381 research outputs found
Friction of Physisorbed Nanotubes: Rolling or Sliding?
The structure and motion of carbon and h-BN nanotubes (NTs) deposited on
graphene is inquired theoretically by simulations based on state-of-the-art
interatomic force fields. Results show that any typical cylinder-over-surface
approximation is essentially inaccurate. NTs tend to flatten at the interface
with the substrate and upon driving they can either roll or slide depending on
their size and on their relative orientation with the substrate. In the
epitaxially aligned orientation we find that rolling is always the main
mechanism of motion, producing a kinetic friction linearly growing with the
number of walls, in turn causing an unprecedented supra-linear scaling with the
contact area. A 30 degrees misalignment raises superlubric effects, making
sliding favorable against rolling. The resulting rolling-to-sliding transition
in misaligned NTs is explained in terms of the faceting appearing in large
multi-wall tubes, which is responsible for the increased rotational stiffness.
Modifying the geometrical conditions provides an additional means of
drastically tailoring the frictional properties in this unique tribological
system
Single-Anchor Localization and Orientation Performance Limits Using Massive Arrays: MIMO vs. Beamforming
open3noIn the next generation of cellular networks, it is
desirable to use single access points both for communication and
localization. This could be made possible thanks to the combination
of femtocells, mm-wave technology and massive antenna
arrays, and would overcome the problem of having an over-sized
infrastructure for positioning which is, nowadays, the bottleneck
for the widespread diffusion of indoor localization systems. In
this context, our paper aims at investigating the localization
and orientation performance limits employing massive arrays
both at the access point and mobile side. To this end, we first
asymptotically demonstrate the tightness of the Cramér-Rao
bound (CRB) in the massive array regime and that the effect
of multipath can be made negligible even for practical values
of SNR levels. Successively, we propose a comparison between
two different transmitter configurations, namely multiple-input
multiple-output (MIMO), where orthogonal waveforms are sent,
and beamforming, which takes advantage of highly correlated
waveforms and directive array patterns. We also consider random
weighting as a trade-off between the diversity gain of MIMO and
the high directivity guaranteed by the beamforming. CRB results
show the interplay between diversity and beamforming gain as
well as the benefits achievable by varying the number of antennas
in terms of localization accuracy and multipath mitigation.embargoed_20181201Guerra, Anna; Guidi, Francesco; Dardari, DavideGuerra, Anna; Guidi, Francesco; Dardari, David
Single-Anchor Localization and Orientation Performance Limits Using Massive Arrays: MIMO vs. Beamforming
In the next generation of cellular networks, it is
desirable to use single access points both for communication and
localization. This could be made possible thanks to the combination
of femtocells, mm-wave technology and massive antenna
arrays, and would overcome the problem of having an over-sized
infrastructure for positioning which is, nowadays, the bottleneck
for the widespread diffusion of indoor localization systems. In
this context, our paper aims at investigating the localization
and orientation performance limits employing massive arrays
both at the access point and mobile side. To this end, we first
asymptotically demonstrate the tightness of the Cram\ue9r-Rao
bound (CRB) in the massive array regime and that the effect
of multipath can be made negligible even for practical values
of SNR levels. Successively, we propose a comparison between
two different transmitter configurations, namely multiple-input
multiple-output (MIMO), where orthogonal waveforms are sent,
and beamforming, which takes advantage of highly correlated
waveforms and directive array patterns. We also consider random
weighting as a trade-off between the diversity gain of MIMO and
the high directivity guaranteed by the beamforming. CRB results
show the interplay between diversity and beamforming gain as
well as the benefits achievable by varying the number of antennas
in terms of localization accuracy and multipath mitigation
