992 research outputs found
DeepTx: Deep Learning Beamforming with Channel Prediction
Machine learning algorithms have recently been considered for many tasks in
the field of wireless communications. Previously, we have proposed the use of a
deep fully convolutional neural network (CNN) for receiver processing and shown
it to provide considerable performance gains. In this study, we focus on
machine learning algorithms for the transmitter. In particular, we consider
beamforming and propose a CNN which, for a given uplink channel estimate as
input, outputs downlink channel information to be used for beamforming. The CNN
is trained in a supervised manner considering both uplink and downlink
transmissions with a loss function that is based on UE receiver performance.
The main task of the neural network is to predict the channel evolution between
uplink and downlink slots, but it can also learn to handle inefficiencies and
errors in the whole chain, including the actual beamforming phase. The provided
numerical experiments demonstrate the improved beamforming performance.Comment: 27 pages, this work has been submitted to the IEEE for possible
publication; v2: Fixed typo in author name, v3: a revisio
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Kinematic frames and "active longitudes": does the Sun have a face?
It has recently been claimed that analysis of Greenwich sunspot data over 120
years reveals that sunspot activity clusters around two longitudes separated by
180 degrees (``active longitudes'') with clearly defined differential rotation
during activity cycles.In the present work we extend this critical examination
of methodology to the actual Greenwich sunspot data and also consider newly
proposed methods of analysis claiming to confirm the original identification of
active longitudes. Our analysis revealed that values obtained for the
parameters of differential rotation are not stable across different methods of
analysis proposed to track persistent active longitudes. Also, despite a very
thorough search in parameter space, we were unable to reproduce results
claiming to reveal the century-persistent active longitudes. We can therefore
say that strong and well substantiated evidence for an essential and
century-scale persistent nonaxisymmetry in the sunspot distribution does not
exist.Comment: 14 pages, 1 table, 21 figures, accepted in A&
Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo
The possibility of the magnetic flux expulsion from the Galaxy in the
superbubble (SB) explosions, important for the Alpha-Omega dynamo, is
considered. Special emphasis is put on the investigation of the downsliding of
the matter from the top of the shell formed by the SB explosion which is able
to influence the kinematics of the shell. It is shown that either Galactic
gravity or the development of the Rayleigh-Taylor instabilities in the shell
limit the SB expansion, thus, making impossible magnetic flux expulsion. The
effect of the cosmic rays in the shell on the sliding is considered and it is
shown that it is negligible compared to Galactic gravity. Thus, the question of
possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure
Effects of Rotation and Input Energy Flux on Convective Overshooting
We study convective overshooting by means of local 3D convection
calculations. Using a mixing length model of the solar convection zone (CZ) as
a guide, we determine the Coriolis number (Co), which is the inverse of the
Rossby number, to be of the order of ten or larger at the base of the solar CZ.
Therefore we perform convection calculations in the range Co = 0...10 and
interpret the value of Co realised in the calculation to represent a depth in
the solar CZ. In order to study the dependence on rotation, we compute the
mixing length parameters alpha_T and alpha_u relating the temperature and
velocity fluctuations, respectively, to the mean thermal stratification. We
find that the mixing length parameters for the rapid rotation case,
corresponding to the base of the solar CZ, are 3-5 times smaller than in the
nonrotating case. Introducing such depth-dependent alpha into a solar structure
model employing a non-local mixing length formalism results in overshooting
which is approximately proportional to alpha at the base of the CZ. Although
overshooting is reduced due to the reduced alpha, a discrepancy with
helioseismology remains due to the steep transition to the radiative
temperature gradient. In comparison to the mixing length models the transition
at the base of the CZ is much gentler in the 3D models. It was suggested
recently (Rempel 2004) that this discrepancy is due to the significantly larger
(up to seven orders of magnitude) input energy flux in the 3D models in
comparison to the Sun and solar models, and that the 3D calculations should be
able to approach the mixing length regime if the input energy flux is decreased
by a moderate amount. We present results from local convection calculations
which support this conjecture.Comment: 6 pages, 3 figures, to appear in Convection in Astrophysics, Proc.
IAUS 239, edited by F. Kupka, I.W. Roxburgh, K.L. Cha
Slight respiratory irritation but not inflammation in mice exposed to (1-->3)-beta-D-glucan aerosols.
Airway irritation effects after single and repeated inhalation exposures to aerosols of beta-glucan (grifolan) were investigated in mice. In addition, the effects on serum total immunoglobulin E (IgE) production and histopathological inflammation in the respiratory tract were studied. The beta-glucan aerosols provoked slight sensory irritation in the airways, but the response was not concentration dependent at the levels studied. Slight pulmonary irritation was observed after repeated exposures. No effect was found on the serum total IgE levels, and no signs of inflammation were seen in the airways 6 h after the final exposure. The results suggest that, irrespective of previous fungal sensitization of the animals, inhaled beta-glucan may cause symptoms of respiratory tract irritation but without apparent inflammation. Respiratory tract irritation reported after inhalation of fungi may not be entirely attributed to beta-glucan
Local models of stellar convection II: Rotation dependence of the mixing length relations
We study the mixing length concept in comparison to three-dimensional
numerical calculations of convection with rotation. In a limited range, the
velocity and temperature fluctuations are linearly proportional to the
superadiabaticity, as predicted by the mixing length concept and in accordance
with published results. The effects of rotation are investigated by varying the
Coriolis number, Co = 2 Omega tau, from zero to roughly ten, and by calculating
models at different latitudes. We find that \alpha decreases monotonically as a
function of the Coriolis number. This can be explained by the decreased spatial
scale of convection and the diminished efficiency of the convective energy
transport, the latter of which leads to a large increase of the
superadibaticity, \delta = \nabla - \nabla_ad as function of Co. Applying a
decreased mixing length parameter in a solar model yields very small
differences in comparison to the standard model within the convection zone. The
main difference is the reduction of the overshooting depth, and thus the depth
of the convection zone, when a non-local version of the mixing length concept
is used. Reduction of \alpha by a factor of roughly 2.5 is sufficient to
reconcile the difference between the model and helioseismic results. The
numerical results indicate reduction of \alpha by this order of magnitude.Comment: Final published version, 8 pages, 9 figure
- …