1,720,986 research outputs found
Free Space Optics for Next Generation Cellular Backhaul
No full textThe exponential increase in the number of mobile users, coupled with the strong
demand for high-speed data services results in a significant growth in the required cellular backhaul capacity. Optimizing the cost efficiency while increasing the capacity
is becoming a key challenge to the cellular backhaul. It refers to connections between
base stations and mobile switching nodes over a variety of transport technologies such
as copper, optical fibers, and radio links. These traditional transmission technologies
are either expensive, or cannot provide high data rates. This work is focused on the
opportunities of free-space-optical (FSO) technology in next generation cellular back-
haul. FSO is a cost effective and wide bandwidth solution as compared with the
traditional radio-frequency (RF) transmission. Moreover, due to its ease of deployment,
license-free operation, high transmission security, and insensitivity to interference,
FSO links are becoming an attractive solution for next generation cellular networks.
However, the widespread deployment of FSO links is hampered by the atmospheric
turbulence-induced fading, weather conditions, and pointing errors. Increasing the
reliability of FSO systems, while still exploiting their high data rate communications,
is a key requirement in the deployment of an FSO-based backhaul. Therefore, the aim
of this work is to provide different approaches to address these technical challenges. In
this context, investigation of hybrid automatic repeat request (HARQ) protocols from
an information-theoretic perspective is undertaken. Moreover, performance analysis
of asymmetric RF/FSO dual-hop systems is studied. In such system models,
multiple RF users can be multiplexed and sent over the FSO link. More specifically, the end-to-end performance metrics are presented in closed-form. This also has increased
the interest to study the performance of dual-hop mixed FSO/RF systems, where the
FSO link is used as a multicast channel that serves different RF users. Having such
interesting results motivates further the analysis of dual-hop FSO fixed-gain relaying
communication systems, and exact closed-form performance metrics are presented in
terms of the bivariate H-Fox function. This model is further enhanced through the
deployment of a multihop FSO relaying system as an efficient technique to mitigate
the turbulence-induced fading as well as pointing errors
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
Multihop Relaying over IM/DD FSO Systems with Pointing Errors
No full textIn this paper, the end-to-end performance of a multihop free-space optical system with amplify-and-forward channelstate- information-assisted or fixed-gain relays using intensity modulation with direct detection technique over Gamma-Gamma turbulence fading with pointing error impairments is studied. More specifically, novel closed-form results for the probability density function and the cumulative distribution function of the end-to-end signal-to-noise ratio (SNR) are derived in terms of the Fox’s H function. Based on these formulas, closed-form bounds for the outage probability, the average bit-error rate (BER) of on-off keying modulation scheme, the moments, and the ergodic capacity are presented. Furthermore, using the momentsbased approach, tight asymptotic approximations at high and low average SNR regimes are derived for the ergodic capacity in terms of simple elementary functions. The obtained results indicate that the overall system performance degrades with an increase of the number of hops. The effects of the atmospheric turbulence conditions and the pointing error are also quantified. All the analytical results are verified via computer-based Monte- Carlo simulations
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
Multihop communications over CSI-assisted relay IM/DD FSO systems with pointing errors
No full textIn this paper, the end-to-end performance of a multihop free-space optical system with amplify-and-forward channel-state-information-assisted relays using intensity modulation with direct detection technique over Gamma-Gamma turbulence fading with pointing error impairments is studied. More specifically, novel closed-form expressions for the moment generating function, the cumulative distribution function, and the probability density function of the end-to-end signal-to-noise ratio (SNR) are derived in terms of the Meijer's G function. Based on these formulas, closed-form bounds for the outage probability, the average bit-error rate (BER) of a variety of modulation schemes, the moments, and the ergodic capacity are presented. Furthermore, by using the asymptotic expansion of the Meijer's G function at high SNR, accurate asymptotic results are introduced for the outage probability, the average BER and the ergodic capacity in terms of simple elementary functions. For the capacity, novel asymptotic results at low and high SNR regimes are also derived through the moments. All the analytical results are verified via computer-based Monte-carlo simulations
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
On the Performance of Multihop Heterodyne FSO Systems With Pointing Errors
No full textThis paper reports the end-to-end performance analysis of a multihop free-space optical system with amplify-and-forward (AF) channel-state-information (CSI)-assisted or fixed-gain relays using heterodyne detection over Gamma–Gamma turbulence fading with pointing error impairments. In particular, we derive new closed-form results for the average bit error rate (BER) of a variety of binary modulation schemes and the ergodic capacity in terms of the Meijer's G function. We then offer new accurate asymptotic results for the average BER and the ergodic capacity at high SNR values in terms of simple elementary functions. For the capacity, novel asymptotic results at low and high average SNR regimes are also obtained via an alternative moments-based approach. All analytical results are verified via computer-based Monte-Carlo simulations
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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