741 research outputs found
Anomalous fluctuation regimes at the FFLO transition
Recently some experimental evidences have been obtained in favour of the
existence of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
superconducting state in heavy-fermion superconductor CeCoIn_{5} and organic
superconductor -(BETS)_{2}FeCl_{4}. However the unambiguous identification of
FFLO state remains very difficult. We present the theoretical studies of the
Gaussian fluctuations near the tricritical point (where the FFLO modulation
appears) and demonstrate that the behavior of the fluctuational specific heat,
paraconductivity and diamagnetism is qualitatively different from the usual
superconducting transition. Special values of the critical exponent and the
crossovers between different fluctuational regimes may provide a unique test
for the FFLO state appearance.Comment: 6 pages, 2 figures. Work supported by ANR Extreme Conditions
Correlated Electrons (ANR-06-BLAN-0220
Weighted Sum Rate Maximization for Downlink OFDMA with Subcarrier-pair based Opportunistic DF Relaying
This paper addresses a weighted sum rate (WSR) maximization problem for
downlink OFDMA aided by a decode-and-forward (DF) relay under a total power
constraint. A novel subcarrier-pair based opportunistic DF relaying protocol is
proposed. Specifically, user message bits are transmitted in two time slots. A
subcarrier in the first slot can be paired with a subcarrier in the second slot
for the DF relay-aided transmission to a user. In particular, the source and
the relay can transmit simultaneously to implement beamforming at the
subcarrier in the second slot. Each unpaired subcarrier in either the first or
second slot is used for the source's direct transmission to a user. A benchmark
protocol, same as the proposed one except that the transmit beamforming is not
used for the relay-aided transmission, is also considered. For each protocol, a
polynomial-complexity algorithm is developed to find at least an approximately
optimum resource allocation (RA), by using continuous relaxation, the dual
method, and Hungarian algorithm. Instrumental to the algorithm design is an
elegant definition of optimization variables, motivated by the idea of
regarding the unpaired subcarriers as virtual subcarrier pairs in the direct
transmission mode. The effectiveness of the RA algorithm and the impact of
relay position and total power on the protocols' performance are illustrated by
numerical experiments. The proposed protocol always leads to a maximum WSR
equal to or greater than that for the benchmark one, and the performance gain
of using the proposed one is significant especially when the relay is in close
proximity to the source and the total power is low. Theoretical analysis is
presented to interpret these observations.Comment: 8 figures, accepted and to be published in IEEE Transactions on
Signal Processing. arXiv admin note: text overlap with arXiv:1301.293
Mathematics education (research) liberated from teaching and learning: Towards (the future of) doing mathematics
Now, what do you make of that? We want to discuss mathematics education, and research in mathematics education (we don’t really need to specify, hence the parenthesis). And we want to address teaching and learning (mathematics, but we skipped the parenthesis this time around because we used the word just before) in terms of not-being. You will notice, however, the provocative presence of “liberated”, a strong verb which alludes to the idea of freedom while acknowledging certain constraints: captivity, dependence, and even liability. Its selection was a bold choice. But having made such a suggestive statement (without actually saying things upfront), we hastily print a colon to bring in a potential alternative – more positive, one should hope. This alternative mentions to be about going “towards” something. If we care enough to read the next parenthesis (yep, another set!), we realize that this something does not exist yet, since it lies in the future. This something we finally name with the title’s last breath: “doing|mathematics”. For those who haven’t read our recent articles (e.g., in French, Maheux & Proulx, 2014, or in English, Maheux & Proulx, 2015), what that is will remain obscure for some time. In a certain way, it is also still mysterious for us: it is the object of our research, so we haven’t “found” it yet. We haven’t really found out what “doing|mathematics” is, or what it does. As a piece of research, this article thus also aims to help us figure some of these things out. As a result, this communication piece is not merely for you. And after all, as Von Foerster used to say, in the end you’ll know more about us than about the topic of the paper
ASTERIA In-Orbit Testing on OPSSAT: An On-Board Autonomous Orbit Control Solution Including Collision Risks Avoidance
For several years, CNES has been working on Flight Dynamics algorithms to ensure high level of autonomy for next generation of space missions. One example of these autonomous techniques is the Autonomous Orbital Control, which consists of delegating to onboard satellite system the identification, planning and realization of orbital corrections to stay in the mission reference orbit. ASTERIA, an application of on-board autonomy combining station keeping and collision risk management for the low earth orbit satellites, enables both in-track and cross-track control for different LEO missions. The on-board collision risk management process is fully integrated into the autonomous station keeping in order to maintain the satellite orbit as best as possible and to minimize mission unavailability resulting from the avoidance maneuvers.
The paper aims to show the completeness of the ASTERIA concept. First, the principles of on-board orbit control with collision risk management are described with the operational concepts of such a solution. Then, the ability to operate ASTERIA is demonstrated through an in-orbit experiment performed last year on the ESA OPSSAT 3-Units CubeSat
AILiveSim : An Extensible Virtual Environment for Training Autonomous Vehicles
Virtualization technologies have become common- place both in software development as well as engineering in a more general sense. Using virtualization offers other benefits than simulation and testing as a virtual environment can often be more liberally configured than the corresponding physical envi- ronment. This, in turn, introduces new possibilities for education and training, including both for humans and artificial intelligence (AI). To this end, we are developing a simulation platform AILiveSim. The platform is built on top of the Unreal Engine game development system, and it is dedicated to training and testing autonomous systems, their sensors and their algorithms in a simulated environment. In this paper, we describe the elements that we have built on top of the engine to realize a Virtual Environment (VE) useful for the design, implementation, application and analysis of autonomous systems. We present the architecture that we have put in place to transform our simulation platform from automotive specific to be domain agnostic and support two new domains of applications: autonomous ships and autonomous mining machines. We describe the important specificity of each domain in regard to simulation. In addition, we also report the challenges encountered when simulating those applications, and the decisions taken to overcome these challenges.Peer reviewe
FLORES: Helping People to Realize Sustainable Futures
People usually know how they want their situation to change to secure a better future, but they do not always know how to change their situation. Initiatives intended to secure a better future do not always work as intended, and may have unintended side effects. Computer models can help advocates explore consequences of proposed initiatives, so they can make informed selections of alternatives, secure in the knowledge that consequences have been thoroughly investigated. By encouraging people to explore scenarios, models empower people to be more innovative and less dependent on technocrats. New software solves technical limitations, but the real issue is not software, but rather the provision of a supportive framework within which people can express and experiment with ideas. FLORES, the Forest Land Oriented Resource Envisioning System, provides such a framework to stimulate interdisciplinary collaboration between researchers, practitioners and clients. A recent prototype demonstrated the feasibility of FLORES. However, FLORES is not about software; it is about providing the means to explore the consequences of alternative scenarios. Ultimately, FLORES is not a physical package, but a user group and the interactions they have amongst themselves, and with the people involved in policy-making. Fostering this emerging network through workshops and technical support will enhance FLORES by offering a better understanding of the concept, and by allowing more people, especially those from developing countries, to influence the development of FLORES and the issues that can be explored within it
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