6,609 research outputs found
Non Binary Low Density Parity Check Codes Decoding Over Galois Field
Conventional LDPC codes have a low decoding complexity but may have high encoding complexity. The encoding complexity is typically of the order O(n2)[5]. Also high storage space may be required to explicitly store the generator matrix. For long blocknbsp lengths the storage space required would be huge. The above factors make the implementation of the Conventional LDPC codes less attractive.
These codes are usually decoded using the sum-product algorithm, which is anbsp message passing algorithm working on the Tanner graph of the code[5]. The sparseness of the parity check matrix is essential for attaining good performance with sum-product decoding. The time complexity of the sum- product algorithm is linear in code length. This property makes it possible to implement a practical decoder for long lengths.nbs
Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective
Two-dimensional (2D) materials have shown extraordinary performances as photocatalysts compared to their bulk counterparts. Simulations have made a great contribution to the deep understanding and design of novel 2D photocatalysts. Ab initio simulations based on density functional theory (DFT) not only show efficiency and reliability in new structure searching, but also can provide a reliable, efficient, and economic way for screening the photocatalytic property space. In this review, we summarize the recent developments in the field of water splitting using 2D materials from a theoretical perspective. We address that DFT-based simulations can fast screen the potential spaces of photocatalytic properties with the accuracy comparable to experiments, by investigating the effects of various physical/chemical perturbations. This, at last, will lead to the enhanced photocatalytic activities of 2D materials, and promote the development of photocatalysis
Asymptotically Optimal Approximation Algorithms for Coflow Scheduling
Many modern datacenter applications involve large-scale computations composed
of multiple data flows that need to be completed over a shared set of
distributed resources. Such a computation completes when all of its flows
complete. A useful abstraction for modeling such scenarios is a {\em coflow},
which is a collection of flows (e.g., tasks, packets, data transmissions) that
all share the same performance goal.
In this paper, we present the first approximation algorithms for scheduling
coflows over general network topologies with the objective of minimizing total
weighted completion time. We consider two different models for coflows based on
the nature of individual flows: circuits, and packets. We design
constant-factor polynomial-time approximation algorithms for scheduling
packet-based coflows with or without given flow paths, and circuit-based
coflows with given flow paths. Furthermore, we give an -approximation polynomial time algorithm for scheduling circuit-based
coflows where flow paths are not given (here is the number of network
edges).
We obtain our results by developing a general framework for coflow schedules,
based on interval-indexed linear programs, which may extend to other coflow
models and objective functions and may also yield improved approximation bounds
for specific network scenarios. We also present an experimental evaluation of
our approach for circuit-based coflows that show a performance improvement of
at least 22% on average over competing heuristics.Comment: Fixed minor typo
Generalized Multi-Camera Scene Reconstruction Using Graph Cuts
Reconstructing a 3-D scene from more than one camera is a classical problem in computer vision. One of the major sources of difficulty is the fact that not all scene elements are visible from all cameras. In the last few years, two promising approaches have been developed [. . .] that formulate the scene reconstruction problem in terms of energy minimization, and minimize the energy using graph cuts. These energy minimization approaches treat the input images symmetrically, handle visibility constraints correctly, and allow spatial smoothness to be enforced. However, these algorithm propose different problem formulations, and handle a limited class of smoothness terms. One algorithm [. . .] uses a problem formulation that is restricted to two-camera stereo, and imposes smoothness between a pair of cameras. The other algorithm [. . .] can handle an arbitrary number of cameras, but imposes smoothness only with respect to a single camera. In this paper we give a more general energy minimization formulation for the problem, which allows a larger class of spatial smoothness constraints. We show that our formulation includes both of the previous approaches as special cases, as well as permitting new energy functions. Experimental results on real data with ground truth are also included.Engineering and Applied Science
Dielectric functions and collective excitations in MgB_2
The frequency- and momentum-dependent dielectric function as well as the energy loss function Im[-\protect{]} are calculated for intermetallic superconductor
by using two {\it ab initio} methods: the plane-wave pseudopotential method and
the tight-binding version of the LMTO method. We find two plasmon modes
dispersing at energies -8 eV and -22 eV. The high energy
plasmon results from a free electron like plasmon mode while the low energy
collective excitation has its origin in a peculiar character of the band
structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both
the peak position and the peak width. In particular, the low energy collective
excitation has practically zero width in the direction perpendicular to boron
layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200
Thermal Stabilization of the HCP Phase in Titanium
We have used a tight-binding model that is fit to first-principles
electronic-structure calculations for titanium to calculate quasi-harmonic
phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega
crystal structures. We show that the true zero-temperature ground-state is the
omega structure, although this has never been observed experimentally at normal
pressure, and that it is the entropy from the thermal population of phonon
states which stabilizes the hcp structure at room temperature. We present the
first completely theoretical prediction of the temperature- and
pressure-dependence of the hcp-omega phase transformation and show that it is
in good agreement with experiment. The quasi-harmonic approximation fails to
adequately treat the bcc phase because the zero-temperature phonons of this
structure are not all stable
Universal phase transitions of B1 structured stoichiometric transition-metal carbides
The high-pressure phase transitions of B1-structured stoichiometric
transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were
systematically investigated using ab initio calculations. These carbides
underwent universal phase transitions along two novel phase-transition routes,
namely, B1\rightarrowdistorted TlI (TlI')\rightarrowTlI and/or
B1\rightarrowdistorted TiB (TiB')\rightarrowTiB, when subjected to pressures.
The two routes can coexist possibly because of the tiny enthalpy differences
between the new phases under corresponding pressures. Four new phases result
from atomic slips of the B1-structured parent phases under pressure. After
completely releasing the pressure, taking TiC as a representative of TMCs, only
its new TlI'-type phase is mechanically and dynamically stable, and may be
recovered.Comment: [email protected]
Development of Photonic Crystal Fiber Based Gas/ Chemical Sensors
The development of highly-sensitive and miniaturized sensors that capable of
real-time analytes detection is highly desirable. Nowadays, toxic or colorless
gas detection, air pollution monitoring, harmful chemical, pressure, strain,
humidity, and temperature sensors based on photonic crystal fiber (PCF) are
increasing rapidly due to its compact structure, fast response and efficient
light controlling capabilities. The propagating light through the PCF can be
controlled by varying the structural parameters and core-cladding materials, as
a result, evanescent field can be enhanced significantly which is the main
component of the PCF based gas/chemical sensors. The aim of this chapter is to
(1) describe the principle operation of PCF based gas/ chemical sensors, (2)
discuss the important PCF properties for optical sensors, (3) extensively
discuss the different types of microstructured optical fiber based gas/
chemical sensors, (4) study the effects of different core-cladding shapes, and
fiber background materials on sensing performance, and (5) highlight the main
challenges of PCF based gas/ chemical sensors and possible solutions
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