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Ponssard C.,ENSTA | Graichen K.,Vienna University of Technology | Petit N.,MINES ParisTech | Laurent-Varin J.,French National Center for Space Studies
2009 European Control Conference, ECC 2009 | Year: 2015

This paper exposes work to optimize the ascent trajectory of a multistage space launcher. Motivated by well-known singular arc effects as in the case of the Goddard rocket, we investigate whether the payload of the launcher can be enhanced by means of a variable thrust during the first phase of the flight. Due to the high complexity and numerical sensitivity of the multistage problem, we use constructive methods for initialization and handling input constraints in optimal control to compute the ascent trajectories. Based on realistic numerical values for key design parameters and flight dynamics, we conclude that the computed trajectories are in fact full thrust. © 2009 EUCA.

Finiasz M.,ENSTA
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2011

This article presents a modification of the CFS code based signature scheme. By producing two (or more generally i) signatures in parallel, we show that it is possible to protect this scheme from "one out of many" decoding attacks. With this modification, and at the cost of slightly larger signatures, it is possible to use smaller parameters for the CFS signature, thus making this new Parallel-CFS construction more practical than standard CFS signatures. © 2011 Springer-Verlag Berlin Heidelberg.

Cluzeau M.,French Institute for Research in Computer Science and Automation | Finiasz M.,ENSTA | Tillich J.-P.,French Institute for Research in Computer Science and Automation
IEEE International Symposium on Information Theory - Proceedings | Year: 2010

We present two new algorithms for the reconstruction of turbo codes from a noisy intercepted bitstream. With these algorithms, we were able to reconstruct various turbo codes with realistic parameter sizes. To the best of our knowledge, these are the first algorithms able to recover the whole permutation of a turbo code in the presence of high noise levels. © 2010 IEEE.

Mahmood A.,ENSTA | Belfiore J.,ENST
IEEE Transactions on Communications | Year: 2010

An efficient algorithm for bit-loading in multicarrier systems is proposed based upon an allocation rhythm/order underlying the classical optimal greedy bit-allocation procedure. An accurate complexity comparison in terms of the exact number of execution cycles over a processor is made with the classical Greedy [1] solution along with another recently proposed optimal bit-loading algorithm [2]. Simulation results verify the significant complexity advantage of our algorithm with no loss in performance. © 2010 IEEE.

Levy-Dit-Vehel F.,ENSTA | Perret L.,French Institute for Research in Computer Science and Automation
Designs, Codes, and Cryptography | Year: 2010

We investigate two schemes based on the word problem on groups. From a complexity-theoretic point of view, we show that the problems underlying those schemes are equivalent. We then present a reaction attack on one of the schemes, thus easily transposed to the other. The attack, besides its efficiency, permits to recover an equivalent secret key. © 2009 Springer Science+Business Media, LLC.

Doare O.,ENSTA
Journal of Sound and Vibration | Year: 2010

In this article, the effect of dissipation on local and global stability of fluid conveying pipes is analyzed. The local approach refers to an infinite medium and uses wave propagation analyses without taking boundary conditions into account. The global approach refers to the same medium, but with finite length and associated with a given set of boundary conditions. The finite length system is generally studied by calculating its eigenmodes and eigenfrequencies. Criteria for local instability are derived in the first part of this paper, and dissipation is found to significantly affect local stability. Moreover, dissipation is found to have a stabilizing or destabilizing effect, depending on the other parameters. Next, numerical computations are presented for finite-length systems and results are analyzed and compared with local stability properties of the corresponding media. When the system is sufficiently long, it is found that critical velocity for global instability tends to a local criterion which can be that of local stability of the damped medium or a local transition criterion of the undamped medium, which is not necessarily the local instability criterion. Finally, a reasoning based on lengthscale ratios is developed. It allows to know which criterion is able to predict the global stability for long systems. © 2008 Elsevier Ltd. All rights reserved.

Jaulin L.,ENSTA
Mathematics in Computer Science | Year: 2012

This paper proposes a new set-membership state estimator for estimating the state vector of a nonlinear dynamic robot. The method combines a symbolic technique based on flatness concepts with rigorous numerical methods based on interval analysis. Two testcases related to the state estimation of a sailboat robot are proposed to illustrate the principle and the efficiency of the approach. © 2012 Springer Basel.

Bonnet-Ben Dhia A.-S.,ENSTA | Goursaud B.,ENSTA | Hazard C.,ENSTA | Prieto A.,ENSTA
Physics Procedia | Year: 2010

We present an original method for the simulation of ultrasonic waves in a varying cross section elastic waveguide embedded in a homogeneous matrix. The so-called multimodal methods consist in expanding the acoustic field in each transverse section as a series which involves the modes of a perfectly cylindrical waveguide with this particular section. For such a cylindrical waveguide, the modal components of the acoustic field are uncoupled. But a varying cross section yields a coupling of these components. A multimodal approach consists in solving the system of differential equations which couples these components instead of the propagation equations. Such methods have been extensively studied for closed waveguides, that is, when no wave can radiate in the transverse direction. In our case, radiation waves occur, because of the presence of the matrix. Actually, such an open waveguide is characterized by a continuum of radiation modes together with a possible finite set of guided modes. The multimodal method proposed here does not take into account this continuum, but uses instead the concept of leaky modes. These modes appear naturally when the computational domain is artificially limited by Perfectly Matched Layers (PMLs). The main advantage of using leaky modes instead of radiation modes is that the formers compose a discrete set, which simplifies considerably the implementation of a multimodal method. In the case of a simple two-dimensional scalar model, the basic ideas of such a method are presented and illustrated by numerical results.

Hammami O.,ENSTA | Benmouhoub R.,ENSTA | Aouadi I.,ENSTA
European Signal Processing Conference | Year: 2015

The JPEG-2000 image compression standard is increasingly gaining widespread importance. The rich variety of features makes it highly suitable for a large spectrum of applications but at the same time its associated complexity makes it hard to optimize for particular implementations. One of the key step during the processing is entropy coding which takes about 70 % of the execution time. We propose in this paper an analysis and hardware design of this entropy coder in the system framework of the xilinx virtex-II pro chips. © 2004 EUSIPCO.

Hammami O.,ENSTA | Houllier M.,ENSTA
8th Annual IEEE International Systems Conference, SysCon 2014 - Proceedings | Year: 2014

The decision process is considered very important in many systems engineering methodologies, and multi-objective optimization is known to be an important tool for decision-making support. However, there is still a lack of a theoretical ground to guide its smart use. In a context of conception of naval systems, we present the outline of a process designed to output a set of alternatives for its architecture, optimizing several objectives and subject to constraints. The key point here is to outline the importance of the development of methods to choose smartly our approach to multi-objective optimization, in order to get the desired quality of results along several criteria, under several cost constraints. These choices include the choice of the algorithm and its parameters, but also the choice of the width of the architecture space. We show how complexity measures may be used to this end. We also show how we can take advantage of design with architecture frameworks in our approach. © 2014 IEEE.

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