24,671 research outputs found
Linear-Codes-Based Lossless Joint Source-Channel Coding for Multiple-Access Channels
A general lossless joint source-channel coding (JSCC) scheme based on linear
codes and random interleavers for multiple-access channels (MACs) is presented
and then analyzed in this paper. By the information-spectrum approach and the
code-spectrum approach, it is shown that a linear code with a good joint
spectrum can be used to establish limit-approaching lossless JSCC schemes for
correlated general sources and general MACs, where the joint spectrum is a
generalization of the input-output weight distribution. Some properties of
linear codes with good joint spectra are investigated. A formula on the
"distance" property of linear codes with good joint spectra is derived, based
on which, it is further proved that, the rate of any systematic codes with good
joint spectra cannot be larger than the reciprocal of the corresponding
alphabet cardinality, and any sparse generator matrices cannot yield linear
codes with good joint spectra. The problem of designing arbitrary rate coding
schemes is also discussed. A novel idea called "generalized puncturing" is
proposed, which makes it possible that one good low-rate linear code is enough
for the design of coding schemes with multiple rates. Finally, various coding
problems of MACs are reviewed in a unified framework established by the
code-spectrum approach, under which, criteria and candidates of good linear
codes in terms of spectrum requirements for such problems are clearly
presented.Comment: 18 pages, 3 figure
The effects of surface modifications of multiwalled carbon nanotubes on their dispersibility in different solvents and poly(ether ether ketone)
The effects of surface modifications of multi-walled carbon nanotubes (MWCNTs) on their dispersibility in different solvents and poly (ether ether ketone) (PEEK) have been studied. MWCNTs were treated by mixed acids to obtain acid-functionalized MWCNTs. The acid-functionalized MWCNTs were modified with different chemical agents separately, such as 1,6-diaminohexane, hexadecyl trimethyl ammonium bromide, silane coupling agent 3-aminopropyltriethoxysilane, anhydrous sulfanilic acid and ethanolamine. MWCNT/PEEK composite films were fabricated in order to explore systematically the dispersibility of differently modified MWCNTs PEEK as well as in different solvents. The morphology and structures of MWCNTs and the compatibility between MWCNTs and PEEK have been investigated. It was observed that the MWCNTs modified with anhydrous sulfanilic acid have an excellent dispersion in the PEEK grafted by sulfonic acid groups and that the MWCNTs modified with ethanolamine are also dispersed well in pure PEEK. The results herein provide useful insights into the development of MWCNT/PEEK composites for a wide variety of applications.Peer reviewe
Weight Distributions of Regular Low-Density Parity-Check Codes over Finite Fields
The average weight distribution of a regular low-density parity-check (LDPC)
code ensemble over a finite field is thoroughly analyzed. In particular, a
precise asymptotic approximation of the average weight distribution is derived
for the small-weight case, and a series of fundamental qualitative properties
of the asymptotic growth rate of the average weight distribution are proved.
Based on this analysis, a general result, including all previous results as
special cases, is established for the minimum distance of individual codes in a
regular LDPC code ensemble.Comment: 15 pages, 5 figures, accepted for publication in IEEE Transactions on
Information Theory, July 201
Stability analysis of the Eulerian-Lagrangian finite volume methods for nonlinear hyperbolic equations in one space dimension
In this paper, we construct a novel Eulerian-Lagrangian finite volume (ELFV)
method for nonlinear scalar hyperbolic equations in one space dimension. It is
well known that the exact solutions to such problems may contain shocks though
the initial conditions are smooth, and direct numerical methods may suffer from
restricted time step sizes. To relieve the restriction, we propose an ELFV
method, where the space-time domain was separated by the partition lines
originated from the cell interfaces whose slopes are obtained following the
Rakine-Hugoniot junmp condition. Unfortunately, to avoid the intersection of
the partition lines, the time step sizes are still limited. To fix this gap, we
detect effective troubled cells (ETCs) and carefully design the influence
region of each ETC, within which the partitioned space-time regions are merged
together to form a new one. Then with the new partition of the space-time
domain, we theoretically prove that the proposed first-order scheme with Euler
forward time discretization is total-variation-diminishing and
maximum-principle-preserving with {at least twice} larger time step constraints
than the classical first order Eulerian method for Burgers' equation. Numerical
experiments verify the optimality of the designed time step sizes.Comment: Submitted to Mathematics of Computatio
BERT with History Answer Embedding for Conversational Question Answering
Conversational search is an emerging topic in the information retrieval
community. One of the major challenges to multi-turn conversational search is
to model the conversation history to answer the current question. Existing
methods either prepend history turns to the current question or use complicated
attention mechanisms to model the history. We propose a conceptually simple yet
highly effective approach referred to as history answer embedding. It enables
seamless integration of conversation history into a conversational question
answering (ConvQA) model built on BERT (Bidirectional Encoder Representations
from Transformers). We first explain our view that ConvQA is a simplified but
concrete setting of conversational search, and then we provide a general
framework to solve ConvQA. We further demonstrate the effectiveness of our
approach under this framework. Finally, we analyze the impact of different
numbers of history turns under different settings to provide new insights into
conversation history modeling in ConvQA.Comment: Accepted to SIGIR 2019 as a short pape
The Reliability Evaluation of Antiskid Valve of CRH2 EMUs Based on Bayes Method
This paper takes the antiskid valve of braking system as the object of the research. Two parameter Weibull distribution is established through analyzing the operational principle of antiskid valve. According to the Bayesian Reliability, the cumulative failure probability of antiskid valve is calculated. Meanwhile, the posterior distribution is obtained through Markov Chain Monte Carlo. By contrast with these two methods that the reliability of antiskid valve is obtained. According to the least square method, the undetermined parameters of the two parameter Weibull distribution were solved. Then, through the sensitivity analysis, the mathematical model is proved more robust. This paper provides a certain theoretical basis for the assessment of antiskid valve’s reliability, and it provides the reference to the other valve in braking system of rail transit vehicle. At the end of the paper, the reliability of the whole antiskid system is calculated through GO method on the basis of the parts' reliability is obtained by Bayes method
Coupled effects of local movement and global interaction on contagion
By incorporating segregated spatial domain and individual-based linkage into
the SIS (susceptible-infected-susceptible) model, we investigate the coupled
effects of random walk and intragroup interaction on contagion. Compared with
the situation where only local movement or individual-based linkage exists, the
coexistence of them leads to a wider spread of infectious disease. The roles of
narrowing segregated spatial domain and reducing mobility in epidemic control
are checked, these two measures are found to be conducive to curbing the spread
of infectious disease. Considering heterogeneous time scales between local
movement and global interaction, a log-log relation between the change in the
number of infected individuals and the timescale is found. A theoretical
analysis indicates that the evolutionary dynamics in the present model is
related to the encounter probability and the encounter time. A functional
relation between the epidemic threshold and the ratio of shortcuts, and a
functional relation between the encounter time and the timescale are
found
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