108,506 research outputs found
Performance Evaluation of Distributed-Antenna Communications Systems Using Beam-Hopping
Digital beamforming (DBF) techniques are capable of improving the performance of communications systems significantly. However, if the transmitted signals are conflicted with strong interference, especially, in the direction of the transmitted beams , these directional jamming signals will severely degrade the system performance. In order to efficiently mitigate the interference of the directional jammers, in this contribution a beam-hopping (BH) communications scheme is proposed. In the proposed BH communications scheme, only one pair of the beams is used for transmission and it hops from one to the next according to an assigned BH pattern. In this contribution a range of expressions in terms of the average SINR performance have been derived, when both the uplink and downlink are considered. The average SINR performance of the proposed BH scheme and that of the conventional single-beam (SB) as well as multiple-beam (MB) assisted beam-processing schemes have been investigated. Our analysis and results show that the proposed BH scheme is capable of efficiently combating the directional jamming, with the aid of utilizing the directional gain of the beams generated by both the transmitter and the receiver. Furthermore, the BH scheme is capable of reducing the intercept probability of the communications. Therefore, the proposed BH scheme is suitable for communications, when several distributed antenna arrays are available around a mobile
Fluctuations of the vacuum energy density of quantum fields in curved spacetime via generalized zeta functions
For quantum fields on a curved spacetime with an Euclidean section, we derive
a general expression for the stress energy tensor two-point function in terms
of the effective action. The renormalized two-point function is given in terms
of the second variation of the Mellin transform of the trace of the heat kernel
for the quantum fields. For systems for which a spectral decomposition of the
wave opearator is possible, we give an exact expression for this two-point
function. Explicit examples of the variance to the mean ratio of the vacuum energy density of a
massless scalar field are computed for the spatial topologies of and , with results of , and
respectively. The large variance signifies the importance
of quantum fluctuations and has important implications for the validity of
semiclassical gravity theories at sub-Planckian scales. The method presented
here can facilitate the calculation of stress-energy fluctuations for quantum
fields useful for the analysis of fluctuation effects and critical phenomena in
problems ranging from atom optics and mesoscopic physics to early universe and
black hole physics.Comment: Uses revte
Thermal And Mechanical Analysis of High-power Light-emitting Diodes with Ceramic Packages
In this paper we present the thermal and mechanical analysis of high-power
light-emitting diodes (LEDs) with ceramic packages. Transient thermal
measurements and thermo-mechanical simulation were performed to study the
thermal and mechanical characteristics of ceramic packages. Thermal resistance
from the junction to the ambient was decreased from 76.1 oC/W to 45.3 oC/W by
replacing plastic mould to ceramic mould for LED packages. Higher level of
thermo-mechanical stresses in the chip were found for LEDs with ceramic
packages despite of less mismatching coefficients of thermal expansion
comparing with plastic packages. The results suggest that the thermal
performance of LEDs can be improved by using ceramic packages, but the mounting
process of the high power LEDs with ceramic packages is critically important
and should be in charge of delaminating interface layers in the packages.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
The asymmetric structure of the Galactic halo
Using the stellar photometry catalogue based on the latest data release (DR4)
of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using
star counts is carried out for selected areas of the sky. The sample areas are
selected along a circle at a Galactic latitude of +60, and 10 strips of
high Galactic latitude along different longitudes. Direct statistics of the
data show that the surface densities of from to
are systematically higher than those of from
to , defining a region of overdensity (in the direction of Virgo)
and another one of underdensity (in the direction of Ursa Major) with respect
to an axisymmetric model. It is shown by comparing the results from star counts
in the colour that the density deviations are due to an asymmetry of
the stellar density in the halo. Theoretical models for the surface density
profile are built and star counts are performed using a triaxial halo of which
the parameters are constrained by observational data. Two possible reasons for
the asymmetric structure are discussed.Comment: 17 pages, 7 figures, 5 tables, MNRAS accepte
Precision of Hubble constant derived using black hole binary absolute distances and statistical redshift information
Measured gravitational waveforms from black hole binary inspiral events
directly determine absolute luminosity distances. To use these data for
cosmology, it is necessary to independently obtain redshifts for the events,
which may be difficult for those without electromagnetic counterparts. Here it
is demonstrated that certainly in principle, and possibly in practice,
clustering of galaxies allows extraction of the redshift information from a
sample statistically for the purpose of estimating mean cosmological
parameters, without identification of host galaxies for individual events. We
extract mock galaxy samples from the 6th Data Release of the Sloan Digital Sky
Survey resembling those that would be associated with inspiral events of
stellar mass black holes falling into massive black holes at redshift z ~ 0.1
to 0.5. A simple statistical procedure is described to estimate a likelihood
function for the Hubble constant H_0: each galaxy in a LISA error volume
contributes linearly to the log likelihood for the source redshift, and the log
likelihood for each source contributes linearly to that of H_0. This procedure
is shown to provide an accurate and unbiased estimator of H_0. It is estimated
that a precision better than one percent in H_0 may be possible if the rate of
such events is sufficiently high, on the order of 20 to z = 0.5.Comment: 9 pages, 4 figures, submitted to Phys. Rev. D; new references adde
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