4,609 research outputs found
Low-Complexity Iterative Detection for Orthogonal Time Frequency Space Modulation
We elaborate on the recently proposed orthogonal time frequency space (OTFS)
modulation technique, which provides significant advantages over orthogonal
frequency division multiplexing (OFDM) in Doppler channels. We first derive the
input--output relation describing OTFS modulation and demodulation (mod/demod)
for delay--Doppler channels with arbitrary number of paths, with given delay
and Doppler values. We then propose a low-complexity message passing (MP)
detection algorithm, which is suitable for large-scale OTFS taking advantage of
the inherent channel sparsity. Since the fractional Doppler paths (i.e., not
exactly aligned with the Doppler taps) produce the inter Doppler interference
(IDI), we adapt the MP detection algorithm to compensate for the effect of IDI
in order to further improve performance. Simulations results illustrate the
superior performance gains of OTFS over OFDM under various channel conditions.Comment: 6 pages, 7 figure
UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors
We present ultraviolet (UV) integrated colors of 44 Galactic globular
clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV
and NUV bands. This data-base is the largest homogeneous catalog of UV colors
ever published for stellar systems in our Galaxy. The proximity of GGCs makes
it possible to resolve many individual stars even with the somewhat low spatial
resolution of GALEX. This allows us to determine how the integrated UV colors
are driven by hot stellar populations, primarily horizontal branch stars and
their progeny. The UV colors are found to be correlated with various parameters
commonly used to define the horizontal branch morphology. We also investigate
how the UV colors vary with parameters like metallicity, age, helium abundance
and concentration. We find for the first time that GCs associated with the
Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs
with the same metallicity. Finally, we speculate about the presence of an
interesting trend, suggesting that the UV color of GCs may be correlated with
the mass of the host galaxy, in the sense that more massive galaxies possess
bluer clusters.Comment: Accepted for publication by The Astronomical Journal. 36 pages, 9
figures, 1 tabl
Mixing in Non-Quasirandom Groups
We initiate a systematic study of mixing in non-quasirandom groups. Let A and B be two independent, high-entropy distributions over a group G. We show that the product distribution AB is statistically close to the distribution F(AB) for several choices of G and F, including:
1) G is the affine group of 2x2 matrices, and F sets the top-right matrix entry to a uniform value,
2) G is the lamplighter group, that is the wreath product of ?? and ?_{n}, and F is multiplication by a certain subgroup,
3) G is H? where H is non-abelian, and F selects a uniform coordinate and takes a uniform conjugate of it.
The obtained bounds for (1) and (2) are tight.
This work is motivated by and applied to problems in communication complexity. We consider the 3-party communication problem of deciding if the product of three group elements multiplies to the identity. We prove lower bounds for the groups above, which are tight for the affine and the lamplighter groups
Indium segregation during III-V quantum wire and quantum dot formation on patterned substrates
We report a model for metalorganic vapor-phase epitaxy on non-planar
substrates, specifically V-grooves and pyramidal recesses, which we apply to
the growth of InGaAs nanostructures. This model, based on a set of coupled
reaction-diffusion equations, one for each facet in the system, accounts for
the facet-dependence of all kinetic processes (e.g., precursor decomposition,
adatom diffusion, and adatom lifetimes) and has been previously applied to
account for the temperature, concentration, and temporal-dependence of AlGaAs
nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses.
In the present study, the growth of InGaAs quantum wires at
the bottom of V-grooves is used to determine a set of optimized kinetic
parameters. Based on these parameters, we have modeled the growth of
InGaAs nanostructures formed in pyramidal site-controlled
quantum-dot systems, successfully producing a qualitative explanation for the
temperature-dependence of their optical properties, which have been reported in
previous studies. Finally, we present scanning electron and cross-sectional
atomic force microscopy images which show previously unreported facetting at
the bottom of the pyramidal recesses that allow quantum dot formation.Comment: 9 pages, 8 figure
Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state
We present an analysis of the 2013 Suzaku campaign on the nearby luminous
quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455
ks. During these observations, the X-ray flux was suppressed by a factor of >10
in the soft X-ray band when compared to other epochs. We investigated the
broadband continuum by constructing a spectral energy distribution, making use
of the optical/UV photometry and hard X-ray spectra from the later
XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state
cannot be accounted for by self-consistent accretion disc and corona models
without attenuation by absorbing gas, which partially covers a substantial
fraction of the line of sight towards the X-ray source. Two absorption layers
are required, of column density and ,
with average covering factors of ~80% (with typical 5% variations) and 60%
(10-15%), respectively. In these observations PDS 456 displays significant
short term X-ray spectral variability, on timescales of ~100 ks, which can be
accounted for by variable covering of the absorbing gas. The partial covering
absorber prefers an outflow velocity of at
the >99.9% confidence level over the case where . This is
consistent with the velocity of the highly ionised outflow responsible for the
blueshifted iron K absorption profile. We therefore suggest that the partial
covering clouds could be the denser, or clumpy part of an inhomogeneous
accretion disc wind. Finally we estimate the size-scale of the X-ray source
from its variability. The radial extent of the X-ray emitter is found to be of
the order ~15-20 , although the hard X-ray (>2 keV) emission may
originate from a more compact or patchy corona of hot electrons, which is ~6-8
in size.Comment: 38 pages, 13 figures, accepted for publication in MNRA
Informative and misinformative interactions in a school of fish
It is generally accepted that, when moving in groups, animals process
information to coordinate their motion. Recent studies have begun to apply
rigorous methods based on Information Theory to quantify such distributed
computation. Following this perspective, we use transfer entropy to quantify
dynamic information flows locally in space and time across a school of fish
during directional changes around a circular tank, i.e. U-turns. This analysis
reveals peaks in information flows during collective U-turns and identifies two
different flows: an informative flow (positive transfer entropy) based on fish
that have already turned about fish that are turning, and a misinformative flow
(negative transfer entropy) based on fish that have not turned yet about fish
that are turning. We also reveal that the information flows are related to
relative position and alignment between fish, and identify spatial patterns of
information and misinformation cascades. This study offers several
methodological contributions and we expect further application of these
methodologies to reveal intricacies of self-organisation in other animal groups
and active matter in general
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