128,579 research outputs found
Localization phenomena in models of ion-conducting glass formers
The mass transport in soft-sphere mixtures of small and big particles as well
as in the disordered Lorentz gas (LG) model is studied using molecular dynamics
(MD) computer simulations. The soft-sphere mixture shows anomalous
small-particle diffusion signifying a localization transition separate from the
big-particle glass transition. Switching off small-particle excluded volume
constraints slows down the small-particle dynamics, as indicated by incoherent
intermediate scattering functions. A comparison of logarithmic time derivatives
of the mean-squared displacements reveals qualitative similarities between the
localization transition in the soft-sphere mixture and its counterpart in the
LG. Nevertheless, qualitative differences emphasize the need for further
research elucidating the connection between both models.Comment: to appear in Eur. Phys. J. Special Topic
Sparse matrix-vector multiplication on GPGPU clusters: A new storage format and a scalable implementation
Sparse matrix-vector multiplication (spMVM) is the dominant operation in many
sparse solvers. We investigate performance properties of spMVM with matrices of
various sparsity patterns on the nVidia "Fermi" class of GPGPUs. A new "padded
jagged diagonals storage" (pJDS) format is proposed which may substantially
reduce the memory overhead intrinsic to the widespread ELLPACK-R scheme. In our
test scenarios the pJDS format cuts the overall spMVM memory footprint on the
GPGPU by up to 70%, and achieves 95% to 130% of the ELLPACK-R performance.
Using a suitable performance model we identify performance bottlenecks on the
node level that invalidate some types of matrix structures for efficient
multi-GPGPU parallelization. For appropriate sparsity patterns we extend
previous work on distributed-memory parallel spMVM to demonstrate a scalable
hybrid MPI-GPGPU code, achieving efficient overlap of communication and
computation.Comment: 10 pages, 5 figures. Added reference to other recent sparse matrix
format
On the Derivation of Optimal Partial Successive Interference Cancellation
The necessity of accurate channel estimation for Successive and Parallel
Interference Cancellation is well known. Iterative channel estimation and
channel decoding (for instance by means of the Expectation-Maximization
algorithm) is particularly important for these multiuser detection schemes in
the presence of time varying channels, where a high density of pilots is
necessary to track the channel. This paper designs a method to analytically
derive a weighting factor , necessary to improve the efficiency of
interference cancellation in the presence of poor channel estimates. Moreover,
this weighting factor effectively mitigates the presence of incorrect decisions
at the output of the channel decoder. The analysis provides insight into the
properties of such interference cancellation scheme and the proposed approach
significantly increases the effectiveness of Successive Interference
Cancellation under the presence of channel estimation errors, which leads to
gains of up to 3 dB.Comment: IEEE GLOBECOM 201
Small crater populations on Vesta
The NASA Dawn mission has extensively examined the surface of asteroid Vesta,
the second most massive body in the main belt. The high quality of the gathered
data provides us with an unique opportunity to determine the surface and
internal properties of one of the most important and intriguing main belt
asteroids (MBAs). In this paper, we focus on the size frequency distributions
(SFDs) of sub-kilometer impact craters observed at high spatial resolution on
several selected young terrains on Vesta. These small crater populations offer
an excellent opportunity to determine the nature of their asteroidal precursors
(namely MBAs) at sizes that are not directly observable from ground-based
telescopes (i.e., below ~100 m diameter). Moreover, unlike many other MBA
surfaces observed by spacecraft thus far, the young terrains examined had
crater spatial densities that were far from empirical saturation. Overall, we
find that the cumulative power-law index (slope) of small crater SFDs on Vesta
is fairly consistent with predictions derived from current collisional and
dynamical models down to a projectile size of ~10 m diameter (Bottke et al.,
2005a,b). The shape of the impactor SFD for small projectile sizes does not
appear to have changed over the last several billions of years, and an argument
can be made that the absolute number of small MBAs has remained roughly
constant (within a factor of 2) over the same time period. The apparent steady
state nature of the main belt population potentially provides us with a set of
intriguing constraints that can be used to glean insights into the physical
evolution of individual MBAs as well as the main belt as an ensemble.Comment: Accepted by PSS, to appear on Vesta cratering special issu
LT Code Design for Inactivation Decoding
We present a simple model of inactivation decoding for LT codes which can be
used to estimate the decoding complexity as a function of the LT code degree
distribution. The model is shown to be accurate in variety of settings of
practical importance. The proposed method allows to perform a numerical
optimization on the degree distribution of a LT code aiming at minimizing the
number of inactivations required for decoding.Comment: 6 pages, 7 figure
GHOST: Building blocks for high performance sparse linear algebra on heterogeneous systems
While many of the architectural details of future exascale-class high
performance computer systems are still a matter of intense research, there
appears to be a general consensus that they will be strongly heterogeneous,
featuring "standard" as well as "accelerated" resources. Today, such resources
are available as multicore processors, graphics processing units (GPUs), and
other accelerators such as the Intel Xeon Phi. Any software infrastructure that
claims usefulness for such environments must be able to meet their inherent
challenges: massive multi-level parallelism, topology, asynchronicity, and
abstraction. The "General, Hybrid, and Optimized Sparse Toolkit" (GHOST) is a
collection of building blocks that targets algorithms dealing with sparse
matrix representations on current and future large-scale systems. It implements
the "MPI+X" paradigm, has a pure C interface, and provides hybrid-parallel
numerical kernels, intelligent resource management, and truly heterogeneous
parallelism for multicore CPUs, Nvidia GPUs, and the Intel Xeon Phi. We
describe the details of its design with respect to the challenges posed by
modern heterogeneous supercomputers and recent algorithmic developments.
