Key Factors for Position Errors in 802.11-based Positioning Systems

Indoor positioning systems based on 802.11 and fingerprints offer reasonably low position errors. We study the key factors for position errors by systematically investigating (1) the number of access points, (2) the number of samples in the training phase, (3) the number of samples in the position determination phase, and (4) the setup of the grid of reference points. Further, we squeeze out the best of the positioning system by selecting advantageous values for these parameters. For our study, we utilize a test environment with a size of about 312 square meters that is covered with 612 reference points arranged in an equally spaced grid

An Energy-Efficient Forwarding Scheme for Wireless Sensor Networks

Energy-efficient forwarding becomes important if resources and battery lifetime are limited such as in Wireless Sensor Networks. Although widely used, simple hop-based forwarding along a path from one node towards a sink can be very inefficient in terms of delivery rate as well as energy efficiency, especially in lossy environments. We will show that just minimizing the expected number of transmissions within the network is not always the most efficient forwarding strategy. Using a realistic link loss model, we derive two new forwarding schemes named Single-Link and Multi-Link Energy-Efficient Forwarding that trade off delivery rate and energy costs best by maximizing energy efficiency. Multi-Link Forwarding further benefits from addressing multiple receivers during packet forwarding, instead of a single one. By mathematical analyses, extensive simulations, and experimental experiments we contrast the performance of our approaches against a comprehensive framework of different forwarding strategies

Enhancing curvature scale space features for robust shape classification

The curvature scale space (CSS) technique, which is also part of the MPEG-7 standard, is a robust method to describe complex shapes. The central idea is to analyze the curvature of a shape and derive features from inflection points. A major drawback of the CSS method is its poor representation of convex segments: Convex objects cannot be represented at all due to missing inflection points. We have extended the CSS approach to generate feature points for concave and convex segments of a shape. This generic approach is applicable to arbitrary objects. In the experimental results, we evaluate as a comprehensive example the automatic recognition of characters in images and videos

Positionierung mit Wireless-LAN und Bluetooth

In diesem Artikel wird ein Überblick über bestehende Wireless-LAN- und Bluetooth-basierte Positionierungssysteme gegeben. Eine detaillierte identifikation der Merkmale und Eigenschaften der unterschiedlichen Systeme soll helfen, Positionierungsverfahren anhand ihrer Eigenschaften einzuordnen

Overhearing the Wireless Interface for 802.11-based Positioning Systems

Not only the proliferation of 802.11, but also the capability to determine the position of mobile devices make 802.11 highly appealing for many application areas. Typically, a mobile device that wants to know its position regularly performs active or passive scans to obtain the signal strength measurements of neighboring access points. Active and passive scanning are survey techniques originally intended to be performed once in a while to learn about the presence and signal reception quality of access points within communication range. Based on this survey the best suitable access point is selected as the gateway to the wired network. However, so far, no investigations are known to have been launched into how regular scanning affects concurrent data transmissions from an end-user point of view. In this paper, we explore how common data communication is affected while actively or passively scanning at the same time. We found that with an active scanning interval of less than 2 seconds the network conditions such as throughput and round trip delay are insufficient for interactive applications. The same is true for passive scanning if a scanning interval of less than 7 seconds is chosen. Furthermore, we present a novel scan scheme called Monitor Sniffing to reduce client service disruptions. Monitor Sniffing exploits the fact that 802.11 operates on overlapping channels by overhearing the wireless interface. We have implemented our Monitor Sniffing algorithm using commodity 802.11g hardware, and we demonstrate that it is faster than active and passive scanning and does not disturb concurrent data communication. Finally, our approach only requires software modifications on the client side, making the adoption process quite easy

The Impact of Forward Error Correction on Wireless Sensor Network Performance

In networks there are basically two methods to tackle the problem of erroneous packets: Automatic Repeat Requests (ARQ) and Forward Error Correction (FEC). While ARQ means packet retransmissions, FEC uses additional bits to detect and correct distorted data. However, extensive field test of our sensor nodes have shown that FEC can take effect only as long as both sender and receiver are bit-wise synchronized. Otherwise, all following bits are misinterpreted which results in an uncorrectable number of errors. We will thus introduce a new resync scheme which is particularly tailored for many sensor network platforms using UARTs in conjunction with radio transmission. We can show that only using an appropriate resync mechanism exploits the full potential of FEC

Signalanalyse-Verfahren zur Segmentierung von Multimediadaten

Im Rahmen der Segmentierung sollten Objekte in digitalen Bildern und (medizinischen) Volumendaten vom Hintergrund abgegrenzt werden. Ein Schwerpunkt dieser Arbeit liegt darauf, ein Bild geeigneten Transformationen zu unterziehen, um die Segmentierung so zuverlässlich wie möglich zu machen. Insbesondere wird die Eignung unterschiedlicher Transformationen analysiert und zu Erkenntnissen aus der mathematischen Statistik in Bezug gesetzt

TECA : A Topology and Energy Control Algorithm for Sensor Networks

A main challenge in the field of sensor networks is energy efficiency to prolong the sensor's operational lifetime. Due to low-cost hardware, nodes' placement or hardware design, recharging might be impossible. Since most energy is spent for radio communication, many approaches exist that put sensor nodes into sleep mode with the communication radio turned off. In this paper, we propose a new Topology and Energy Control Algorithm called TECA. We will show the performance of TECA by means of extensive simulations compared to two other approaches. In terms of operational lifetime, packet delivery and network connectivity, TECA shows promising results. Unlike many other simulations, we use an appropriate link loss model that was verified in reality. By measuring packet delivery rates, TECA is able to adapt to different environments while still maintaining network connectivity

Automatic Generation of Video Summaries for Historical Films

A video summary is a sequence of video clips extracted from a longer video. Much shorter than the original, the summary preserves its essential messages. In the project ECHO (European Chronicles On-line) a system was developed to store and manage large collections of historical films for the preservation of cultural heritage. At the University of Mannheim we have developed the video summarization component of the ECHO system. In this paper we discuss the particular challenges the historical film material poses, and how we have designed new video processing algorithms and modified existing ones to cope with noisy black-and-white films. We also report empirical results from the use of our summarization tool at the four major European national video archives