24,113 research outputs found
An Improved Variable Structure Adaptive Filter Design and Analysis for Acoustic Echo Cancellation
In this research an advance variable structure adaptive Multiple Sub-Filters (MSF) based algorithm for single channel Acoustic Echo Cancellation (AEC) is proposed and analyzed. This work suggests a new and improved direction to find the optimum tap-length of adaptive filter employed for AEC. The structure adaptation, supported by a tap-length based weight update approach helps the designed echo canceller to maintain a trade-off between the Mean Square Error (MSE) and time taken to attain the steady state MSE. The work done in this paper focuses on replacing the fixed length sub-filters in existing MSF based AEC algorithms which brings refinements in terms of convergence, steady state error and tracking over the single long filter, different error and common error algorithms. A dynamic structure selective coefficient update approach to reduce the structural and computational cost of adaptive design is discussed in context with the proposed algorithm. Simulated results reveal a comparative performance analysis over proposed variable structure multiple sub-filters designs and existing fixed tap-length sub-filters based acoustic echo cancellers
Symmetric space description of carbon nanotubes
Using an innovative technique arising from the theory of symmetric spaces, we
obtain an approximate analytic solution of the Dorokhov-Mello-Pereyra-Kumar
(DMPK) equation in the insulating regime of a metallic carbon nanotube with
symplectic symmetry and an odd number of conducting channels. This symmetry
class is characterized by the presence of a perfectly conducting channel in the
limit of infinite length of the nanotube. The derivation of the DMPK equation
for this system has recently been performed by Takane, who also obtained the
average conductance both analytically and numerically. Using the Jacobian
corresponding to the transformation to radial coordinates and the
parameterization of the transfer matrix given by Takane, we identify the
ensemble of transfer matrices as the symmetric space of negative curvature
SO^*(4m+2)/[SU(2m+1)xU(1)] belonging to the DIII-odd Cartan class. We rederive
the leading-order correction to the conductance of the perfectly conducting
channel and its variance Var(log(delta g)). Our results are in
complete agreement with Takane's. In addition, our approach based on the
mapping to a symmetric space enables us to obtain new universal quantities: a
universal group theoretical expression for the ratio Var(log(delta
g)/ and as a byproduct, a novel expression for the localization
length for the most general case of a symmetric space with BC_m root system, in
which all three types of roots are present.Comment: 23 pages. Text concerning symmetric space description augmented,
table and references added. Version to be published on JSTA
Numerical simulation of the effect of pellet injection on ELMs
We report on numerical simulation studies of the dynamical behavior of edge
localized modes (ELMs) under the influence of repetitive injection of pellets.
In our nonlinear 2-fluid model the ELMs are excited by introducing a particle
source in the confinement region and a particle sink in the edge region. The
injection of pellets is simulated by periodically raising the edge density in a
pulsed manner. We find that when the edge density is raised to twice the normal
edge density with a duty cycle (on time:off time) of 1:2, the ELMs are
generated on an average at a faster rate and with reduced amplitudes. These
changes lead to significant improvements in the plasma beta indicative of an
improvement in the energy confinement due to pellet injection. Concurrently,
the plasma density and temperature profiles also get significantly modified. A
comparative study is made of the nature of ELM dynamics for different
magnitudes of edge density enhancements. We also discuss the relative impact on
ELMs from resonant magnetic perturbations (RMPs) compared to pellet injection
in terms of changes in the plasma temperature, density, location of the ELMs
and the nonlinear spectral transfer of energies
Towards a Classification of Two-Character Rational Conformal Field Theories
We provide a simple and general construction of infinite families of
consistent, modular-covariant pairs of characters satisfying the basic
requirements to describe two-character RCFT. These correspond to solutions of
generic second-order modular linear differential equations. To find these
solutions, we first construct "quasi-characters" from the Kaneko-Zagier
equation and subsequent works by Kaneko and collaborators, together with coset
dual generalisations that we provide in this paper. We relate our construction
to the Hecke images recently discussed by Harvey and Wu.Comment: 64 pages, typos corrected, proofs now provided for two claims that
were previously conjectures, final version to appear in JHE
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