5,977 research outputs found
Hierarchical and High-Girth QC LDPC Codes
We present a general approach to designing capacity-approaching high-girth
low-density parity-check (LDPC) codes that are friendly to hardware
implementation. Our methodology starts by defining a new class of
"hierarchical" quasi-cyclic (HQC) LDPC codes that generalizes the structure of
quasi-cyclic (QC) LDPC codes. Whereas the parity check matrices of QC LDPC
codes are composed of circulant sub-matrices, those of HQC LDPC codes are
composed of a hierarchy of circulant sub-matrices that are in turn constructed
from circulant sub-matrices, and so on, through some number of levels. We show
how to map any class of codes defined using a protograph into a family of HQC
LDPC codes. Next, we present a girth-maximizing algorithm that optimizes the
degrees of freedom within the family of codes to yield a high-girth HQC LDPC
code. Finally, we discuss how certain characteristics of a code protograph will
lead to inevitable short cycles, and show that these short cycles can be
eliminated using a "squashing" procedure that results in a high-girth QC LDPC
code, although not a hierarchical one. We illustrate our approach with designed
examples of girth-10 QC LDPC codes obtained from protographs of one-sided
spatially-coupled codes.Comment: Submitted to IEEE Transactions on Information THeor
The evolution of aerospace guidance technology at the Massachusetts Institute of Technology, 1935 - 1951: A memoir
Early developments in inertial guidance systems design are described
The Sonic Altimeter for Aircraft
Discussed here are results already achieved with sonic altimeters in light of the theoretical possibilities of such instruments. From the information gained in this investigation, a procedure is outlined to determine whether or not a further development program is justified by the value of the sonic altimeter as an aircraft instrument. The information available in the literature is reviewed and condensed into a summary of sonic altimeter developments. Various methods of receiving the echo and timing the interval between the signal and the echo are considered. A theoretical discussion is given of sonic altimeter errors due to uncertainties in timing, variations in sound velocity, aircraft speed, location of the sending and receiving units, and inclinations of the flight path with respect to the ground surface. Plots are included which summarize the results in each case. An analysis is given of the effect of an inclined flight path on the frequency of the echo. A brief study of the acoustical phases of the sonic altimeter problem is carried through. The results of this analysis are used to predict approximately the maximum operating altitudes of a reasonably designed sonic altimeter under very good and very bad conditions. A final comparison is made between the estimated and experimental maximum operating altitudes which shows good agreement where quantitative information is available
The Physical Effects of Detonation in a Closed Cylindrical Chamber
Detonation in the internal-combustion engine is studied as a physical process. It is shown that detonation is accompanied by pressure waves within the cylinder charge. Sound theory is applied to the calculation of resonant pressure-wave frequencies. Apparatus is described for direct measurement of pressure-wave frequencies. Frequencies determined from two engines of different cylinder sizes are shown to agree with the values calculated from sound theory. An outline of the theoretically possible modes of vibration in a right circular cylinder with flat ends is included. An appendix by John P. Elting gives a method of calculating pressure in the sound wave following detonation
The meteorological airplane ascents of the Massachusetts Institute of Technology. Part I. On the technique of meteorological airplane ascents. Part II. Aircraft instruments in meteorological flying
The aerological flights at Boston are part of the general research program of the
Meteorological Division of Massachusetts Institute of Technology, which program since
1929 has been directed especially toward the study of American air masses and fronts.
Recently, some results of these studies were published by Willett, who based his
investigations on a series continuous over three years of mornìng and evening weather
maps, analyzed at the Institute, together with upper air soundings from the United
States Weather Bureau stations at Dallas, Omaha, Chicago, Groesbeck, Atlanta and
from the United States Navy at Seattle, Anacostia, Pensacola and San Diego. These upper
air data facilitated the determination of the properties of the air masses and so proved
of inestimable value for the study. But the use of the data also showed that improvement
both in the number of stations and in the quality of observations was highly desirable.
Ascents in the northeastern part of the United States were lacking. Knowledge of the
vertical structure of air masses reaching this region, however, is of special interest in
forecasting for this densely populated district. For these reasons and since the direct
comparison of actual local weather developments with upper air conditions is also con- sidered to be very valuable, the Institute started its own airplane station at Boston. In
addition to "regular" ascents at the time of the morning surface observations, special
flights were made when particularly interesting weather situations prevailed. On a
number of days series of ascents were carried out to obtain cross sections through fronts
passing over Boston. Other special flights were made to obtain information on atmospheric
turbulence. For this same purpose and also in order to study the diurnal changes of
temperature in the lowest 5,000 feet, several series are planned of a number of comparatively
low altitude flights at short intervals throughout the day
Are Ferroan Anorthosites Direct Products of the Lunar Magma Ocean?
According to Lunar Magma Ocean (LMO) theory, lunar samples that fall into the ferroan anorthosite (FAN) category represent the only samples we have of of the primordial crust of the Moon. Modeling indicates that plagioclase crystallizes after >70% LMO crystallization and formed a flotation crust, depending upon starting composition. The FAN group of highlands materials has been subdivided into mafic-magnesian, mafic-ferroan, anorthositic- sodic, and anorthositic-ferroan, although it is not clear how these subgroups are related. Recent radiogenic isotope work has suggested the range in FAN ages and isotopic systematics are inconsistent with formation of all FANs from the LMO. While an insulating lid could have theoretically extend the life of the LMO to explain the range of the published ages, are the FAN compositions consistent with crystallization from the LMO? As part of a funded Emerging Worlds proposal (NNX15AH76G), we examine this question through analysis of FAN samples. We compare the results with various LMO crystallization models, including those that incorporate the influence of garnet
Searching for tetraquarks on the lattice
We address the question whether the lightest scalar mesons sigma and kappa
are tetraquarks. We present a search for possible light tetraquark states with
J^PC=0^++ and I=0, 1/2, 3/2, 2 in the dynamical and the quenched lattice
simulations using tetraquark interpolators. In all the channels, we unavoidably
find lowest scattering states pi(k)pi(-k) or K(k)pi(-k) with back-to-back
momentum k=0,2*pi/L,.. . However, we find an additional light state in the I=0
and I=1/2 channels, which may be related to the observed resonances sigma and
kappa with a strong tetraquark component. In the exotic repulsive channels I=2
and I=3/2, where no resonance is observed, we find no light state in addition
to the scattering states.Comment: 3 pages, 1 figure, proceedings of Lepton-Photon 2009, Hambur
Glueball matrix elements on anisotropic lattices
We describe a lattice calculation of the matrix elements relevant for
glueball production in radiative decays. The techniques for such a
calculation on anisotropic lattices with an improved action are outlined. We
present preliminary results showing the efficacy of the computational method.Comment: 3 pages (LaTeX), 3 figures (PostScript), Presented at Lattice '9
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