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Search Results: 1 - 10 of 290 matches for " Vasilis Raptis "
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Design and Measurements of Ultra-Wideband Antenna  [PDF]
Giorgos TATSIS, Vasilis RAPTIS, Panos KOSTARAKIS
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2010, DOI: 10.4236/ijcns.2010.32017
Abstract: This paper describes the design, realization and experimental measurements of an antenna element to operate at ultra-wideband (UWB) spectrum. The type of this antenna is a circular disk monopole (CDM), with two notches opposite to each other at two sides of the disk. The feed of the antenna is a coplanar waveguide (CPW). The effect of the presence of the notches is studied through simulations and tested experimentally.
A/D Restrictions (Errors) in Ultra-Wideband Impulse Radios  [PDF]
Giorgos Tatsis, Constantinos Votis, Vasilis Raptis, Vasilis Christofilakis, Panos Kostarakis
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2010, DOI: 10.4236/ijcns.2010.35055
Abstract: Ultra-Wideband Impulse Radio (UWB-IR) technologies, although are relatively easy in transmission but they present difficulties in reception, in fact the reception of such waveform is a quite complicated matter. The main reason is that in fully digital receiver the received waveform must be sampled at a rate of several GHz. This paper focuses on the impact of the Analog to Digital (A/D) conversion stage that is used to sample the received waveform. More specifically we focus on the impact of the two main parameters that affect the performance of the Software Defined Radio (SDR) system. These parameters are the bit resolution and the time jittering. The influence of these parameters is deeply examined.
Enhanced PAPR in OFDM without Deteriorating BER Performance  [PDF]
Spyridon K. Chronopoulos, Giorgos Tatsis, Vasilis Raptis, Panos Kostarakis
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2011, DOI: 10.4236/ijcns.2011.43020
Abstract: Orthogonal frequency division multiplexing (OFDM) is vastly used in wireless networks. Its superiority relies on the fact that information can be split in large amount of frequencies. Each frequency is called information subcarrier. OFDM exhibits excellent annotation in channel fades and interferers as only a few subcarriers can be affected and consequently a small part of the original data stream can be lost. Orthogonality between frequencies ensures better spectrum management and obviates the danger of intersymbol interference. However, an essential problem exists. OFDM systems have high peak to average power ratio. This implies large fluctuations in signal power, ending up in increasing complexity of ADCs and DACs. Also, power amplifiers must work in a larger linear dynamic region. In this paper we present two new techniques for reducing Peak to Average Power Ratio (PAPR), that can be added in any OFDM system and we compare them with other existing schemes.
Development and Experimental Measurements of a Tunable Antenna  [PDF]
Vasilis Raptis, Giorgos Tatsis, Spyridon K. Chronopoulos, Stavros Mallios, Panos Kostarakis
Communications and Network (CN) , 2013, DOI: 10.4236/cn.2013.53026
Abstract:

Modern telecommunication systems need to be equipped with antennas that are precisely tuned to more than one frequency in order to allow operation in several bands. Antenna precise tuning to the desired frequency is very important for system performance. In this paper the operating frequency of a PIFA antenna is adjusted using a varactor. This configuration has the advantage of continuous tuning, thus correcting any frequency deviation due to environmental or other changes. The PIFA antenna’s tuning ranges from 860 MHz to 1025 MHz. Also, the geometry of the antenna is studied through simulations and the effect of the varactor is tested experimentally.

Reduced Ecological Footprints of Modern Facilities Introducing the Implementation of Advanced Wireless Technologies, and Human Resources’ Benefits  [PDF]
Spyridon K. Chronopoulos, Evangelia I. Kosma, Dionysios Tafiadis, Dimitrios Dimopoulos, Vasilis Raptis, Evaggelos C. Karvounis, Pantelis Angelidis, Panos Kostarakis
Communications and Network (CN) , 2018, DOI: 10.4236/cn.2018.101002
Abstract: The necessity of lowering the mean power consumption of various facilities, due to the lack of their enormous future energy needs, led to an ongoing advance of various technologies. These technologies have been oriented towards the concept of a Reduced Ecological Footprint. Massive structures (such as building complexes and hospitals) have been redesigned and upgraded; many interior designs have been dramatically altered while new electronic devices are constantly being produced in order to revolutionize a long term perspective towards a “Green Planet” while they exhibit astonishing signal processing. Consequently, an enormous technology already exists which needs to be properly combined to a proposed methodology and to new ideas relevant to systems’ administration through automatic wireless control. This paper intends to reduce the gap between design and realization of the aforementioned research. Consequently, the primary contribution of this research is the proposal of a complete design protocol with minimized defects relevant to Reduced Ecological Footprints of Facilities (REFF) along with its beneficial advantages relevant to providing a healthy and productive work environment. This protocol consists of four main parts which are 1) the main key points-guidelines, 2) its objectives, 3) the know-how methodology for implementation in existing installations and 4) the description of the imminent benefits in workforce/human resources.
