1,721,267 research outputs found
Strategies for rapid congestion recovery using ramp metering
No full textThis research investigated strategies for motorway congestion management from a different angle: that is, how to quickly recover motorway from congestion at the end of peak hours, given congestion cannot be eliminated due to excessive demand during the long peak hours nowadays. The project developed a zone recovery strategy using ramp metering for rapid congestion recovery, and a serious of traffic simulation investigations were included to evaluate the developed strategy. The results, from both microscopic and macroscopic simulation, demonstrated the effectiveness of the zone recovery strategy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
An optimal scaling scheme for DCO-OFDM based visible light communications
No full textDC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) is widely used in visible light communication (VLC) systems to provide high data rate transmission. As intensity modulation with direct detection (IM/DD) is employed to modulate the OFDM signal, scale up the amplitude of the signal can increase the effective transmitted electrical power whereas more signals are likely to be clipped due to the limited dynamic range of LEDs, resulting in severe clipping distortion. Thus, it is crucial to scale the signal to find a tradeoff between the effective electrical power and the clipping distortion. In this paper, an optimal scaling scheme is proposed to maximize the received signal-to-noise-plus-distortion ratio (SNDR) with the constraint of the radiated optical power in a practical scenario where DC bias is fixed for a desired dimming level. Simulation results show that the system with the optimal scaling factor outperforms that with fixed scaling factor under different equivalent noise power in terms of the bit error ratio (BER) performance
Multi-user sum-rate optimization for visible light communications with lighting constraints
No full textIn visible light communication (VLC) systems, white light emitting diodes (LEDs) are used as illumination sources and transmitters simultaneously. Compared to the phosphor-converted LEDs, multi-chip LEDs have higher modulation bandwidth. Consequently, the multi-chip based VLC systems have great potential for high data rate transmission. Since each chip of the multi-chip LEDs can be modulated independently, parallel communication channels are viable for information transmission. In this paper, in order to maximize the multi-user sum-rate for the multi-chip based multi-input single-output VLC systems, an electrical and optical power allocation scheme is proposed in consideration of the luminance, chromaticity, amplitude and bit error rate constraints. From the perspective of human color vision, the chromaticity constraint is defined within a MacAdam ellipse. As a result, the degree of freedom can be achieved by relaxing the chromaticity constraint from a fixed color point to an elliptic region. Numerical results demonstrate that with the increase of the total luminous flux, the maximum sum-rates present an open-down parabolic tendency due to the limited dynamic range of LEDs. Higher data rate can be achieved under higher correlated color temperature (CCT) for the variation of light components. In addition, the simulation results indicate that the shapes of the chromaticity constrained region (either ellipse or quadrangle) have little impact on the multi-user sum-rate at the same CCT
A tight upper bound on channel capacity for visible light communications
Full text linkSince the optical wireless channel in visible light communication (VLC) systems is subject to the non-negativity of the signal and the average optical power, the classic Shannon channel capacity formula is not applicable to VLC systems. To derive a simple closed-form upper bound on channel capacity, sphere packing argument method has been applied previously. However, there is an obvious gap between the existing sphere-packing upper bounds and the lower bounds at high optical signal-to-noise-ratios (OSNRs), which is mainly caused by the inaccurate mathematical approximation of the intrinsic volumes of the simplex. In this letter, a tight sphere-packing upper bound is derived with a new approximation method. Numerical results demonstrate that compared to the existing sphere-packing upper bounds, our proposed upper bound is tighter at high OSNRs
Interference-free LED allocation for visible light communications with fisheye lens
No full textDue to the limited modulation bandwidth of commercial light emitting diodes (LEDs), imaging optical multiple-input multiple-output (MIMO) technology is applied in visible light communication (VLC) system to achieve high data rate. Since a receiver with a wide angle/field-of-view is preferred in the imaging optical MIMO VLC system, the fisheye lens can be utilized to concentrate the lights from the LEDs. To eliminate the inter-user interference and satisfy their target bit error rate (BER) requirements with the minimum number of LEDs, an interference-free LED allocation scheme is investigated in this paper, which is formulated as a combinatorial problem. The cost criterion of the combinatorial problem is defined as the number of the LEDs used to serve all users, and its discrete alternatives (i.e., feasible solutions) are the disjoint sets consisting of the LEDs that can be cooperatively utilized to satisfy the BER requirements for all users. For each LED in the disjoint set, its neighboring LEDs projected onto the same pixel are forbidden to serve different users. Moreover, due to the NP-hardness of the formulated problem, a location-based greedy algorithm is proposed, where the LEDs are allocated to the users sequentially based on their distances to the center of the LED array. Simulation results verify the effectiveness of our proposed algorithm and show that there exists no interference among all users while the target BER requirements for all users are satisfied with the proposed algorithm.</p
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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