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Papadakis P.,Institute of Informatics and Telecommunications | Pratikakis I.,Institute of Informatics and Telecommunications | Theoharis T.,National and Kapodistrian University of Athens | Perantonis S.,Institute of Informatics and Telecommunications
International Journal of Computer Vision | Year: 2010

We present a novel 3D shape descriptor that uses a set of panoramic views of a 3D object which describe the position and orientation of the object's surface in 3D space. We obtain a panoramic view of a 3D object by projecting it to the lateral surface of a cylinder parallel to one of its three principal axes and centered at the centroid of the object. The object is projected to three perpendicular cylinders, each one aligned with one of its principal axes in order to capture the global shape of the object. For each projection we compute the corresponding 2D Discrete Fourier Transform as well as 2D Discrete Wavelet Transform. We further increase the retrieval performance by employing a local (unsupervised) relevance feedback technique that shifts the descriptor of an object closer to its cluster centroid in feature space. The effectiveness of the proposed 3D object retrieval methodology is demonstrated via an extensive consistent evaluation in standard benchmarks that clearly shows better performance against state-of-the-art 3D object retrieval methods. © 2009 Springer Science+Business Media, LLC.


Peppas K.P.,Institute of Informatics and Telecommunications | Datsikas C.K.,National and Kapodistrian University of Athens
Journal of Optical Communications and Networking | Year: 2010

Using an accurate exponential bound for the Gaussian Q-function, we derive simple approximate closed-form expressions for the average symbol error probability (ASEP) of a free-space optical communication link using subcarrier intensity modulation (SIM) with general-order rectangular quadrature amplitude modulation (QAM) over atmospheric turbulence channels. To model the atmospheric turbulence conditions, the log-normal and the gamma-gamma distribution are used. Extensive numerical and computer simulation results are presented in order to verify the accuracy of the proposed mathematical analysis. © 2010 Optical Society of America.


Peppas K.P.,Institute of Informatics and Telecommunications
IEEE Wireless Communications Letters | Year: 2012

In this letter we propose a unified analytical expression for the average bit error probability of dual-hop amplify-and-forward relaying systems operating in the presence of both fast fading and shadowing. In order to provide a realistic end-to-end performance of the considered system we assume that both hops are subject to independent but not necessarily identically distributed Extended Generalized-\mathcal{K} fading. Our newly derived formula is obtained in terms of the bivariate H-Fox function. A computationally efficient algorithm to evaluate the average bit error probability is also proposed. The accuracy of the proposed method is substantiated by various numerically evaluated and computer simulation results. © 2012 IEEE.


Peppas K.P.,Institute of Informatics and Telecommunications
IEEE Transactions on Vehicular Technology | Year: 2012

In this paper, a generalized Laguerre polynomial expansion for the probability density function and the cumulative distribution function of the sum of independent nonidentically distributed squared κ-μ random variables is proposed. Based on these statistical results, we investigate the performance of maximal-ratio-combining diversity techniques operating over κ-μ fading channels with arbitrary fading parameters. Our newly derived formulas are mathematically tractable and include as special cases several results available in the technical literature, namely, those of Rice and Nakagami-m fading channels. The proposed analysis is substantiated by numerically evaluated results compared with Monte Carlo simulations. © 2011 IEEE.


Peppas K.P.,Institute of Informatics and Telecommunications
IEEE Photonics Technology Letters | Year: 2011

In this letter, we propose a simple accurate closed-form approximation to the probability density function (pdf) of the sum of independent, identically distributed gammagamma (G-G) random variables. It is shown that the pdf of the G-G sum can be efficiently approximated by the pdf of an α- μdistribution. Based on this approach, simple precise approximations for important performance metrics of multiple-input multiple-output (MIMO) free-space optical systems operating over G-G fading are presented. The accuracy of the proposed method is substantiated by various numerically evaluated and computer simulation results. © 2011 IEEE.


In this study, highly accurate closed-form approximations to the probability density function of the sum of independent identically distributed (i.i.d.) generalised-K fading envelopes are derived. These approximations are valid for a wide range of values of the distribution parameters and number of summands. Based on the previously derived formulas, simple precise approximations for the outage probability and the average bit error probability of equal-gain combining receivers operating on i.i.d. generalised-K fading channels are proposed. Extensive numerically evaluating and computer simulation results are presented to demonstrate the proposed analysis. © 2011 The Institution of Engineering and Technology.


Peppas K.P.,Institute of Informatics and Telecommunications
IEEE Transactions on Vehicular Technology | Year: 2013

This paper provides an analytical framework for the performance evaluation of dual-hop decode-and-forward (DF) cooperative systems operating over η-μ fading channels in the presence of cochannel interference (CCI) and additive white Gaussian noise (AWGN). The interferers are subject to independent but not necessarily identically distributed η-μ fading. For the considered system, assuming multiple interferers at both the relay and the destination, simple accurate analytical expressions for the outage probability (OP), average bit error probability (ABEP), and average capacity are derived. Various numerically evaluated results accompanied with Monte Carlo simulations are presented to corroborate the accuracy of the proposed approximations. © 2013 IEEE.


Valavanis I.,Zenon Automation Technologies | Kosmopoulos D.,Institute of Informatics and Telecommunications
Expert Systems with Applications | Year: 2010

In this paper, a method for the detection and classification of defects in weld radiographs is presented. The method has been applied for detecting and discriminating discontinuities in the weld images that may correspond to false alarms or defects such as worm holes, porosity, linear slag inclusion, gas pores, lack of fusion or crack. A set of 43 descriptors corresponding to texture measurements and geometrical features is extracted for each segmented object and given as input to a classifier. The classifier is trained to classify each of the objects it into one of the defect classes or characterize it as non-defect. Three fold cross validation was utilized and experimental results are reported for three different classifiers (Support Vector Machine, Neural Network, k-NN). © 2010 Elsevier Ltd. All rights reserved.


Peppas K.P.,Institute of Informatics and Telecommunications
IET Communications | Year: 2010

An analysis for the Shannon channel capacity of digital receivers operating over η-μ fading channels is presented under four adaptive policies: constant power with optimal rate adaptation, optimal power and rate adaptation, channel inversion with fixed rate and truncated channel inversion with fixed rate. In this context, useful formulae for the average channel capacity for maximal-ratio combining receivers with not necessarily identically distributed branches are derived. The analysis also includes the performance of single-branch receivers. These expressions provide an effective tool to assess the spectral efficiency of the aforementioned adaptive transmission techniques over practical fading channels. Our newly derived expressions include several special cases that arise from the η-μ distribution, namely the Nakagami-m and the Hoyt distribution. Extensive numerical and computer simulation results are also presented that illustrate the proposed mathematical analysis. © 2010 The Institution of Engineering and Technology.


Chatzis S.P.,Imperial College London | Kosmopoulos D.I.,Institute of Informatics and Telecommunications
Pattern Recognition | Year: 2011

The Student's-t hidden Markov model (SHMM) has been recently proposed as a robust to outliers form of conventional continuous density hidden Markov models, trained by means of the expectationmaximization algorithm. In this paper, we derive a tractable variational Bayesian inference algorithm for this model. Our innovative approach provides an efficient and more robust alternative to EM-based methods, tackling their singularity and overfitting proneness, while allowing for the automatic determination of the optimal model size without cross-validation. We highlight the superiority of the proposed model over the competition using synthetic and real data. We also demonstrate the merits of our methodology in applications from diverse research fields, such as human computer interaction, robotics and semantic audio analysis. © 2010 Elsevier Ltd. All rights reserved.

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