28,731 research outputs found
Node synchronization of viterbi decoders using state metrics
The concept of node synchronization using state metrics is investigated. The branch metrics are integrated over a fixed time interval and the results are compared to the detection threshold. If the threshold is exceeded, the out-of-sync hypothesis is accepted; otherwise, the in-sync hypothesis is accepted. It is shown that the detection threshold can be chosen independent of any particular convolutional code with fixed code rate and constant length if the code has reasonably good bit error rate performance. Three node synchronization schemes are compared: (1) a scheme using the syndrome; (2) a scheme using the frame-sync patterns; and (3) a scheme using the state metrics. At very low signal to noise ratios (SNR), scheme 2 can be faster than scheme 1. For Voyager's rate 1/2 and constraint length 7 convolutional code, this happens for SNRs of less than 0.7 dB. This result is obtained by assuming that the code frame-sync pattern has good aperiodic autocorrelation properties. For a fixed false alarm probability, the sequential detection scheme based on the syndrome is faster than scheme 3 with fixed integration time. A sequential detection technique is needed to improve the speed of scheme 3
Software package for performing experiments about the convolutionally encoded Voyager 1 link
A software package enabling engineers to conduct experiments to determine the actual performance of long constraint-length convolutional codes over the Voyager 1 communication link directly from the Jet Propulsion Laboratory (JPL) has been developed. Using this software, engineers are able to enter test data from the Laboratory in Pasadena, California. The software encodes the data and then sends the encoded data to a personal computer (PC) at the Goldstone Deep Space Complex (GDSC) over telephone lines. The encoded data are sent to the transmitter by the PC at GDSC. The received data, after being echoed back by Voyager 1, are first sent to the PC at GDSC, and then are sent back to the PC at the Laboratory over telephone lines for decoding and further analysis. All of these operations are fully integrated and are completely automatic. Engineers can control the entire software system from the Laboratory. The software encoder and the hardware decoder interface were developed for other applications, and have been modified appropriately for integration into the system so that their existence is transparent to the users. This software provides: (1) data entry facilities, (2) communication protocol for telephone links, (3) data displaying facilities, (4) integration with the software encoder and the hardware decoder, and (5) control functions
Performance of the all-digital data-transition tracking loop in the advanced receiver
The performance of the all-digital data-transition tracking loop (DTTL) with coherent or noncoherent sampling is described. The effects of few samples per symbol and of noncommensurate sampling rates and symbol rates are addressed and analyzed. Their impacts on the loop phase-error variance and the mean time to lose lock (MTLL) are quantified through computer simulations. The analysis and preliminary simulations indicate that with three to four samples per symbol, the DTTL can track with negligible jitter because of the presence of earth Doppler rate. Furthermore, the MTLL is also expected to be large engough to maintain lock over a Deep Space Network track
Spontaneous Formation of Stable Capillary Bridges for Firming Compact Colloidal Microstructures in Phase Separating Liquids: A Computational Study
Computer modeling and simulations are performed to investigate capillary
bridges spontaneously formed between closely packed colloidal particles in
phase separating liquids. The simulations reveal a self-stabilization mechanism
that operates through diffusive equilibrium of two-phase liquid morphologies.
Such mechanism renders desired microstructural stability and uniformity to the
capillary bridges that are spontaneously formed during liquid solution phase
separation. This self-stabilization behavior is in contrast to conventional
coarsening processes during phase separation. The volume fraction limit of the
separated liquid phases as well as the adhesion strength and thermodynamic
stability of the capillary bridges are discussed. Capillary bridge formations
in various compact colloid assemblies are considered. The study sheds light on
a promising route to in-situ (in-liquid) firming of fragile colloidal crystals
and other compact colloidal microstructures via capillary bridges
Theoretical Study of Sodium and Potassium Resonance Lines Pressure Broadened by Helium Atoms
We perform fully quantum mechanical calculations in the binary approximation
of the emission and absorption profiles of the sodium - and potassium
- resonance lines under the influence of a helium perturbing gas. We
use carefully constructed potential energy surfaces and transition dipole
moments to compute the emission and absorption coefficients at temperatures
from 158 to 3000 K. Contributions from quasi-bound states are included. The
resulting red and blue wing profiles agree well with previous theoretical
calculations and with experimental measurements.Comment: 16 figure
Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface
Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive "funnel"
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte
Point configurations that are asymmetric yet balanced
A configuration of particles confined to a sphere is balanced if it is in
equilibrium under all force laws (that act between pairs of points with
strength given by a fixed function of distance). It is straightforward to show
that every sufficiently symmetrical configuration is balanced, but the converse
is far from obvious. In 1957 Leech completely classified the balanced
configurations in R^3, and his classification is equivalent to the converse for
R^3. In this paper we disprove the converse in high dimensions. We construct
several counterexamples, including one with trivial symmetry group.Comment: 10 page
Catalytic Conversion of Methane and Carbon Dioxide (Greenhouse Gases) into Syngas over Samarium-Cobalt-Trioxides Perovskite Catalyst
The catalytic behaviour of samarium-cobalt-trioxides perovskite catalyst was investigated for the mitigation of greenhouse gases (carbon dioxide and methane) to provide cleaner energy (hydrogen and carbon monoxide) and environment. X-ray photoelectron spectroscopy analysis of the as-synthesized catalyst showed peaks corresponding to complexes of cobalt (II), dual oxygen species, and samarium (III) ions. X-ray diffraction pattern showed a monophasic samarium-cobalt-trioxides perovskite structure, while post-reaction analysis showed modification of the perovskite. The temperature-programmed reduction analysis showed peaks corresponding to reduction of cobalt (II) to Coo. The temperature-programmed desorption displayed peaks ascribed to medium strength basic and acidic sites. Performance test carried out on the catalyst via methane dry reforming, showed excellent reactants conversions of above 90% which was maintained for the duration (30 h) of the experiment. The catalyst remained active over the time of experiment, even though the temperature-programmed oxidation, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses of the used catalyst showed evidence of carbon deposit
Gauge Invariance of Resummation Schemes: The QCD Partition Function
We pick up a method originally developed by Cheng and Tsai for vacuum
perturbation theory which allows to test the consistency of different sets of
Feynman rules on a purely diagrammatic level, making explicit loop calculations
superfluous. We generalize it to perturbative calculations in thermal field
theory and we show that it can be adapted to check the gauge invariance of
physical quantities calculated in improved perturbation schemes. Specifically,
we extend this diagrammatic technique to a simple resummation scheme in
imaginary time perturbation theory. As an application, we check up to O(g^4) in
general covariant gauge the gauge invariance of the result for the QCD
partition function which was recently obtained in Feynman gauge.Comment: 29 pages, LaTeX, using RevTeX and feynmf.sty, Replacement: NO changes
to the paper, TeX-source now additionally avaibl
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