Institute for Space Applications and Remote Sensing ISARS

Athens, Greece

Institute for Space Applications and Remote Sensing ISARS

Athens, Greece
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Berthomier M.,Laboratoire Of Physique Des Plasmas Lpp | Fazakerley A.N.,University College London | Forsyth C.,University College London | Pottelette R.,Laboratoire Of Physique Des Plasmas Lpp | And 59 more authors.
Experimental Astronomy | Year: 2012

The aurorae are dynamic, luminous displays that grace the night skies of Earth's high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth's atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The "ideal magnetohydrodynamics" description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfvén concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets. © 2011 Springer Science+Business Media B.V.


Perakakis P.,University of Granada | Perakakis P.,Jaume I University | Taylor M.,Institute for Space Applications and Remote Sensing ISARS | Mazza M.G.,TU Berlin | Trachana V.,National Hellenic Research Foundation
Scientometrics | Year: 2011

We welcome the commentary by L. Egghe (Scientometrics, this issue) stimulating discussion on our recent article "Natural selection of academic papers" (NSAP) (Scientometrics, 85(2):553-559, 2010) that focuses on an important modern issue at the heart of the scientific enterprise-the open and continuous evaluation and evolution of research. We are also grateful to the editor of Scientometrics for giving us the opportunity to respond to some of the arguments by L. Egghe that we believe are inaccurate or require further comment. © 2011 Akadémiai Kiadó, Budapest, Hungary.


Perakakis P.,University of Granada | Taylor M.,Institute for Space Applications and Remote Sensing ISARS | Mazza M.,TU Berlin | Trachana V.,National Hellenic Research Foundation
Scientometrics | Year: 2010

Academic papers, like genes, code for ideas or technological innovations that structure and transform the scientific organism and consequently the society at large. Genes are subject to the process of natural selection which ensures that only the fittest survive and contribute to the phenotype of the organism. The process of selection of academic papers, however, is far from natural. Commercial for-profit publishing houses have taken control over the evaluation and access to scientific information with serious consequences for the dissemination and advancement of knowledge. Academic authors and librarians are reacting by developing an alternative publishing system based on free-access journals and self-archiving in institutional repositories and global disciplinary libraries. Despite the emergence of such trends, the journal monopoly, rather than the scientific community, is still in control of selecting papers and setting academic standards. Here we propose a dynamical and transparent peer review process, which we believe will accelerate the transition to a fully open and free-for-all science that will allow the natural selection of the fittest ideas. © 2010 Akadémiai Kiadó, Budapest, Hungary.


Benmayor D.,Institute for Space Applications and Remote Sensing ISARS | Benmayor D.,National Technical University of Athens | Papaharalabos S.,Institute for Space Applications and Remote Sensing ISARS | Mathiopoulos P.T.,Institute for Space Applications and Remote Sensing ISARS | And 2 more authors.
Wireless Personal Communications | Year: 2011

Alow-complexity algorithm for the design of efficiently-encodable rate-compatible (RC) low-density parity-check (LDPC) codes by deterministically extending an irregular repeat-accumulate (IRA) is introduced. The extending structure is based on circulants shifted according a truncated Vandermonde matrix (VM) and therefore termed as "extended VM" (eVM). The novel extending algorithm is significantly less computationally complex than other known similar methods since it does not require any optimization of the extending profile or any post-construction girth conditioning. To improve the codes' properties and correcting capabilities in low code rate applications, the optimal proportions of degree-1 and degree-2 parity bits for the extended nodes are investigated and, in contrast to existing deterministic extending approaches for RC-IRA codes, an extending increment step equal to half the information block length is chosen. Various bit error rate (BER) and frame error rate (FER) have been obtained for different code rates, R, and information block length k 0 = 512 and 1024 bits considering an additive white Gaussian noise (AWGN) channel. The results have demonstrated that the proposed eVM RC-LDPC codes, despite their very simple structure and very low computational complexity, exhibit excellent performance only slightly inferior to both dedicated IRA and previously known RC-IRA codes for different data block sizes. © Springer Science+Business Media, LLC. 2010.


Taylor M.,Institute for Space Applications and Remote Sensing ISARS | Daglis I.A.,Institute for Space Applications and Remote Sensing ISARS | Anastasiadis A.,Institute for Space Applications and Remote Sensing ISARS | Vassiliadis D.,West Virginia University
AIP Conference Proceedings | Year: 2010

We show how a general class of spatio-temporal nonlinear impulse-response forecast networks (Volterra networks) can be constructed from a taxonomy of nonlinear autoregressive integrated moving average with exogenous inputs (NARMAX) input-output equations, and used to model the evolution of energetic particle f uxes in the Van Allen radiation belts. We present initial results for the nonlinear response of the radiation belts to conditions a month earlier. The essential features of spatio-temporal observations are recovered with the model echoing the results of state space models and linear f nite impulse-response models whereby the strongest coupling peak occurs in the preceding 1-2 days. It appears that such networks hold promise for the development of accurate and fully data-driven space weather modelling, monitoring and forecast tools. © 2010 American Institute of Physics.