Modélisation du transfert d'un aérocontaminant dans un local ventilé en champ proche d'une source d'émission accidentelle
La prévision de l'évolution spatio-temporelle de la concentration en polluant dans un local ventilé est encore de nos jours difficile à obtenir. Les travaux réalisés consistent en la modélisation de la dispersion d'un polluant dans une partie du local, correspondant au champ proche d'une source d'émission, suite à une situation accidentelle conduisant à la rupture du confinement assuré par une enceinte. La recherche de modèles s'appuie à la fois sur des expérimentations utilisant des techniques de traçage gazeux et des simulations multidimensionnelles à l'aide de codes de mécanique des fluides. Un modèle décrivant l'évolution spatio-temporelle de la concentration d'un polluant gazeux dilué en champ proche de la fuite est proposé. Il tient compte des différents paramètres d'étude : géométrie de la fuite (fente ou orifice circulaire), type de l'émission (continue ou transitoire), vitesse et durée d'émission. Enfin, l'influence des effets de densité (polluant en forte concentration) et celle des effets de la ventilation d'un local sont également abordées dans le cas de fuites continues. L'ensemble des modèles élaborés, pour un polluant gazeux, se présente sous la forme de corrélations s'inspirant de la théorie des jets d'écoulements turbulents libres. Ces modèles sont facilement utilisables dans le cadre des évaluations de sûreté traitant du confinement des substances et de la protection des opérateurs dans les installations, nucléaires ou autres. ABSTRACT : Nowadays, predicting the space-time evolution of a pollutant released in a ventilated room including a process operation remains hard to achieve. However this prediction is imperative in hazardous activities, such as nuclear ones. The study consists in predicting space-time evolution of an airborne contaminant dispersion in the near-field emission source around a workplace, following an accidental rupture of a containment enclosure. The whole work is based on experiments of gas tracing, and on multidimensional simulations using CFD tools. The proposed model is written as a correlated function of various parameters: leak geometry (slot or circular opening), emission type (continuous or puff), initial velocity and emission duration. Influence of ventilation and obstructions (room walls) have been also studied in the case of continuous leaks. All final models, for gaseous pollutants, are written as correlations inspired by the theory of free turbulent jet flows. These models are easy to use within the framework of safety evaluations dealing with radioactive material containment and radiological protection inside nuclear facilities
Functional characterization of an E3 ubiquitin ligase involved in plant response to abiotic stress
Protein ubiquitination is a post-translational modification that targets protein substrates for 26S proteasome-mediated degradation. It is based on the covalent attachment of the 76-amino acid eukaryotic molecule, ubiquitin, to substrate proteins. Protein ubiquitination plays a key role in a wide variety of cellular processes such as hormone signaling, DNA repair, biotic and abiotic stress
response, cell cycle regulation. Ubiquitin conjugation is a multistep reaction, sequentially involving
three enzymes referred to as E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme)
and E3 (ubiquitin ligase). In Arabidopsis thaliana more than one thousand of genes code for E3 ubiquitin enzymes that specifically recognize target proteins. In a previous work we isolated an E3
ubiquitin ligase early induced during cold/light stress in durum wheat. In this work several approaches were initiated to functional characterize the E3. To test its functionality in vitro an ubiquitination assay was carried out and the subcellular localization was determined. The involvement of the E3 in drought stress was also investigated. Moreover a wheat cDNA library from cold treated leaves of Triticum durum has been produced and screened by two-hybrid system to isolate potential E3 interactors and ubiquitination targets. In parallel was developed a system to identify E3 targets on large scale in Arabidopsis. Preliminary results of this work will be shown
Dynamic Radar Network of UAVs: A Joint Navigation and Tracking Approach
Nowadays there is a growing research interest on the possibility of enriching
small flying robots with autonomous sensing and online navigation capabilities.
