33,019 research outputs found
Lossy Source Coding via Spatially Coupled LDGM Ensembles
We study a new encoding scheme for lossy source compression based on
spatially coupled low-density generator-matrix codes. We develop a
belief-propagation guided-decimation algorithm, and show that this algorithm
allows to approach the optimal distortion of spatially coupled ensembles.
Moreover, using the survey propagation formalism, we also observe that the
optimal distortions of the spatially coupled and individual code ensembles are
the same. Since regular low-density generator-matrix codes are known to achieve
the Shannon rate-distortion bound under optimal encoding as the degrees grow,
our results suggest that spatial coupling can be used to reach the
rate-distortion bound, under a {\it low complexity} belief-propagation
guided-decimation algorithm.
This problem is analogous to the MAX-XORSAT problem in computer science.Comment: Submitted to ISIT 201
Performance Analysis and Optimization of Sparse Matrix-Vector Multiplication on Modern Multi- and Many-Core Processors
This paper presents a low-overhead optimizer for the ubiquitous sparse
matrix-vector multiplication (SpMV) kernel. Architectural diversity among
different processors together with structural diversity among different sparse
matrices lead to bottleneck diversity. This justifies an SpMV optimizer that is
both matrix- and architecture-adaptive through runtime specialization. To this
direction, we present an approach that first identifies the performance
bottlenecks of SpMV for a given sparse matrix on the target platform either
through profiling or by matrix property inspection, and then selects suitable
optimizations to tackle those bottlenecks. Our optimization pool is based on
the widely used Compressed Sparse Row (CSR) sparse matrix storage format and
has low preprocessing overheads, making our overall approach practical even in
cases where fast decision making and optimization setup is required. We
evaluate our optimizer on three x86-based computing platforms and demonstrate
that it is able to distinguish and appropriately optimize SpMV for the majority
of matrices in a representative test suite, leading to significant speedups
over the CSR and Inspector-Executor CSR SpMV kernels available in the latest
release of the Intel MKL library.Comment: 10 pages, 7 figures, ICPP 201
Self-guided wakefield experiments driven by petawatt class ultra-short laser pulses
We investigate the extension of self-injecting laser wakefield experiments to
the regime that will be accessible with the next generation of petawatt class
ultra-short pulse laser systems. Using linear scalings, current experimental
trends and numerical simulations we determine the optimal laser and target
parameters, i.e. focusing geometry, plasma density and target length, that are
required to increase the electron beam energy (to > 1 GeV) without the use of
external guiding structures.Comment: 15 pages, 8 figure
Information preserved guided scan pixel difference coding for medical images
This paper analyzes the information content of medical images, with 3-D MRI
images as an example, in terms of information entropy. The results of the
analysis justify the use of Pixel Difference Coding for preserving all
information contained in the original pictures, lossless coding in other words.
The experimental results also indicate that the compression ratio CR=2:1 can be
achieved under the lossless constraints. A pratical implementation of Pixel
Difference Coding which allows interactive retrieval of local ROI (Region of
Interest), while maintaining the near low bound information entropy, is
discussed.Comment: 5 pages and 5 figures. Published in IEEE Wescanex proceeding
EuPRAXIA@SPARC_LAB: the high-brightness RF photo-injector layout proposal
At EuPRAXIA@SPARC_LAB, the unique combination of an advanced high-brightness
RF injector and a plasma-based accelerator will drive a new multi-disciplinary
user-facility. The facility, that is currently under study at INFN-LNF
Laboratories (Frascati, Italy) in synergy with the EuPRAXIA collaboration, will
operate the plasma-based accelerator in the external injection configuration.
Since in this configuration the stability and reproducibility of the
acceleration process in the plasma stage is strongly influenced by the
RF-generated electron beam, the main challenge for the RF injector design is
related to generating and handling high quality electron beams. In the last
decades of R&D activity, the crucial role of high-brightness RF photo-injectors
in the fields of radiation generation and advanced acceleration schemes has
been largely established, making them effective candidates to drive
plasma-based accelerators as pilots for user facilities. An RF injector
consisting in a high-brightness S-band photo-injector followed by an advanced
X-band linac has been proposed for the EuPRAXIA@SPARC_LAB project. The electron
beam dynamics in the photo-injector has been explored by means of simulations,
resulting in high-brightness, ultra-short bunches with up to 3 kA peak current
at the entrance of the advanced X-band linac booster. The EuPRAXIA@SPARC_LAB
high-brightness photo-injector is described here together with performance
optimisation and sensitivity studies aiming to actual check the robustness and
reliability of the desired working point.Comment: 5 pages,5 figures, EAAC201
One-to-one full scale simulations of laser wakefield acceleration using QuickPIC
We use the quasi-static particle-in-cell code QuickPIC to perform full-scale,
one-to-one LWFA numerical experiments, with parameters that closely follow
current experimental conditions. The propagation of state-of-the-art laser
pulses in both preformed and uniform plasma channels is examined. We show that
the presence of the channel is important whenever the laser self-modulations do
not dominate the propagation. We examine the acceleration of an externally
injected electron beam in the wake generated by 10 J laser pulses, showing that
by using ten-centimeter-scale plasma channels it is possible to accelerate
electrons to more than 4 GeV. A comparison between QuickPIC and 2D OSIRIS is
provided. Good qualitative agreement between the two codes is found, but the 2D
full PIC simulations fail to predict the correct laser and wakefield
amplitudes.Comment: 5 pages, 5 figures, accepted for publication IEEE TPS, Special Issue
- Laser & Plasma Accelerators - 8/200
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