Papaharalabos S.,Institute for Space Applications and Remote Sensing ISARS | Sybis M.,Poznan University of Technology | Tyczka P.,Poznan University of Technology | Mathiopoulos P.T.,Institute for Space Applications and Remote Sensing ISARS
Wireless Personal Communications | Year: 2013

A novel method for approximating the optimal max operator used in Log- MAP decoding of turbo and turbo trellis-coded modulation (TTCM) codes is proposed being derived from a well-known inequality, which has not been published before. The max operation is generalized, for the first time, and performed on n > 2 rather than n = 2 arguments, as it is the conventional approach. Complexity comparisons reveal a significant reduction in the number of operations required per decoding step for the proposed approximation, as compared with the optimal Log-MAP algorithm. Performance evaluation results are presented for both turbo and TTCM codes, showing the near optimal performance of the novel approximation method in both the additive white Gaussian noise (AWGN) and uncorrelated, i.e. fully interleaved, Rayleigh fading channels. © 2012 Springer Science+Business Media, LLC.


Papaharalabos S.,Institute for Space Applications and Remote Sensing ISARS | Mathiopoulos P.T.,Institute for Space Applications and Remote Sensing ISARS | Masera G.,Polytechnic University of Turin | Martina M.,Polytechnic University of Turin
IET Communications | Year: 2012

In this study, the authors deal with the problem of how to effectively approximate the max * operator when having n>2 input values, with the aim of reducing implementation complexity of conventional Log-MAP turbo decoders. They show that, contrary to previous approaches, it is not necessary to apply the max * operator recursively over pairs of values. Instead, a simple, yet effective, solution for the max * operator is revealed having the advantage of being in non-recursive form and thus, requiring less computational effort. Hardware synthesis results for practical turbo decoders have shown implementation savings for the proposed method against the most recent published efficient turbo decoding algorithms by providing near optimal bit error rate (BER) performance. © 2012 The Institution of Engineering and Technology.


Papaharalabos S.,Institute for Space Applications and Remote Sensing ISARS | Benmayor D.,Institute for Space Applications and Remote Sensing ISARS | Benmayor D.,National Technical University of Athens | Mathiopoulos P.T.,Institute for Space Applications and Remote Sensing ISARS | Fan P.,Southwest Jiaotong University
IEEE Transactions on Broadcasting | Year: 2011

Digital satellite multimedia broadcasting services in Europe are specified in the Mobile Satellite Services (MSS) frequency band by two recently developed standards, namely the European Telecommunications Standards Institute (ETSI) Satellite Digital Radio (SDR) and the Digital Video BroadcastingSatellite services to Handhelds (DVB-SH) standards. Commercial deployment of operational systems based on these standards is foreseen in the coming years targeting mainly six large European markets. For these standards, a state-of-the-art channel code is deployed based on the 3rd Generation Partnership Project 2 (3GPP2) specifications having a wide range of coding rates. The purpose of this paper is twofold: (i) It studies and compares, for the first time, the Bit Error Rate (BER) performance of the 3GPP2 turbo code used in the aforementioned systems, in order to serve as a benchmark for system design engineers; and (ii) It investigates novel alternative channel coding schemes, including other state-of-the-art turbo code configurations and also Low-Density Parity-Check (LDPC) codes with linear-time encoding, such as the Irregular Repeat-Accumulate (IRA) codes, in order to improve the performance and/or reduce the complexity in future mobile satellite broadcasting systems. Extensive performance evaluation results in the Additive White Gaussian Noise (AWGN) and uncorrelated Rayleigh/Rician fading channels have shown that the use of turbo codes with higher number of states improves performance against the 3GPP2 turbo codes of up to 0.25 dB but the decoding complexity is almost doubled. Furthermore, the use of duo-binary turbo codes reduces decoding latency and makes them more robust in puncturing with performance improvement against the 3GPP2 turbo codes of up to 0.3 dB but performance degrades at very low coding rates. On the other hand, the use of Rate Compatible (RC)-IRA codes results in smaller performance improvement against the 3GPP2 turbo codes, i.e. up to 0.1 dB, but rate compatibility is used to obtain different coding rates with simple puncturing/extending method. © 2006 IEEE.

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