for product monitoring. The individual identification struggles with data overload and inefficient channel access due to the high number of tags involved. We address these problems with the concept of Radio Channel Computing, combining approaches from information theory, such as the method of types and multiple access adder channels. We realise data pre-processing on the physical layer and significantly improve response time and scalability. With mathematical formulation, simulations and a real world implementation, we evaluate and prove the usefulness of the proposed system.

Inexpensive and automatic calibration for acceleration sensors

In this paper, we present two methods for calibration of acceleration sensors that are inexpensive, in-situ, require minimum user interaction and are targeted to a broad set of acceleration sensor applications and devices. We overcome the necessity of orthogonal axes alignment by extending existing calibration methods with a non-orthogonal axes model. Our non-orthogonal method can furthermore be used to enable automatic calibration for 1- or 2-axes accelerometers or realize a simultaneous mass-calibration of sensors with minimum effort. The influence of noise to the presented calibration methods is analysed.

eSeal – A System for Enhanced Electronic Assertion of Authenticity and Integrity

Ensuring authenticity and integrity are important tasks when dealing with goods. While in the past seal wax was used to ensure the integrity, electronic devices are now able to take over this functionality and provide better, more fine grained, more automated and more secure supervision. This paper presents eSeal, a system with a computational device at its core that can be attached to a good, services in the network and a communication protocol. The system is able to control various kinds of integrity settings and to notify authenticated instances about consequent violations of integrity. The system works without infrastructure so that goods can be supervised that are only accessible in certain locations. The paper motivates the eSeal system and its design decisions, lists several types of integrity scenarios, presents the communication protocol and identifies practical conditions for design and implementation. An implementation in a business relevant scenario is presented as a proof of concept.

The uPart experience: building a wireless sensor network

This paper presents an experience report illustrating the design of the uPart tiny low-power sensor network platform: from the analysis phase over the definition of the application, design and construction of hardware, the implementation of the software and network to the application set-up. uPart sensor nodes were given away in the conference badge to 500 voluntary attendees of the Ubicomp 2005. In our demo application, uParts were able to recognize activities of attendees of the Ubicomp 2005 conference. Design was carried out under serve time and budget restrictions. The paper focuses on reporting design decisions and presents technical details of uPart hardware, firmware and applications. It also shows first qualitative experiences with the run of the system at the conference. The outcome of the paper is a general meta-guideline for designing sensor network systems under similar conditions.

Syncob: Collaborative Time Synchronization in Wireless Sensor Networks

This paper analyses the problem of time-synchronization of distributed sensor nodes. We present a Syncob, a method for synchronizing an arbitrary number of nodes in a distributed setting without the requirements of an infrastructure, master node or time and resource consuming protocol overhead. Syncob is therefore also very good suited for highly mobile settings with ad-hoc communication. Syncob is implemented as a physical layer protocol and provides a time synchronization deviation of max. 4 mus between any participating node. Our implementation on low-cost pPart sensor nodes shows that Syncob requires very low overhead and very low complexity for hardware and software.

Using fine-grained infrared positioning to support the surface-based activities of mobile users

Knowledge of the fine-grained location and orientation of devices on a surface can be used to enhance the surface-based computing tasks of mobile users in the home and workplace. However existing systems, which provide surface-based positioning information are often not a practical solution for mobile users, since the systems all rely upon pre-installed and calibrated environmental infrastructure. In this paper, we present prototype positioning devices for surfaces which do not rely on such infrastructure. We show that inexpensive infrared transducers can be used to effectively sense relative location and orientation of surface devices. We evaluate the novel approach of using intensity of light pulses for fine-grained location measurements.