3,141 research outputs found
A Language and Toolset for the Synthesis and Efficient Simulation of Clock-Cycle-True Signal-Processing Algorithms
Optimal simulation speed and synthesizability are contradictory requirements for a hardware description language. This paper presents a language and toolset that enables both synthesis and fast simulation of fixed-point signal processing algorithms at the register-transfer level using a single system description. This is achieved by separate code generators for different purposes. Code-generators have been developed for fast simulation (using ANSI-C) and for synthesis (using VHDL). The simulation performance of the proposed approach has been compared with other known methods and turns out to be comparable in speed to the fastest among them
Avoiding dark states in open quantum systems by tailored initial correlations
We study the transport of excitations on a V-shaped network of three coupled
two-level systems that are subjected to an environment that induces incoherent
hopping between the nodes. Two of the nodes are coupled to a source while the
third node is coupled to a drain. A common feature of these networks is the
existence of a dark-state that blocks the transport to the drain. Here we
propose a means to avoid this state by a suitable choice of initial
correlations, induced by a source that is common to both coupled nodes.Comment: 5 pages, 3 figure
Generalized Fokker-Planck equation, Brownian motion, and ergodicity
Microscopic theory of Brownian motion of a particle of mass in a bath of
molecules of mass is considered beyond lowest order in the mass ratio
. The corresponding Langevin equation contains nonlinear corrections to
the dissipative force, and the generalized Fokker-Planck equation involves
derivatives of order higher than two. These equations are derived from first
principles with coefficients expressed in terms of correlation functions of
microscopic force on the particle. The coefficients are evaluated explicitly
for a generalized Rayleigh model with a finite time of molecule-particle
collisions. In the limit of a low-density bath, we recover the results obtained
previously for a model with instantaneous binary collisions. In general case,
the equations contain additional corrections, quadratic in bath density,
originating from a finite collision time. These corrections survive to order
and are found to make the stationary distribution non-Maxwellian.
Some relevant numerical simulations are also presented
Implementation of a Combined OFDM-Demodulation and WCDMA-Equalization Module
For a dual-mode baseband receiver for the OFDMWireless LAN andWCDMA standards, integration of the demodulation and equalization tasks on a dedicated hardware module has been investigated. For OFDM demodulation, an FFT algorithm based on cascaded twiddle factor decomposition has been selected. This type of algorithm combines high spatial and temporal regularity in the FFT data-flow graphs with a minimal number of computations. A frequency-domain algorithm based on a circulant channel approximation has been selected for WCDMA equalization. It has good performance, low hardware complexity and a low number of computations. Its main advantage is the reuse of the FFT kernel, which contributes to the integration of both tasks. The demodulation and equalization module has been described at the register transfer level with the in-house developed Arx language. The core of the module is a pipelined radix-23 butterfly combined with a complex multiplier and complex divider. The module has an area of 0.447 mm2 in 0.18 ¿m technology and a power consumption of 10.6 mW. The proposed module compares favorably with solutions reported in literature
The Accuracy of Perturbative Master Equations
We consider open quantum systems with dynamics described by master equations
that have perturbative expansions in the system-environment interaction. We
show that, contrary to intuition, full-time solutions of order-2n accuracy
require an order-(2n+2) master equation. We give two examples of such
inaccuracies in the solutions to an order-2n master equation: order-2n
inaccuracies in the steady state of the system and order-2n positivity
violations, and we show how these arise in a specific example for which exact
solutions are available. This result has a wide-ranging impact on the validity
of coupling (or friction) sensitive results derived from second-order
convolutionless, Nakajima-Zwanzig, Redfield, and Born-Markov master equations.Comment: 6 pages, 0 figures; v2 updated references; v3 updated references,
extension to full-time and nonlocal regime
Effect of Inhomogeneity in Translocation of Polymers through Nanopores
The motion of polymers with inhomogeneous structure through nanopores is
discussed theoretically. Specifically, we consider the translocation dynamics
of polymers consisting of double-stranded and single-stranded blocks. Since
only the single-stranded chain can go through the nanopore the double-stranded
