1,063 research outputs found
Precoder design for space-time coded systems over correlated Rayleigh fading channels using convex optimization
A class of computationally efficient linear precoders for space-time block coded multiple-input multiple-output wireless systems is derived based on the minimization of the exact symbol error rate (SER) and its upper bound. Both correlations at the transmitter and receiver are assumed to be present, and only statistical channel state information in the form of the transmit and receive correlation matrices is assumed to be available at the transmitter. The convexity of the design based on SER minimization is established and exploited. The advantage of the developed technique is its low complexity. We also find various relationships of the proposed designs to the existing precoding techniques, and derive very simple closed-form precoders for special cases such as two or three receive antennas and constant receive correlation. The numerical simulations illustrate the excellent SER performance of the proposed precoders
Resonant Enhancement of Inelastic Light Scattering in Strongly Correlated Materials
We use dynamical mean field theory to find an exact solution for inelastic
light scattering in strongly correlated materials such as those near a
quantum-critical metal-insulator transition. We evaluate the results for
(Raman) scattering and find that resonant effects can be quite
large, and yield a triple resonance, a significant enhancement of nonresonant
scattering peaks, a joint resonance of both peaks when the incident photon
frequency is on the order of , and the appearance of an isosbestic point in
all symmetry channels for an intermediate range of incident photon frequencies.Comment: 5 pages RevTex, 4 Figures ep
On the Optimal Precoding for MIMO Gaussian Wire-Tap Channels
We consider the problem of finding secrecy rate of a multiple-input
multiple-output (MIMO) wire-tap channel. A transmitter, a legitimate receiver,
and an eavesdropper are all equipped with multiple antennas. The channel states
from the transmitter to the legitimate user and to the eavesdropper are assumed
to be known at the transmitter. In this contribution, we address the problem of
finding the optimal precoder/transmit covariance matrix maximizing the secrecy
rate of the given wiretap channel. The problem formulation is shown to be
equivalent to a difference of convex functions programming problem and an
efficient algorithm for addressing this problem is developed.Comment: Published in Proceedings of the Tenth International Symposium on
Wireless Communication Systems (ISWCS 2013), Ilmenau, Germany, August 201
Surface Brightness Gradients Produced by the Ring Waves of Star Formation
We compute surface brightness profiles of galactic disks for outwardly
propagating waves of star formation with a view to investigate the stellar
populations in ring galaxies. We consider two mechanisms which can create
outwardly propagating star forming rings in a purely gaseous disk --- a
self-induced wave and a density wave. We show that the surface brightness
profiles produced by both scenarios of ring formation are similar and are
strongly sensitive to the velocity of the wave. The results of our computations
are compared with the observational quantities sensitive to the young and old
stellar populations in the ring galaxies A0035-335 (the Cartwheel galaxy) and
VIIZw466. The best fit to the observed radial H_alpha surface brightness
distribution in the Cartwheel galaxy is obtained for a wave velocity of about
90 km/s. The red continuum brightness of the ring can be fully explained by the
evolving stars present in the trailing part of the wave. However the red
continuum brightness in regions internal to the ring indicates that the wave of
star formation propagates in a pre-existing stellar disk in the Cartwheel. The
H_alpha and K-band surface brightness profiles in VIIZw466 match the values
expected from stellar populations produced by a wave of star formation
propagating in a purely gaseous disk very well. We conclude that VIIZw466 is
probably experiencing the first event of star formation in the disk.Comment: Uses aas2pp4.sty and epsfig.sty, 15 pages To appear in Astrophysical
Journal, March 10, 199
Chemical Abundance Gradients in the Star-Forming Ring Galaxies
Ring waves of star formation, propagating outwardly in the galactic disks,
leave chemical abundance gradients in their wakes. We show that the relative
[Fe/O] abundance gradients in ring galaxies can be used as a tool for
determining the role of the SNIa explosions in their chemical enrichment. We
consider two mechanisms which can create outwardly propagating star forming
rings in a purely gaseous disk -- a self-induced wave and a density wave, and
demonstrate that the radial distribution of the relative [Fe/O] abundance
gradients does not depend on the particular mechanism of the wave formation or
on the parameters of the star-forming process. We show that the [Fe/O] profile
is determined by the velocity of the wave, initial mass function, and the
initial chemical composition of the star-forming gas. If the role of SNIa
explosions is negligible in the chemical enrichment, the ratio [Fe/O] remains
constant throughout the galactic disk with a steep gradient at the wave front.
