2,280 research outputs found
Crystal-controlled multivibrator
Method is described for designing crystal oscillator which has frequency and symmetry stability suitable for system-clock-generators used in spacecraft data systems. Simple multivibrator is used in which timing capacitor is replaced by a crystal
Local Lagrangian Formalism and Discretization of the Heisenberg Magnet Model
In this paper we develop the Lagrangian and multisymplectic structures of the
Heisenberg magnet (HM) model which are then used as the basis for geometric
discretizations of HM. Despite a topological obstruction to the existence of a
global Lagrangian density, a local variational formulation allows one to derive
local conservation laws using a version of N\"other's theorem from the formal
variational calculus of Gelfand-Dikii. Using the local Lagrangian form we
extend the method of Marsden, Patrick and Schkoller to derive local
multisymplectic discretizations directly from the variational principle. We
employ a version of the finite element method to discretize the space of
sections of the trivial magnetic spin bundle over an
appropriate space-time . Since sections do not form a vector space, the
usual FEM bases can be used only locally with coordinate transformations
intervening on element boundaries, and conservation properties are guaranteed
only within an element. We discuss possible ways of circumventing this problem,
including the use of a local version of the method of characteristics,
non-polynomial FEM bases and Lie-group discretization methods.Comment: 12 pages, accepted Math. and Comp. Simul., May 200
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Error Rate Analysis for Coded Multicarrier Systems over Quasi-Static Fading Channels
This paper presents two methods for approximating the performance of coded
multicarrier systems operating over frequency-selective, quasi-static fading
channels with non-ideal interleaving. The first method is based on
approximating the performance of the system over each realization of the
channel, and is suitable for obtaining the outage performance of this type of
system. The second method is based on knowledge of the correlation matrix of
the frequency-domain channel gains and can be used to directly obtain the
average performance. Both of the methods are applicable for
convolutionally-coded interleaved systems employing Quadrature Amplitude
Modulation (QAM). As examples, both methods are used to study the performance
of the Multiband Orthogonal Frequency Division Multiplexing (OFDM) proposal for
high data-rate Ultra-Wideband (UWB) communication.Comment: 5 pages, 3 figures, 2 tables. Submitted to Globecom 200
Improving Receiver Performance of Diffusive Molecular Communication with Enzymes
This paper studies the mitigation of intersymbol interference in a diffusive
molecular communication system using enzymes that freely diffuse in the
propagation environment. The enzymes form reaction intermediates with
information molecules and then degrade them so that they cannot interfere with
future transmissions. A lower bound expression on the expected number of
molecules measured at the receiver is derived. A simple binary receiver
detection scheme is proposed where the number of observed molecules is sampled
at the time when the maximum number of molecules is expected. Insight is also
provided into the selection of an appropriate bit interval. The expected bit
error probability is derived as a function of the current and all previously
transmitted bits. Simulation results show the accuracy of the bit error
probability expression and the improvement in communication performance by
having active enzymes present.Comment: 13 pages, 8 figures, 1 table. To appear in IEEE Transactions on
Nanobioscience (submitted January 22, 2013; minor revision October 16, 2013;
accepted December 4, 2013
Using Dimensional Analysis to Assess Scalability and Accuracy in Molecular Communication
In this paper, we apply dimensional analysis to study a diffusive molecular
communication system that uses diffusing enzymes in the propagation environment
to mitigate intersymbol interference. The enzymes bind to information molecules
and then degrade them so that they cannot interfere with the detection of
future transmissions at the receiver. We determine when it is accurate to
assume that the concentration of information molecules throughout the receiver
is constant and equal to that expected at the center of the receiver. We show
that a lower bound on the expected number of molecules observed at the receiver
can be arbitrarily scaled over the environmental parameters, and generalize how
the accuracy of the lower bound is qualitatively impacted by those parameters.Comment: 6 pages, 2 figures, will be presented at the 3rd IEEE International
Workshop on Molecular and Nanoscale Communications (MoNaCom 2013) in
Budapest, Hungar
On the Statistics of Reaction-Diffusion Simulations for Molecular Communication
A molecule traveling in a realistic propagation environment can experience
stochastic interactions with other molecules and the environment boundary. The
statistical behavior of some isolated phenomena, such as dilute unbounded
molecular diffusion, are well understood. However, the coupling of multiple
interactions can impede closed-form analysis, such that simulations are
required to determine the statistics. This paper compares the statistics of
molecular reaction-diffusion simulation models from the perspective of
molecular communication systems. Microscopic methods track the location and
state of every molecule, whereas mesoscopic methods partition the environment
into virtual containers that hold molecules. The properties of each model are
described and compared with a hybrid of both models. Simulation results also
assess the accuracy of Poisson and Gaussian approximations of the underlying
Binomial statistics.Comment: 6 pages, 1 table, 10 figures. Submitted to the 2nd ACM International
Conference on Nanoscale Computing and Communication (ACM NANOCOM 2015) on May
16, 201
Improving Diffusion-Based Molecular Communication with Unanchored Enzymes
In this paper, we propose adding enzymes to the propagation environment of a
diffusive molecular communication system as a strategy for mitigating
intersymbol interference. The enzymes form reaction intermediates with
information molecules and then degrade them so that they have a smaller chance
of interfering with future transmissions. We present the reaction-diffusion
dynamics of this proposed system and derive a lower bound expression for the
expected number of molecules observed at the receiver. We justify a
particle-based simulation framework, and present simulation results that show
both the accuracy of our expression and the potential for enzymes to improve
communication performance.Comment: 15 pages, 4 figures, presented at the 7th International Conference on
Bio-Inspired Models of Network, Information, and Computing Systems (BIONETICS
2012) in Lugano, Switzerlan
Optimal Receiver Design for Diffusive Molecular Communication With Flow and Additive Noise
In this paper, we perform receiver design for a diffusive molecular
communication environment. Our model includes flow in any direction, sources of
information molecules in addition to the transmitter, and enzymes in the
propagation environment to mitigate intersymbol interference. We characterize
the mutual information between receiver observations to show how often
independent observations can be made. We derive the maximum likelihood sequence
detector to provide a lower bound on the bit error probability. We propose the
family of weighted sum detectors for more practical implementation and derive
their expected bit error probability. Under certain conditions, the performance
of the optimal weighted sum detector is shown to be equivalent to a matched
filter. Receiver simulation results show the tradeoff in detector complexity
versus achievable bit error probability, and that a slow flow in any direction
can improve the performance of a weighted sum detector.Comment: 14 pages, 7 figures, 1 appendix. To appear in IEEE Transactions on
NanoBioscience (submitted July 31, 2013, revised June 18, 2014, accepted July
7, 2014
Bounds on Distance Estimation via Diffusive Molecular Communication
This paper studies distance estimation for diffusive molecular communication.
The Cramer-Rao lower bound on the variance of the distance estimation error is
derived. The lower bound is derived for a physically unbounded environment with
molecule degradation and steady uniform flow. The maximum likelihood distance
estimator is derived and its accuracy is shown via simulation to perform very
close to the Cramer-Rao lower bound. An existing protocol is shown to be
equivalent to the maximum likelihood distance estimator if only one observation
is made. Simulation results also show the accuracy of existing protocols with
respect to the Cramer-Rao lower bound.Comment: 7 pages, 5 figures, 1 table. Will be presented at the 2014 IEEE
Global Communications Conference (GLOBECOM) in Austin, TX, USA, on December
9, 201
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