segment has to unzip before the translocation. Utilizing a simple analytical
model, translocation times are calculated explicitly for different polymer
orientations, i.e., when the single-stranded block enters the pore first and
when the double-stranded segment is a leading one. The dependence of the
translocation dynamics on external fields, energy of interaction in the
double-stranded segment, size of the polymer and the fraction of
double-stranded monomers is analyzed. It is found that the order of entrance
into the pore has a significant effect on the translocation dynamics. The
theoretical results are discussed using free-energy landscape arguments.Comment: 12 pages, 5 figures, submitted to J. Chem. Phy
Slow transport by continuous time quantum walks
Continuous time quantum walks (CTQW) do not necessarily perform better than
their classical counterparts, the continuous time random walks (CTRW). For one
special graph, where a recent analysis showed that in a particular direction of
propagation the penetration of the graph is faster by CTQWs than by CTRWs, we
demonstrate that in another direction of propagation the opposite is true; In
this case a CTQW initially localized at one site displays a slow transport. We
furthermore show that when the CTQW's initial condition is a totally symmetric
superposition of states of equivalent sites, the transport gets to be much more
rapid.Comment: 5 pages, 7 figures, accepted for publication in Phys. Rev.
The Three Faces of the Second Law: II. Fokker-Planck Formulation
The total entropy production is the sum of two contributions, the so-called
adiabatic and nonadiabatic entropy production, each of which is non-negative.
We derive their explicit expressions for continuous Markov processes, discuss
their properties and illustrate their behavior on two exactly solvable models.Comment: 7 pages, 1 figur
Nonlinear response theory for Markov processes: Simple models for glassy relaxation
The theory of nonlinear response for Markov processes obeying a master
equation is formulated in terms of time-dependent perturbation theory for the
Green's functions and general expressions for the response functions up to
third order in the external field are given. The nonlinear response is
calculated for a model of dipole reorientations in an asymmetric double well
potential, a standard model in the field of dielectric spectroscopy. The static
nonlinear response is finite with the exception of a certain temperature
determined by the value of the asymmetry. In a narrow temperature range around
, the modulus of the frequency-dependent cubic response shows a peak at a
frequency on the order of the relaxation rate and it vanishes for both, low
frequencies and high frequencies. At temperatures at which the static response
is finite (lower and higher than ), the modulus is found to decay
monotonously from the static limit to zero at high frequencies. In addition,
results of calculations for a trap model with a Gaussian density of states are
presented. In this case, the cubic response depends on the specific dynamical
variable considered and also on the way the external field is coupled to the
kinetics of the model. In particular, a set of different dynamical variables is
considered that gives rise to identical shapes of the linear susceptibility and
only to different temperature dependencies of the relaxation times. It is found
that the frequency dependence of the nonlinear response functions, however,
strongly depends on the particular choice of the variables. The results are
discussed in the context of recent theoretical and experimental findings
regarding the nonlinear response of supercooled liquids and glasses.Comment: 23 pages, 10 figure
Positive Feedback Regulation Results in Spatial Clustering and Fast Spreading of Active Signaling Molecules on a Cell Membrane
Positive feedback regulation is ubiquitous in cell signaling networks, often
leading to binary outcomes in response to graded stimuli. However, the role of
such feedbacks in clustering, and in spatial spreading of activated molecules,
has come to be appreciated only recently. We focus on the latter, using a
simple model developed in the context of Ras activation with competing negative
and positive feedback mechanisms. We find that positive feedback, in the
presence of slow diffusion, results in clustering of activated molecules on the
plasma membrane, and rapid spatial spreading as the front of the cluster
propagates with a constant velocity (dependent on the feedback strength). The
advancing fronts of the clusters of the activated species are rough, with
scaling consistent with the Kardar-Parisi-Zhang (KPZ) equation in one
dimension. Our minimal model is general enough to describe signal transduction
in a wide variety of biological networks where activity in the
membrane-proximal region is subject to feedback regulation.Comment: 37 pages, 8 figures. Journal of Chemical Physics (in press
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