1,721,077 research outputs found
Coupling-Independent Capacitive Wireless Power Transfer Using Frequency Bifurcation
Get PDFCapacitive wireless power transfer can be realized by mutually coupled capacitors operating at a common resonant frequency. An optimal load exists that maximizes either the efficiency or the power transfer to the load. In this work, we utilize the frequency bifurcation effect to propose a frequency agile mode that allows for a nearly coupling-independent regime. We analytically determine the operating conditions of the coupling-independent mode based on the different system gains. In this way, we obtain a solution that achieves nearly constant efficiency and power transfer, even at varying coupling. We compare our results to inductive wireless power transfer where a perfect coupling-independent mode is achievable
Constant capacitive wireless power transfer at variable coupling
No full textCapacitive wireless power transfer uses the varying electric field for wireless power transfer from a transmitter plate to a receiver plate. The amount of power transferred to the load and the system efficiency are dependent of the coupling between transmitter and receiver. However, by changing the operating frequency, it is possible to achieve constant power transfer and efficiency, even at variable coupling. In this work, the analytical expressions for the three different gains of a general capacitive wireless power transfer system are calculated. Based on the gains, an analytical solution is determined for achieving constant power transfer and efficiency at variable coupling. The analytical results are validated by circuital simulation in SPICE
Onderzoek van Visible Light Communication voor Indoor Lokatiebepaling
No full textUntil today, the existing technological solutions for accurate indoor positioning do not have a sufficient accuracy and require expensive equipment to be practically useful. At the same time, there is the arrival of the Light Emitting Diode (LED) which provides a new form of lighting for different applications. Compared to other lighting technologies, LEDs have an additional interesting property, namely that the light can be modulated at high speed. This has led to a new technological challenge, namely the usage of the existing infrastructure of LED lighting in combination with a wireless link for data communication. Visible Light Communication (VLC) has a number of advantages compared to RF communication: the frequency band is not licensed and a number of orders wider, the data is better protected due to the local character, there are no EMC problems with other electronic equipment, ...
In this PhD, we want to explore the unique properties of LEDs (especially the high modulation speeds and the local character of the light) for indoor positioning. Starting with the characterization of the LED, receiver and channel, we will gradually progress to the realization of a working proof-of-concept for indoor positioning. The solution should work without disturbing the original function of the LEDs, namely to illuminate the room according to the lighting standards (when required with possibility for dimming) when there is communication. For a robust solution, it is important to characterize all kinds of optical noise sources, which comes down to the definition of a good channel model. Another aspect of the PhD is the research of a multipoint (set of LED fixtures in the lighting infrastructure) to point (mobile) topology, as it is perfectly possible to receive the emitted light from different LEDs at the same time. An appropriate coding technique and a solution to determine the relative signal strength is essential for the technology to succeed. Therefore, the definition of a robust multiple access technology for a multipoint-to-point topology is required with light as a carrier of information. In many cases, the encoding is oriented towards the maximization of the data rate in point-to-point links. These technologies typically target applications for the establishment of a bi-directional high data rate link using VLC over short distances (typically a desk lamp to a computer, the headlights in an aircraft, ...). It is clear that for these applications, the optical interference (noise) and multiple access does not play an important role. However, we wish to develop a coding technique which is specifically designed for use in combination with a robust multiple access technology in a multi-point-to-point topology. The solution should not only be examined in a static environment but also the dynamic behavior (using soft handovers) might be critical for some application.status: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Robuuste netwerken met communicatie via zichtbaar licht: systeem analyse en implementatie
No full textWireless networks are dominating our daily lives. By 2023, over 70% of the global population will have mobile connectivity and there will be 3.6 networked devices per capita. To accommodate this increasing density in wireless devices, further advancements in communication technologies are required. Despite the continuous efforts spent on Radio Frequency (RF) technologies, they lack the sufficient RF spectrum to meet this ongoing and ever-expanding traffic demand. To tackle this, Visible Light Communication (VLC) has recently attracted significant attention on complementing RF networks to provide robust communication in ultra-dense scenarios. In VLC, data is transmitted by rapidly modulating the light intensity of Light Emitting Diodes (LEDs), with receivers able to decode the information at data rates up to Gbit/s. Nonetheless, VLC brings extra challenges as the propagation is predominantly line-of-sight (LOS), the channel is highly subject to the geometric properties of the environment, the small confinement of light limits user mobility and overlapping light sources require tight interference coordination.
The goal of this thesis is to provide deeper insights in overcoming these challenges and making VLC networks robust in all circumstances. To achieve this, the thesis starts from a dense transmitter infrastructure and aims to optimize several representing and important performance metrics. The outcome of this thesis can be summarized in five main contributions.
As a first contribution, I design and build a proof-of-concept VLC testbed consisting of 36 densely deployed transmitters and 4 mobile receivers. By supporting dynamic adaptation of the transmitter (known as beamsteering) and receiver orientations, the VLC channel can be precisely characterized in a wide range of experiments. This testbed functions as a fundamental building block in this thesis for the experimental validation of the proposed systems and algorithm designs.
Relying on this testbed, the second contribution includes the design of a multi-user cell-free VLC network. Unlike conventional cellular networks, there is no partitioning in cells and all users are simultaneously served by all transmitters within the receiver's field-of-view (FOV). I optimize the power efficiency in the network and conclude from the evaluation results that a dense cell-free network is a suitable candidate for VLC.
As the LOS link can easily be blocked by the surroundings, the third contribution investigates the impact of blockage on the performance and proposes two techniques to mitigate it: 1) transmitter densification and 2) a user-in-the-loop system that optimizes the rotation of users holding the receivers.
Since the VLC channel is very sensitive to the geometry of the transmitters and receivers, misalignment can cause severe connectivity outages. To avoid this, as a fourth contribution, I optimize their orientation in a low-complexity manner. Rotating the receiver can greatly increase the lowest signal strengths and thus boosting the robustness. At the transmitter's side, steering the beams allows to generate bright beamspots that follow the users. I show that a blended architecture, consisting of a mixture of fixed and steerable light sources, can successfully boost the data rate while also supporting user mobility.
Lastly, as a fifth contribution, I propose a low-complexity yet powerful interference coordination technique for VLC. To accomplish this, I develop a mathematical framework to decide when interference is harmful. Relying on this, I design a time division contention-free scheduling protocol. The protocol is semi-distributed, executing the coarse-grained interference control centrally and fine-grained control distributively on the transmitters. By taking this divide-and-conquer approach, I can significantly reduce the complexity compared to the state of the art without sacrificing performance.
Despite these advances in VLC communication, next-generation networks are not expected to count on a single technology. That is, realizing ultra-robust wireless communication will require seamless integration of multiple technologies, including RF. Nevertheless, this work takes a big step forward in analyzing and improving the robustness of VLC networks, paving the way towards ultra-robust wireless communication with visible light.status: Publishe
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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