17 research outputs found
Adaptive beamforming for large arrays in satellite communications systems with dispersed coverage
Conventional multibeam satellite communications systems ensure coverage of wide areas through multiple fixed beams where all users inside a beam share the same bandwidth. We consider a new and more flexible system where each user is assigned his own beam, and the users can be very geographically dispersed. This is achieved through the use of a large direct radiating array (DRA) coupled with adaptive beamforming so as to reject interferences and to provide a maximal gain to the user of interest. New fast-converging adaptive beamforming algorithms are presented, which allow to obtain good signal to interference and noise ratio (SINR) with a number of snapshots much lower than the number of antennas in the array. These beamformers are evaluated on reference scenarios
On convergence of the auxiliary-vector beamformer with rank-deficient covariance matrices
The auxiliary-vector beamformer is an algorithm that generates iteratively a sequence of beamformers which,
under the assumption of a positive definite covariance matrix R, converges to the minimum variance distortionless response beamformer, without resorting to any matrix inversion. In the case where R is rank-deficient, e.g., when R is substituted for the sample covariance matrix and the number of snapshots is less than the number of array elements, the behavior of the AV beamformer is not known theoretically. In this letter, we derive a new convergence result and show that the AV beamformer weights converge when R is rank-deficient, and that the limit belongs to the class of reduced-rank beamformers
SYNERGISTIC ON AUXIN AND CYTOKININ 1 positively regulates growth and attenuates soil pathogen resistance
Plants as non-mobile organisms constantly integrate varying environmental signals to flexibly adapt their growth and development. Local fluctuations in water and nutrient availability, sudden changes in temperature or other abiotic and biotic stresses can trigger changes in the growth of plant organs. Multiple mutually interconnected hormonal signaling cascades act as essential endogenous translators of these exogenous signals in the adaptive responses of plants. Although the molecular backbones of hormone transduction pathways have been identified, the mechanisms underlying their interactions are largely unknown. Here, using genome wide transcriptome profiling we identify an auxin and cytokinin cross-talk component; SYNERGISTIC ON AUXIN AND CYTOKININ 1 (SYAC1), whose expression in roots is strictly dependent on both of these hormonal pathways. We show that SYAC1 is a regulator of secretory pathway, whose enhanced activity interferes with deposition of cell wall components and can fine-tune organ growth and sensitivity to soil pathogens. Cytokinin and auxin are two major hormonal regulators of plant growth. Here the authors identify SYAC1, a gene that is synergistically activated by the two hormones being applied together, and show that it is required for normal growth while negatively impacting pathogen resistance
NaCo polarimetric observations of Sz 91 transitional disc: a remarkable case of dust filtering
We present polarized light observations of the transitional disc around Sz 91 acquired with
VLT/NaCo at H (1.7ÎŒm) and Ks (2.2ÎŒm) bands. We resolve the disc and detect polarized
emission up to âŒ0.5 arcsec (âŒ80 au) along with a central cavity at both bands. We computed a
radiative transfer model that accounts for the main characteristics of the polarized observations.
