Publications

2020

• Precoding in Massive MU-MIMO Systems Based on New CSI Accuracy Indicator Reporting
  
  • Askri Aymen
  • Rekaya-Ben Othman Ghaya
  , 2020.
• Lattice Codes for C-RAN Based Sectored Cellular Networks
  
  • Gelincik Samet
  • Rekaya-Ben Othman Ghaya
  , 2020.
• From formal test objectives to TTCN-3 for verifying ETCS complex software control systems
  
  • Ameur-Boulifa Rabéa
  • Cavalli Ana R
  • Maag Stephane
  , 2020, 1250, pp.156-178. The design of a practical but accurate software methodology to guarantee systems correctness and safety is still a big challenge. Where test coverage is dissatisfying, formal analysis grants much higher potential to discover errors or safety vulnerabilities during the design phase of a system. However, formal verification methods often require a strong technical background that limits their usage. In this paper, we present a framework based on testing and verification to ensure the correctness and safety of complex distributed software systems. As a result of the application of our methodology we obtain a more reliable system, in terms of functionality, safety and robustness and a reduction of the time necessary for verification. In order to show the applicability of our solution we applied it on a real industrial case study, that is the European Train Control System (ETCS) [14]. We specify the system using the SDL language [24], and we use a test generation tool to generate abstract test cases in TTCN-3. Based on these standardized tests, we verify using model-checking, some critical properties of the system, in particular these regarding safety requirements. We analyse a real train accident and we demonstrate how the accident could have been avoided if the ETCS system was used. (10.1007/978-3-030-52991-8_8)
  DOI : 10.1007/978-3-030-52991-8_8
• Fractional Underdamped Langevin Dynamics: Retargeting SGD with Momentum under Heavy-Tailed Gradient Noise
  
  • Şimşekli Umut
  • Zhu Lingjiong
  • Teh Yee Whye
  • Gürbüzbalaban Mert
  , 2020. Stochastic gradient descent with momentum (SGDm) is one of the most popular optimization algorithms in deep learning. While there is a rich theory of SGDm for convex problems, the theory is considerably less developed in the context of deep learning where the problem is non-convex and the gradient noise might exhibit a heavy-tailed behavior, as empirically observed in recent studies. In this study, we consider a \emph{continuous-time} variant of SGDm, known as the underdamped Langevin dynamics (ULD), and investigate its asymptotic properties under heavy-tailed perturbations. Supported by recent studies from statistical physics, we argue both theoretically and empirically that the heavy-tails of such perturbations can result in a bias even when the step-size is small, in the sense that \emph{the optima of stationary distribution} of the dynamics might not match \emph{the optima of the cost function to be optimized}. As a remedy, we develop a novel framework, which we coin as \emph{fractional} ULD (FULD), and prove that FULD targets the so-called Gibbs distribution, whose optima exactly match the optima of the original cost. We observe that the Euler discretization of FULD has noteworthy algorithmic similarities with \emph{natural gradient} methods and \emph{gradient clipping}, bringing a new perspective on understanding their role in deep learning. We support our theory with experiments conducted on a synthetic model and neural networks.
• Holarchic structures for decentralized deep learning: a performance analysis
  
  • Pournaras Evangelos
  • Yadhunathan Srivatsan
  • Diaconescu Ada
  Cluster Computing, Springer Verlag, 2020.
• Software Acceleration Techniques for High-speed Programmable Networks
  
  • Linguaglossa Leonardo
  , 2020, 6, pp.203-216. Network programmability has provided an effective approach to enable innovation in network systems. By replacing static, expensive middleboxes with equivalent pieces of software implementing the same functionality, operators can significantly reduce their CAPEX/OPEX expenditures, and engineers can rapidly design, test and deploy novel architectures and services, thus reducing the time-to-market for network applications. However, the flexibility provided by software solutions comes at a cost: purposespecific hardware has the clear advantage of optimized performance (i.e., throughput, latency) with respect to pure software-based solutions. The introduction of software acceleration techniques represented an essential step towards the increasing popularity of the SDN/NFV paradigm shift, by reducing the performance gap between hardware-based and software-based systems. Thanks to such techniques, modern software-networking solutions can operate at multi-10Gbps rate (up to hundreds of Gbps) on commodity servers equipped with regular COTS components. In this chapter, we cover the aspects related to software acceleration techniques in a bottom-up fashion: we first provide an overview of high-speed software networking on COTS architectures, and we then explore the evolution of softwarized networking by focusing on performance acceleration and the design space for high-speed programmable networks. (10.57620/CNIT-Report_06)
  DOI : 10.57620/CNIT-Report_06
• Epitaxial quantum dot lasers on silicon with high thermal stability and strong resistance to optical feedback
  