If SNIa stars are important in the production of cosmic iron, the [Fe/O] ratio
has gradient in the wake of the star-forming wave with the value depending on
the frequency of SNIa explosions.Comment: Uses aas2pp4.sty and epsfig.sty, 7 pages including one figure To
appear in Astrophysical Journa
Outbursts in Global Protoplanetary Disk Simulations
While accreting through a circumstellar disk, young stellar objects are observed to undergo sudden and powerful accretion events known as FUor or EXor outbursts. Although such episodic accretion is considered to be an integral part of the star formation process, the triggers and mechanisms behind them remain uncertain. We conducted global numerical hydrodynamics simulations of protoplanetary disk formation and evolution in the thin-disk limit, assuming both magnetically layered and fully magnetorotational instability (MRI)-active disk structure. In this paper, we characterize the nature of the outbursts occurring in these simulations. The instability in the dead zone of a typical layered disk results in "MRI outbursts." We explore their progression and their dependence on the layered disk parameters as well as cloud core mass. The simulations of fully MRI-active disks showed an instability analogous to the classical thermal instability. This instability manifested at two temperatures - above approximately 1400 K and 3500 K - due to the steep dependence of Rosseland opacity on the temperature. The origin of these thermally unstable regions is related to the bump in opacity resulting from molecular absorption by water vapor and may be viewed as a novel mechanism behind some of the shorter duration accretion events. Although we demonstrated local thermal instability in the disk, more investigations are needed to confirm that a large-scale global instability will ensue. We conclude that the magnetic structure of a disk, its composition, as well as the stellar mass, can significantly affect the nature of episodic accretion in young stellar objects. © 2020. The American Astronomical Society. All rights reserved.Horizon 2020 Framework Programme, H2020: 716155
Science with an ngVLA: Resolving the Radio Complexity of EXor and FUor-type Systems with the ngVLA
Episodic accretion may be a common occurrence in the evolution of young
pre-main sequence stars and has important implications for our understanding of
star and planet formation. Many fundamental aspects of what drives the
accretion physics, however, are still unknown. The ngVLA will be a key tool in
understanding the nature of these events. The high spatial resolution, broad
spectral coverage, and unprecedented sensitivity will allow for the detailed
analysis of outburst systems. The proposed frequency range of the ngVLA allows
for observations of the gas, dust, and non-thermal emission from the star and
disk.Comment: 8 pages, 1 figure, To be published in the ASP Monograph Series,
"Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco,
CA
U-band study of the accretion properties in the sigma Ori star-forming region
This paper presents the results of an U band survey with FORS1/VLT of a large
area in the sigma Orionis star-forming region. We combine the U-band photometry
with literature data to compute accretion luminosity and mass accretion rates
from the U-band excess emission for all objects (187) detected by Spitzer in
the FORS1 field and classified by Hernandez et al. (2007) as likely members of
the cluster. The sample stars range in mass from ~0.06 to ~1.2 Msun; 72 of them
show evidence of disks and we measure mass accretion rates Macc between
<10^{-11} and few 10^{-9} Msun/yr, using the colors of the diskless stars as
photospheric templates. Our results confirm the dependence of Macc on the mass
of the central object, which is stronger for low-mass stars and flattens out
for masses larger than ~0.3 Msun; the spread of Macc for any value of the
stellar mass is ~2 orders of magnitude. We discuss the implications of these
results in the context of disk evolution models. Finally, we analyze the
relation between Macc and the excess emission in the Spitzer bands, and find
that at Macc ~10^{-10} Msun/yr the inner disks change from optically thin to
optically thick.Comment: 19 pages, 17 figures, accepted by A&
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