7,914 research outputs found
Nonlinearity Mitigation in WDM Systems: Models, Strategies, and Achievable Rates
After reviewing models and mitigation strategies for interchannel nonlinear
interference (NLI), we focus on the frequency-resolved logarithmic perturbation
model to study the coherence properties of NLI. Based on this study, we devise
an NLI mitigation strategy which exploits the synergic effect of phase and
polarization noise compensation (PPN) and subcarrier multiplexing with
symbol-rate optimization. This synergy persists even for high-order modulation
alphabets and Gaussian symbols. A particle method for the computation of the
resulting achievable information rate and spectral efficiency (SE) is presented
and employed to lower-bound the channel capacity. The dependence of the SE on
the link length, amplifier spacing, and presence or absence of inline
dispersion compensation is studied. Single-polarization and dual-polarization
scenarios with either independent or joint processing of the two polarizations
are considered. Numerical results show that, in links with ideal distributed
amplification, an SE gain of about 1 bit/s/Hz/polarization can be obtained (or,
in alternative, the system reach can be doubled at a given SE) with respect to
single-carrier systems without PPN mitigation. The gain is lower with lumped
amplification, increases with the number of spans, decreases with the span
length, and is further reduced by in-line dispersion compensation. For
instance, considering a dispersion-unmanaged link with lumped amplification and
an amplifier spacing of 60 km, the SE after 80 spans can be be increased from
4.5 to 4.8 bit/s/Hz/polarization, or the reach raised up to 100 spans (+25%)
for a fixed SE.Comment: Submitted to Journal of Lightwave Technolog
A hierarchical model of transcriptional dynamics allows robust estimation of transcription rates in populations of single cells with variable gene copy number
Motivation: cis-regulatory DNA sequence elements, such as enhancers and silencers, function to control the spatial and temporal expression of their target genes. Although the overall levels of gene expression in large cell populations seem to be precisely controlled, transcription of individual genes in single cells is extremely variable in real time. It is, therefore, important to understand how these cis-regulatory elements function to dynamically control transcription at single-cell resolution. Recently, statistical methods have been proposed to back calculate the rates involved in mRNA transcription using parameter estimation of a mathematical model of transcription and translation. However, a major complication in these approaches is that some of the parameters, particularly those corresponding to the gene copy number and transcription rate, cannot be distinguished; therefore, these methods cannot be used when the copy number is unknown.
Results: Here, we develop a hierarchical Bayesian model to estimate biokinetic parameters from live cell enhancerâpromoter reporter measurements performed on a population of single cells. This allows us to investigate transcriptional dynamics when the copy number is variable across the population. We validate our method using synthetic data and then apply it to quantify the function of two known developmental enhancers in real time and in single cells
Bayesian Nonparametric Unmixing of Hyperspectral Images
Hyperspectral imaging is an important tool in remote sensing, allowing for
accurate analysis of vast areas. Due to a low spatial resolution, a pixel of a
hyperspectral image rarely represents a single material, but rather a mixture
of different spectra. HSU aims at estimating the pure spectra present in the
scene of interest, referred to as endmembers, and their fractions in each
pixel, referred to as abundances. Today, many HSU algorithms have been
proposed, based either on a geometrical or statistical model. While most
methods assume that the number of endmembers present in the scene is known,
there is only little work about estimating this number from the observed data.
In this work, we propose a Bayesian nonparametric framework that jointly
estimates the number of endmembers, the endmembers itself, and their
abundances, by making use of the Indian Buffet Process as a prior for the
endmembers. Simulation results and experiments on real data demonstrate the
effectiveness of the proposed algorithm, yielding results comparable with
state-of-the-art methods while being able to reliably infer the number of
endmembers. In scenarios with strong noise, where other algorithms provide only
poor results, the proposed approach tends to overestimate the number of
endmembers slightly. The additional endmembers, however, often simply represent
noisy replicas of present endmembers and could easily be merged in a
post-processing step
Theta dependence of SU(N) gauge theories in the presence of a topological term
We review results concerning the theta dependence of 4D SU(N) gauge theories
and QCD, where theta is the coefficient of the CP-violating topological term in
the Lagrangian. In particular, we discuss theta dependence in the large-N
limit.
Most results have been obtained within the lattice formulation of the theory
via numerical simulations, which allow to investigate the theta dependence of
the ground-state energy and the spectrum around theta=0 by determining the
moments of the topological charge distribution, and their correlations with
other observables. We discuss the various methods which have been employed to
determine the topological susceptibility, and higher-order terms of the theta
expansion. We review results at zero and finite temperature. We show that the
results support the scenario obtained by general large-N scaling arguments, and
in particular the Witten-Veneziano mechanism to explain the U(1)_A problem. We
also compare with results obtained by other approaches, especially in the
large-N limit, where the issue has been also addressed using, for example, the
AdS/CFT correspondence.
We discuss issues related to theta dependence in full QCD: the neutron
electric dipole moment, the dependence of the topological susceptibility on the
quark masses, the U(1)_A symmetry breaking at finite temperature.
We also consider the 2D CP(N) model, which is an interesting theoretical
laboratory to study issues related to topology. We review analytical results in
the large-N limit, and numerical results within its lattice formulation.
Finally, we discuss the main features of the two-point correlation function
of the topological charge density.Comment: A typo in Eq. (3.9) has been corrected. An additional subsection
(5.2) has been inserted to demonstrate the nonrenormalizability of the
relevant theta parameter in the presence of massive fermions, which implies
that the continuum (a -> 0) limit must be taken keeping theta fixe
Quantitative assessment of intrinsic noise for visually guided behaviour in zebrafish
Supported by Royal Society of London (University Research Fellowship), Medical Research Council (New Investigator Research Grant) and CNRS.Peer reviewedPostprin
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