This will enable a large number of applications spanning from remote
surveillance to logistics, smarter cities and emergency aid in hazardous
environments. In this context, an emerging problem is to track unauthorized
small unmanned aerial vehicles (UAVs) hiding behind buildings or concealing in
large UAV networks. In contrast with current solutions mainly based on static
and on-ground radars, this paper proposes the idea of a dynamic radar network
of UAVs for real-time and high-accuracy tracking of malicious targets. To this
end, we describe a solution for real-time navigation of UAVs to track a dynamic
target using heterogeneously sensed information. Such information is shared by
the UAVs with their neighbors via multi-hops, allowing tracking the target by a
local Bayesian estimator running at each agent. Since not all the paths are
equal in terms of information gathering point-of-view, the UAVs plan their own
trajectory by minimizing the posterior covariance matrix of the target state
under UAV kinematic and anti-collision constraints. Our results show how a
dynamic network of radars attains better localization results compared to a
fixed configuration and how the on-board sensor technology impacts the accuracy
in tracking a target with different radar cross sections, especially in non
line-of-sight (NLOS) situations
In silico analyses of mitochondrial ORFans in freshwater mussels (Bivalvia: Unionoida) provide a framework for future studies of their origin and function
Protein sequence alignment of Cumberlandia monodonta M-ORF and ATP8, along with ATP8 from the most diverse members of the Mt_ATP-synt_B superfamily (pfam02326). Homo sapiens ATP8 has also been included for comparison. The alignment was generated using T-COFFEE. The most conserved N-terminal domain, i.e. the best aligned portion, is in red; the rest of the sequences are rather badly aligned (in green). Consensus is shown and indicates good (red), intermediate (yellow), and bad alignment (green), and insertion/deletion (in blue). Cumberland, Cumberlandia monodonta; H_sapiens, Homo sapiens; Malawimonas, Malawimonas sp. (Excavate); Thraustoch, Thraustochytrium sp. (Stramenopiles); Mesostigma, Mesostigma sp. (Streptophyta); Reclinomon, Reclinomonas sp. (Protozoa); Porphyra, Porphyra sp. (Rhodophyta); Cyanidiosc, Cyanidioschyzon sp. (Rhodophyta); Pseudendoc, Pseudendoclonium sp. (Chlorophyta); Acanthamoe, Acanthamoeba sp. (Amoebozoa); Nephroselm, Nephroselmis sp. (Streptophyta). (PDF 236 kb
Occupancy Grid Mapping for Personal Radar Applications
Next fifth generation (5G) of mobile wireless communication foresees the use of mm-wave technology to boost communication at an unprecedented scale, thanks to the large available bandwidth [1] . In addition, the move-up in the frequency spectrum allows to include a large number of antennas into a small area, thus enabling their integration into portable devices [2 , 3] . In this way, such a technological perspective can be exploited to add new functionalities in addition to communication. For example, the laser-like beamsteering allowed by massive arrays at mm-wave can be used to automatically scan and reconstruct the topology of the surrounding environment. Such an idea, namely personal radar, has been recently proposed in theory and its feasibility assessed by experiments [4 \u2013 [5]6] . In these works, the performance has been investigated through the adoption of a grid-based mapping approach relying on an extended Kalman-Filter (EKF): the environment has been discretized in a grid of cells whose root-radar cross section (RCS) values constitute the state vector to be estimated starting from the backscattered radar response [7 , 8] . To simplify the analysis, the state of the system has been modeled as a Gaussian random vector whose mean vector and covariance matrix are updated during the mapping process as soon as new measurements are collected [5 , 9] . The main limitation of this model is that the Gaussian assumption does not capture the underlaying bimodal nature of the phenomenon, i.e., each cell is empty or occupied. In laser-based mapping systems, occupancy grid (OG) methods are usually considered to model this bi-modality by exploiting the basic assumption that laser beam illuminates only one cell per time [10] . This is not the case in radio-based radars where the shape of the radiation pattern is such to illuminate an area composed of several cells, thus making existing OG methods not appropriate due to the inherent cross-correlation between cells that is not zero [11]
Two-Timescale Joint Precoding Design and RIS Optimization for User Tracking in Near-Field MIMO Systems
In this paper, we propose a novel framework that aims to jointly design the
reflection coefficients of multiple RISs and the precoding strategy of a single
BS to optimize the self-tracking of the position and the velocity of a single
multi-antenna UE that moves either in the far- or near-field region.
Differently from the literature, and to keep the overall complexity affordable,
we assume that RIS optimization is performed less frequently than localization
and precoding adaptation. The proposed procedure leads to minimize the inverse
of the received power in the UE position uncertainty area between two
subsequent optimization steps.
The optimal RIS and precoder strategy are compared with the classical
beam-focusing strategy and with a scheme that maximizes the communication rate.
It is shown that if the RISs are optimized for communications, their
configuration is suboptimal when used for tracking purposes.
Numerical results show that in typical indoor environments with only one BS
and a few RISs operating on millimeter waves, high location accuracy in the
range of less than half a meter can be achieved
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