RDCI: A novel method of cluster analysis and applications thereof in sample molecular simulations
Theophanes Raptis,Vasilios Raptis
Physics , 2013,
Abstract: A novel method, termed Reduced Dimensionality Cluster Identification, RDCI, is presented, for the identification and quantitative description of clusters formed by N objects in three dimensional space. The method consists of finding a path, as short as possible, connecting the objects, and then tracking down the size s, of a subgroup i-n, i-n+1, ..., i+n, of 2n+1 < N particles for i varying from n+1 to N-n. Clusters are located where local minima of s(i) occur whereas local maxima serve as delimiters partitioning the path in subsets containing the clusters. Minimal post-processing allows for the removal of outliers on the basis of user-defined criteria and the identification of clearly defined clusters. The advantage of the method is that it requires no predetermined input or criteria of "clusterness" such as number of objects or size of aggregates. Among the numerous possible applications of the method, results are herein reported, from Molecular Dynamics simulations of a binary mixture of Lennard-Jones fluids and of a model polymeric system with a gas-like substance dispersed in it. The method is shown to allow the extraction of meaningful quantitative information regarding the tendency of molecules to cluster or de-cluster and the dynamics of the clustering processes.
Exploring Secondary School Pupils’ ICT Engagement: A Validation Study  [PDF]
Kleopatra Nikolopoulou, Vasilis Gialamas
Creative Education (CE) , 2016, DOI: 10.4236/ce.2016.74059
Abstract: This paper regards a validation study aiming to explore secondary school pupils’ ICT engagement. A 36-item questionnaire was administered to 246 adolescents (12 - 15 years old) of an experimental school, in Greece. Four factors were extracted: “ICT self-concept”, “social exposure to ICT”, “interest in computers” and “interest in mobile devices”. The factorial structure of the “ICT engagement” questionnaire was revealed. The majority of the pupils expressed strong interest towards both computers and mobile devices. Over 90% of the adolescents believe that the internet is very useful to find practical information, that they can handle mobile phones confidently, and that they know how to download new applications for a mobile phone. Gender was statistically significant correlated to “ICT self-concept” and “social exposure to ICT”, where the boys had higher mean values in comparison to girls. The frequency of computer use had positive correlations with the factors “ICT self-concept”, “social exposure to ICT” and “interest in computers”. It is suggested to describe adolescents’ ICT engagement with respect to discrete dimensions. In particular, the dimensions-factors “interest in computers” and “interest in mobile devices” should be distinct when defining concepts related to ICT engagement. The questionnaire is suggested to be used with other adolescent populations of different countries, in order to reveal possible similarities and differences.
Traducción condensada características de la autoría en cinco revistas científicas internacionales de bibliotecología y ciencias de la información 1
Paschalis Raptis
ACIMED , 1993,
Abstract: Este estudio bibliométrico proporciona una base útil para obtener un perfil de las características de la autoría en revistas Internacionales dedicadas a la bibliotecología y a las ciencias de la información. A tal efecto se revisaron cinco revistas para obtener detalles acerca de los autores que en ellas aparecen, entre los que figuran el sexo, la afiliación, la categoría científica y el país de procedencia. Como base para el estudio se tomaron materiales, publicados en las cinco revistas mencionadas, que se editan en idioma inglés. Se estudiaron también las referencias bibliográficas, la distribución geográfica, la ed y los tipos de documentos citados por los autores al escribir sus artículos.
Free convective oscillatory flow and mass transfer past a porous plate in the presence of radiation for an optically thin fluid
Raptis A.
Thermal Science , 2011, DOI: 10.2298/tsci101208032r
Abstract: We study the two dimensional free convective oscillatory flow and mass transfer of a viscous and optically thin gray fluid over a porous vertical plate in the presence of radiation. The governing partial differential equations have been transformed to ordinary differential equations. Numerical solutions are obtained for different values of radiation parameter, Grashof number and Schmidt number.
Finitary Topos for Locally Finite, Causal and Quantal Vacuum Einstein Gravity
Ioannis Raptis
Physics , 2005, DOI: 10.1007/s10773-006-9240-y
Abstract: Previous work on applications of Abstract Differential Geometry (ADG) to discrete Lorentzian quantum gravity is brought to its categorical climax by organizing the curved finitary spacetime sheaves of quantum causal sets involved therein, on which a finitary (:locally finite), singularity-free, background manifold independent and geometrically prequantized version of the gravitational vacuum Einstein field equations were seen to hold, into a topos structure. This topos is seen to be a finitary instance of both an elementary and a Grothendieck topos, generalizing in a differential geometric setting, as befits ADG, Sorkin's finitary substitutes of continuous spacetime topologies. The paper closes with a thorough discussion of four future routes we could take in order to further develop our topos-theoretic perspective on ADG-gravity along certain categorical trends in current quantum gravity research.
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