  • Huang H.
  • Duan J.
  • Dong B.
  • Norman J.
  • Jung D.
  • Bowers J. E
  • Grillot F.
  APL Photonics, AIP Publishing LLC, 2020, 5 (1), pp.016103. (10.1063/1.5120029)
  DOI : 10.1063/1.5120029
• An Indirect Determination of the Polarization Anisotropy in a Quantum Cascade Laser Under Strong Cross-Polarization Feedback
  
  • Spitz O
  • Herdt A
  • Carras M
  • Maisons G
  • Elsässer W
  • Grillot F
  , 2020. This work demonstrates that a non TM-polarized wave can be generated by a quantum cascade laser subjected to strong cross-polarization optical feedback. This finding is used to determine the anisotropy between the two existing polarizations Acknowledgments: this work is supported by the French Defense Agency (DGA), the French ANR program under grant ANR-17-ASMA-0006
• Optical noise of dual-state lasing quantum dot lasers
  
  • Zhou Yueguang
  • Duan Jianan
  • Grillot Frederic
  • Wang Cheng
  IEEE Journal of Quantum Electronics, Institute of Electrical and Electronics Engineers, 2020, pp.1-1. (10.1109/JQE.2020.3026090)
  DOI : 10.1109/JQE.2020.3026090
• Constructions of optimal locally recoverable codes via Dickson polynomials.
  
  • Liu J.
  • Mesnager Sihem
  • Tang D.
  Journal of Designs, Codes, and Cryptography, 2020.
• An Experimental Study of State-of-the-Art Entity Alignment Approaches
  
  • Zhao Xiang
  • Zeng Weixin
  • Tang Jiuyang
  • Wang​ Wei
  • Suchanek Fabian
  IEEE Transactions on Knowledge and Data Engineering, Institute of Electrical and Electronics Engineers, 2020. Entity alignment (EA) finds equivalent entities that are located in different knowledge graphs (KGs), which is an essential step to enhance the quality of KGs, and hence of significance to downstream applications (e.g., question answering and recommendation). Recent years have witnessed a rapid increase of EA approaches, yet the relative performance of them remains unclear, partly due to the incomplete empirical evaluations, as well as the fact that comparisons were carried out under different settings (i.e., datasets, information used as input, etc.). In this paper, we fill in the gap by conducting a comprehensive evaluation and detailed analysis of state-of-the-art EA approaches. We first propose a general EA framework that encompasses all the current methods, and then group existing methods into three major categories. Next, we judiciously evaluate these solutions on a wide range of use cases, based on their effectiveness, efficiency and robustness. Finally, we construct a new EA dataset to mirror the real-life challenges of alignment, which were largely overlooked by existing literature. This study strives to provide a clear picture of the strengths and weaknesses of current EA approaches, so as to inspire quality follow-up research. (10.1109/TKDE.2020.3018741)
  DOI : 10.1109/TKDE.2020.3018741
• Constructions of self-orthogonal codes from hulls of BCH codes and their parameters
  
  • Du Z.
  • Li C.
  • Mesnager Sihem
  IEEE Transactions on Information Theory, Institute of Electrical and Electronics Engineers, 2020.
• Recent results and problems on constructions of linear codes from cryptographic functions
  
  • Li N.
  • Mesnager Sihem
  Journal of Cryptography and Communications- Discrete Structures, Boolean Functions, and Sequences, 2020.
• Processing Simple Geometric Attributes with Autoencoders
  
  • Newson Alasdair
  • Almansa Andrés
  • Gousseau Yann
  • Ladjal Saïd
  Journal of Mathematical Imaging and Vision, Springer Verlag, 2020, 62 (3), pp.293-312. Image synthesis is a core problem in modern deep learning, and many recent architectures such as autoencoders and Generative Adversarial networks produce spectacular results on highly complex data, such as images of faces or landscapes. While these results open up a wide range of new, advanced synthesis applications, there is also a severe lack of theoretical understanding of how these networks work. This results in a wide range of practical problems, such as difficulties in training, the tendency to sample images with little or no variability, and generalisation problems. In this paper, we propose to analyse the ability of the simplest generative network, the autoencoder, to encode and decode two simple geometric attributes : size and position. We believe that, in order to understand more complicated tasks, it is necessary to first understand how these networks process simple attributes. For the first property, we analyse the case of images of centred disks with variable radii. We explain how the autoencoder projects these images to and from a latent space of smallest possible dimension, a scalar. In particular, we describe a closed-form solution to the decoding training problem in a network without biases, and show that during training, the network indeed finds this solution. We then investigate the best regularisation approaches which yield networks that generalise well. For the second property, position, we look at the encoding and decoding of Dirac delta functions, also known as `one-hot' vectors. We describe a hand-crafted filter that achieves encoding perfectly, and show that the network naturally finds this filter during training. We also show experimentally that the decoding can be achieved if the dataset is sampled in an appropriate manner. (10.1007/s10851-019-00924-w)
  DOI : 10.1007/s10851-019-00924-w