We found that the emission is best explained by small, porous grains distributed in a disc with
a âŒ45 au cavity. Previous ALMA observations have revealed a large sub-mm cavity (âŒ83
au) and extended gas emission from the innermost (<16 au) regions up to almost 400 au
from the star. Dynamical clearing by multiple low-mass planets arises as the most probable
mechanism for the origin of Sz 91âs peculiar structure. Using new L
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band ADI observations,
we can rule out companions more massive than Mp â„ 8 MJup beyond 45 au assuming hot-start
models. The disc is clearly asymmetric in polarized light along the minor axis, with the north
side brighter than the south side. Differences in position angle between the disc observed at
sub-mm wavelengths with ALMA and our NaCo observations were found. This suggests that
the disc around Sz 91 could be highly structured. Higher signal-to-noise near-IR and sub-mm
observations are needed to confirm the existence of such structures and to improve the current
understanding of the origin of transitional discs.IndexaciĂłn: Scopu
Traitement d'antenne SDMA pour systÚme de télécommunications par satellite avec couverture dispersée
Pour les organismes nationaux et/ou internationaux, disposer d'un moyen de communication flexible et rapidement dĂ©ployable sur une zone de service Ă©tendue reprĂ©sente un enjeu majeur. Pour assurer la sĂ©curitĂ© des personnes dans des zones parfois difficiles d'accĂšs, il est Ă©galement important de possĂ©der une infrastructure de communication qui fonctionne mĂȘme en prĂ©sence de brouilleurs. Un systĂšme satellite disposant d'une antenne rĂ©seau Ă rayonnement direct (DRA) associĂ©e Ă un formateur de faisceaux numĂ©rique (DBFN) et Ă un algorithme d'allocation de la ressource temps-frĂ©quence-position permet d'atteindre ces objectifs, en rĂ©alisant un accĂšs multiple Ă rĂ©partition spatiale (SDMA). Le SDMA est le couplage d'une allocation flexible de la ressource et de la formation de faisceaux adaptative. Compte tenu des contraintes spatiales, l'utilisation de traitements Ă faible complexitĂ© et faible support d'entraĂźnement est nĂ©cessaire pour la mise en place d'une stratĂ©gie SDMA afin de permettre une utilisation efficace de la ressource en frĂ©quence. Les conditions d'utilisation de tels algorithmes de formation de faisceaux requis dans le SDMA sont dĂ©terminĂ©es en fonction du contexte opĂ©rationnel rencontrĂ©. La quantification des bĂ©nĂ©fices apportĂ©s par le SDMA montre qu'en temps de paix l'efficacitĂ© spectrale, et donc le dĂ©bit des liens sont augmentĂ©s, et qu'en prĂ©sence de brouilleurs, le maintien d'une partie des communications est rĂ©alisĂ©.TOULOUSE-ISAE (315552318) / SudocSudocFranceF
Subjective-C : Bringing Context to Mobile Platform Programming
Thanks to steady advances in hardware, mobile computing platforms are nowadays much more connected to their physical and logical environment than ever before. To ease the construction of adaptable applications that are smarter with respect to their execution environment,the context-oriented programming paradigm has emerged. However,
up until now there has been no proof that this emerging paradigm can be implemented and used effectively on mobile devices, probably the kind of platform which is most subject to dynamically changing contexts.
In this paper we study how to effectively realise core context-oriented abstractions on top of Objective-C, a mainstream language for mobile device programming. The result is Subjective-C, a language which goes beyond existing context-oriented languages by providing a rich encoding of context interdependencies. Our initial validation cases and efficiency benchmarks make us confident that context-oriented programming can become mainstream in mobile application development
The vertical structure of debris discs and the impact of gas
International audienceThe vertical structure of debris discs provides clues about their dynamical evolution and the collision rate of the unseen planetesimals. Thanks to the ever-increasing angular resolution of contemporary instruments and facilities, we are beginning to constrain the scale height of a handful of debris discs, either at near-infrared or millimeter wavelengths. None the less, this is often done for individual targets only. We present here the geometric modeling of eight discs close to edge-on, all observed with the same instrument (SPHERE) and using the same mode (dual-beam polarimetric imaging). Motivated by the presence of CO gas in two out of the eight discs, we then investigate the impact that gas can have on the scale height by performing N-body simulations including gas drag and collisions. We show that gas can quickly alter the dynamics of particles (both in the radial and vertical directions), otherwise governed by gravity and radiation pressure. We find that, in the presence of gas, particles smaller than a few tens of microns can efficiently settle toward the midplane at the same time as they migrate outward beyond the birth ring. For second generation gas (Mgas †0.1 Mâ), the vertical settling should be best observed in scattered light images compared to observations at millimeter wavelengths. But if the gas has a primordial origin (Mgas â„ 1 Mâ), the disc will appear very flat both at near-infrared and sub-mm wavelengths. Finally, far beyond the birth ring, our results suggest that the surface brightness profile can be as shallow as ~-2.25