266 research outputs found
Lifetime Maximization for Amplify-and-Forward Cooperative Networks
[[abstract]]Power allocation strategies are devised to maximize the network lifetime of amplify-and-forward (AF) cooperative networks. We consider the scenario where one source and multiple partners cooperate to transmit messages to the destination. The powers emitted by the users are subject to the SNR requirement at the destination. First, the power allocation strategy that demands the minimum instantaneous aggregate transmit power of all cooperating partners is described and analyzed. The optimal solution results in a form of selective relaying; namely, the user with the best channel condition is selected to help in relaying the message. However, this instantaneous power minimization strategy does not necessarily maximize the lifetime of battery-limited systems. Then, we propose three AF cooperative schemes to exploit the channel state information (CSI), the residual battery energy and the QoS requirement. It is shown that the network lifetime can be extended considerably by taking all these three factors into account.[[fileno]]2030137030021[[department]]電機工程學
Improved Lattice Gauge Field Hamiltonian
Lepage's improvement scheme is a recent major progress in lattice ,
allowing to obtain continuum physics on very coarse lattices. Here we discuss
improvement in the Hamiltonian formulation, and we derive an improved
Hamiltonian from a lattice Lagrangian free of errors. We do this by
the transfer matrix method, but we also show that the alternative via Legendre
transformation gives identical results. We consider classical improvement,
tadpole improvement and also the structure of L{\"u}scher-Weisz improvement.
The resulting color-electric energy is an infinite series, which is expected to
be rapidly convergent. For the purpose of practical calculations, we construct
a simpler improved Hamiltonian, which includes only nearest-neighbor
interactions.Comment: 30 pages, LaTe
Gravitons and Lightcone Fluctuations
Gravitons in a squeezed vacuum state, the natural result of quantum creation
in the early universe or by black holes, will introduce metric fluctuations.
These metric fluctuations will introduce fluctuations of the lightcone. It is
shown that when the various two-point functions of a quantized field are
averaged over the metric fluctuations, the lightcone singularity disappears for
distinct points. The metric averaged functions remain singular in the limit of
coincident points. The metric averaged retarded Green's function for a massless
field becomes a Gaussian which is nonzero both inside and outside of the
classical lightcone. This implies some photons propagate faster than the
classical light speed, whereas others propagate slower. The possible effects of
metric fluctuations upon one-loop quantum processes are discussed and
illustrated by the calculation of the one-loop electron self-energy.Comment: 18pp, LATEX, TUTP-94-1
The Quantum Interest Conjecture
Although quantum field theory allows local negative energy densities and
fluxes, it also places severe restrictions upon the magnitude and extent of the
negative energy. The restrictions take the form of quantum inequalities. These
inequalities imply that a pulse of negative energy must not only be followed by
a compensating pulse of positive energy, but that the temporal separation
between the pulses is inversely proportional to their amplitude. In an earlier
paper we conjectured that there is a further constraint upon a negative and
positive energy delta-function pulse pair. This conjecture (the quantum
interest conjecture) states that a positive energy pulse must overcompensate
the negative energy pulse by an amount which is a monotonically increasing
function of the pulse separation. In the present paper we prove the conjecture
for massless quantized scalar fields in two and four-dimensional flat
spacetime, and show that it is implied by the quantum inequalities.Comment: 17 pages, Latex, 3 figures, uses eps
SPIDER: Probing the Early Universe with a Suborbital Polarimeter
We evaluate the ability of SPIDER, a balloon-borne polarimeter, to detect a
divergence-free polarization pattern ("B-modes") in the Cosmic Microwave
Background (CMB). In the inflationary scenario, the amplitude of this signal is
proportional to that of the primordial scalar perturbations through the
tensor-to-scalar ratio r. We show that the expected level of systematic error
in the SPIDER instrument is significantly below the amplitude of an interesting
cosmological signal with r=0.03. We present a scanning strategy that enables us
to minimize uncertainty in the reconstruction of the Stokes parameters used to
characterize the CMB, while accessing a relatively wide range of angular
scales. Evaluating the amplitude of the polarized Galactic emission in the
SPIDER field, we conclude that the polarized emission from interstellar dust is
as bright or brighter than the cosmological signal at all SPIDER frequencies
(90 GHz, 150 GHz, and 280 GHz), a situation similar to that found in the
"Southern Hole." We show that two ~20-day flights of the SPIDER instrument can
constrain the amplitude of the B-mode signal to r<0.03 (99% CL) even when
foreground contamination is taken into account. In the absence of foregrounds,
the same limit can be reached after one 20-day flight.Comment: 29 pages, 8 figures, 4 tables; v2: matches published version, flight
schedule updated, two typos fixed in Table 2, references and minor
clarifications added, results unchange
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
Constraining Cut-off Physics in the Cosmic Microwave Background
We investigate the ability to constrain oscillatory features in the
primordial power spectrum using current and future cosmic microwave background
observations. In particular, we study the observability of an oscillation
arising from imprints of physics at the cut-off energy scale. We perform a
likelihood analysis on the WMAP data set, and find that the current data set
constrains the amplitude of the oscillations to be less than 0.77 at 2-sigma,
consistent with a power spectrum without oscillations. In addition, we
investigate the fundamental limitations in the measurement of oscillation
parameters by studying the constraints from a cosmic variance limited
experiment. We find that such an experiment is capable of constraining the
amplitude of such oscillations to be below 0.005, implying that reasonable
models with cut-off energy scales Lambda>200 H_infl are unobservable through
the microwave background.Comment: 16 pages, 7 figures; PRD accepted versio
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
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