749 research outputs found
Noncoherent Space-Time Coding: An Algebraic Perspective
Cataloged from PDF version of article.The design of spaceātime signals for noncoherent
block-fading channels where the channel state information is
not known a priori at the transmitter and the receiver is considered.
In particular, a new algebraic formulation for the diversity
advantage design criterion is developed. The new criterion encompasses,
as a special case, the well-known diversity advantage
for unitary spaceātime signals and, more importantly, applies to
arbitrary signaling schemes and arbitrary channel distributions.
This criterion is used to establish the optimal diversity-versus-rate
tradeoff for training based schemes in block-fading channels.
Our results are then specialized to the class of affine spaceātime
signals which allows for a low complexity decoder. Within this
class, spaceātime constellations based on the threaded algebraic
spaceātime (TAST) architecture are considered. These constellations
achieve the optimal diversity-versus-rate tradeoff over
noncoherent block-fading channels and outperform previously
proposed codes in the considered scenarios as demonstrated by
the numerical results. Using the analytical and numerical results
developed in this paper, nonunitary spaceātime codes are argued
to offer certain advantages in block-fading channels where the appropriate
use of coherent spaceātime codes is shown to offer a very
efficient solution to the noncoherent spaceātime communication
paradigm
Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process
Manufacturing superconducting circuits out of ultrathin films is a
challenging task when it comes to patterning complex compounds, which are
likely to be deteriorated by the patterning process. With the purpose of
developing high-T superconducting photon detectors, we designed a novel
route to pattern ultrathin YBCO films down to the nanometric scale. We believe
that our method, based on a specific use of a focused-ion beam, consists in
locally implanting Ga^{3+} ions and/or defects instead of etching the film.
This protocol could be of interest to engineer high-T superconducting
devices (SQUIDS, SIS/SIN junctions and Josephson junctions), as well as to
treat other sensitive compounds.Comment: 13 pages, 7 figure
Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells
We have shown experimentally that an electric field decreases the energy
separation between the two components of a dense spin-polarized exciton gas in
a coupled double quantum well, from a maximum splitting of meV to
zero, at a field of 35 kV/cm. This decrease, due to the field-induced
deformation of the exciton wavefunction, is explained by an existing
calculation of the change in the spin-dependent exciton-exciton interaction
with the electron-hole separation. However, a new theory that considers the
modification of screening with that separation is needed to account for the
observed dependence on excitation power of the individual energies of the two
exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press
Topology of six degrees of freedom magnetic bearing
A novel magnetic topology has been designed for a six degrees of freedom, magnetically levitated and driven mirror, to be used in a three dimensional (3D) measurement system based on laser interferometry. The translations of the mirror are to be kept small, whereas the rotations are to be controlled over a large range with a high bandwidth and high accuracy. Finite element modelling (FEM) is used to analyze the proposed topology. For computational load reduction, a 2D FEM model has been derived from the actual 3D topology, which incorporates most of the magnetic subsystems. Simulations show that cross-influence between the actuators is small, that the forces and torques are proportional to the applied currents and that the angle of the rotor is of little influence. This allows the multiple in multiple out system to be regarded as multiple linear single in single out systems. Ā©2000 American Institute of Physics
Spin sensitive bleaching and monopolar spin orientation in quantum wells
Spin sensitive bleaching of the absorption of far-infrared radiation has been
observed in -type GaAs/AlGaAs quantum well structures. The absorption of
circularly polarized radiation saturates at lower intensities than that of
linearly polarized light due to monopolar spin orientation in the first heavy
hole subband. Spin relaxation times of holes in -type material in the range
of tens of ps were derived from the intensity dependence of the absorption.Comment: Figures have been updated due to technical printing problems
(Postscript mismatch
P2Y12 blocker monotherapy after percutaneous coronary intervention
For secondary prevention of coronary artery disease (CAD) antiplatelet therapy is essential. For patients undergoing a percutaneous coronary intervention (PCI) temporary dual antiplatelet platelet therapy (DAPT: aspirin combined with a P2Y12 blocker) is mandatory, but leads to more bleeding than single antiplatelet therapy with aspirin. Therefore, to reduce bleeding after a PCI the duration of DAPT is usually kept as short as clinically acceptable; thereafter aspirin monotherapy is administered. Another option to reduce bleeding is to discontinue aspirin at the time of DAPT cessation and thereafter to administer P2Y12 blocker monotherapy. To date, five randomised trials have been published comparing DAPT with P2Y12 blocker monotherapy in 32,181 stented patients. Also two meta-analyses addressing this novel therapy have been presented. P2Y12 blocker monotherapy showed a 50-60% reduction in major bleeding when compared to DAPT without a significant increase in ischaemic outcomes, including stent thrombosis. This survey reviews the findings in the current literature concerning P2Y12 blocker monotherapy after PCI
Rescaling Egocentric Vision: Collection, Pipeline and Challenges for EPIC-KITCHENS-100
This paper introduces the pipeline to extend the largest dataset in egocentric vision, EPIC-KITCHENS. The effort culminates in EPIC-KITCHENS-100, a collection of 100Ā hours, 20M frames, 90K actions in 700 variable-length videos, capturing long-term unscripted activities in 45 environments, using head-mounted cameras. Compared to its previous version (Damen in Scaling egocentric vision: ECCV, 2018), EPIC-KITCHENS-100 has been annotated using a novel pipeline that allows denser (54% more actions per minute) and more complete annotations of fine-grained actions (+128% more action segments). This collection enables new challenges such as action detection and evaluating the ātest of timeāāi.e. whether models trained on data collected in 2018 can generalise to new footage collected two years later. The dataset is aligned with 6 challenges: action recognition (full and weak supervision), action detection, action anticipation, cross-modal retrievalĀ (from captions), as well as unsupervised domain adaptation for action recognition. For each challenge, we define the task, provide baselines and evaluation metrics.Published versionResearch at Bristol is supported by Engineering and Physical Sciences Research Council (EPSRC) Doctoral Training Program (DTP), EPSRC Fellowship UMPIRE (EP/T004991/1). Research at Catania is sponsored by Piano della Ricerca 2016-2018 linea di Intervento 2 of DMI, by MISE - PON I&C 2014-2020, ENIGMA project (CUP: B61B19000520008) and by MIUR AIM - Attrazione e Mobilita Internazionale Linea 1 - AIM1893589 - CUP E64118002540007
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