Implementation details which are indispensable for achieving high efficiency
are pointed out and their necessity is justified by performance measurements or
predictions based on performance models. The library code and several
applications are available as open source. We also provide instructions on how
to make use of GHOST in existing software packages, together with a case study
which demonstrates the applicability and performance of GHOST as a component
within a larger software stack.Comment: 32 pages, 11 figure
Glass transition of charged particles in two-dimensional confinement
The glass transition of mesoscopic charged particles in two-dimensional
confinement is studied by mode-coupling theory. We consider two types of
effective interactions between the particles, corresponding to two different
models for the distribution of surrounding ions that are integrated out in
coarse-grained descriptions. In the first model, a planar monolayer of charged
particles is immersed in an unbounded isotropic bath of ions, giving rise to an
isotropically screened Debye-H\"uckel- (Yukawa-) type effective interaction.
The second, experimentally more relevant system is a monolayer of negatively
charged particles that levitate atop a flat horizontal electrode, as frequently
encountered in laboratory experiments with complex (dusty) plasmas. A steady
plasma current towards the electrode gives rise to an anisotropic effective
interaction potential between the particles, with an algebraically long-ranged
in-plane decay. In a comprehensive parameter scan that covers the typical range
of experimentally accessible plasma conditions, we calculate and compare the
mode-coupling predictions for the glass transition in both kinds of systems.Comment: 10 pages, 8 figure
Performance Evaluation of Vision-Based Algorithms for MAVs
An important focus of current research in the field of Micro Aerial Vehicles
(MAVs) is to increase the safety of their operation in general unstructured
environments. Especially indoors, where GPS cannot be used for localization,
reliable algorithms for localization and mapping of the environment are
necessary in order to keep an MAV airborne safely. In this paper, we compare
vision-based real-time capable methods for localization and mapping and point
out their strengths and weaknesses. Additionally, we describe algorithms for
state estimation, control and navigation, which use the localization and
mapping results of our vision-based algorithms as input.Comment: Presented at OAGM Workshop, 2015 (arXiv:1505.01065
The Voigt and complex error function: Huml\'i\v{c}ek's rational approximation generalized
Accurate yet efficient computation of the Voigt and complex error function is
a challenge since decades in astrophysics and other areas of physics. Rational
approximations have attracted considerable attention and are used in many
codes, often in combination with other techniques. The 12-term code "cpf12" of
Huml\'i\v{c}ek (1979) achieves an accuracy of five to six significant digits
throughout the entire complex plane. Here we generalize this algorithm to a
larger (even) number of terms. The approximation has a relative accuracy
better than for almost the entire complex plane except for very small
imaginary values of the argument even without the correction term required for
the cpf12 algorithm. With 20 terms the accuracy is better than . In
addition to the accuracy assessment we discuss methods for optimization and
propose a combination of the 16-term approximation with the asymptotic
approximation of Huml\'i\v{c}ek (1982) for high efficiency.Comment: 9 pages, 5 figure
Coupling of non-crossing wave modes in a two-dimensional plasma crystal
We report an experimental observation of coupling of the transverse vertical
and longitudinal in-plane dust-lattice wave modes in a two-dimensional complex
plasma crystal in the absence of mode crossing. A new large diameter rf plasma
chamber was used to suspend the plasma crystal. The observations are confirmed
with molecular-dynamics simulations. The coupling manifests itself in traces of
the transverse vertical mode appearing in the measured longitudinal spectra and
vice versa. We calculate the expected ratio of the trace to the principal mode
with a theoretical analysis of the modes in a crystal with finite temperature
and find good agreement with the experiment and simulations.Comment: 4 figures, 5 pages, accepted for publication in PRL Nov 201
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