5,356 research outputs found
A Novel Network NOMA Scheme for Downlink Coordinated Three-Point Systems
In this paper, we propose a network non-orthogonal multiple access (N-NOMA)
technique for the downlink coordinated multipoint (CoMP) communication scenario
of a cellular network, with randomly deployed users. In the considered N-NOMA
scheme, superposition coding (SC) is employed to serve cell-edge users as well
as users close to base stations (BSs) simultaneously, and distributed analog
beamforming by the BSs to meet the cell-edge user's quality of service (QoS)
requirements. The combination of SC and distributed analog beamforming
significantly complicates the expressions for the
signal-to-interference-plus-noise ratio (SINR) at the reveiver, which makes the
performance analysis particularly challenging. However, by using rational
approximations, insightful analytical results are obtained in order to
characterize the outage performance of the considered N-NOMA scheme. Computer
simulation results are provided to show the superior performance of the
proposed scheme as well as to demonstrate the accuracy of the analytical
results
On the Multivariate Gamma-Gamma () Distribution with Arbitrary Correlation and Applications in Wireless Communications
The statistical properties of the multivariate Gamma-Gamma ()
distribution with arbitrary correlation have remained unknown. In this paper,
we provide analytical expressions for the joint probability density function
(PDF), cumulative distribution function (CDF) and moment generation function of
the multivariate distribution with arbitrary correlation.
Furthermore, we present novel approximating expressions for the PDF and CDF of
the sum of random variables with arbitrary correlation. Based
on this statistical analysis, we investigate the performance of radio frequency
and optical wireless communication systems. It is noteworthy that the presented
expressions include several previous results in the literature as special
cases.Comment: 7 pages, 6 figures, accepted by IEEE Transactions on Vehicular
Technolog
Correcting for the solar wind in pulsar timing observations: the role of simultaneous a nd l ow-frequency observations
The primary goal of the pulsar timing array projects is to detect
ultra-low-frequency gravitational waves. The pulsar data sets are affected by
numerous noise processes including varying dispersive delays in the
interstellar medium and from the solar wind. The solar wind can lead to rapidly
changing variations that, with existing telescopes, can be hard to measure and
then remove. In this paper we study the possibility of using a low frequency
telescope to aid in such correction for the Parkes Pulsar Timing Array (PPTA)
and also discuss whether the ultra-wide-bandwidth receiver for the FAST
telescope is sufficient to model the solar wind variations. Our key result is
that a single wide-bandwidth receiver can be used to model and remove the
effect of the solar wind. However, for pulsars that pass close to the Sun such
as PSR J1022+1022, the solar wind is so variable that observations at two
telescopes separated by a day are insufficient to correct the solar wind
effect.Comment: accepted by RA
Gravitational Microlensing by Neutron Stars and Radio Pulsars: Event Rates, Timescale Distributions, and Mass Measurements
We investigate properties of Galactic microlensing events in which a stellar
object is lensed by a neutron star. For an all-sky photometric microlensing
survey, we determine the number of lensing events caused by
potentially-observable radio pulsars to be for
background stellar sources. We expect a few detectable events per
year for the same number of background sources from an astrometric microlensing
survey. We show that such a study could lead to precise measurements of radio
pulsar masses. For instance, if a pulsar distance could be constrained through
radio observations, then its mass would be determined with a precision of
. We also investigate the time-scale distributions for neutron star
events, finding that they are much shorter than had been previously thought.
For photometric events towards the Galactic centre that last days,
around will have a neutron star lens. This fraction drops rapidly for
longer time-scales. Away from the bulge region we find that neutron stars will
contribute of the events that last less than days. These
results are in contrast to earlier work which found that the maximum fraction
of neutron star events would occur on time-scales of hundreds of days.Comment: 10 pages, accepted for publication in ApJ. v2 updated to reflect
change of title in proof stag
X-ray Monitoring of Gravitational Lenses With Chandra
We present \emph{Chandra} monitoring data for six gravitationally lensed
quasars: QJ 01584325, HE 04351223, HE 11041805, SDSS 0924+0219, SDSS
1004+4112, and Q 2237+0305. We detect X-ray microlensing variability in all six
lenses with high confidence. We detect energy dependent microlensing in HE
04351223, SDSS 1004+4112, SDSS 0924+0219 and Q 2237+0305. We present a
detailed spectral analysis for each lens, and find that simple power-law models
plus Gaussian emission lines give good fits to the spectra. We detect intrinsic
spectral variability in two epochs of Q 2237+0305. We detect differential
absorption between images in four lenses. We also detect the \feka\ emission
line in all six lenses, and the Ni XXVII K line in two images of Q
2237+0305. The rest frame equivalent widths of the \feka\ lines are measured to
be 0.4--1.2 keV, significantly higher than those measured in typical active
galactic nuclei of similar X-ray luminosities. This suggests that the \feka\
emission region is more compact or centrally concentrated than the continuum
emission region.Comment: 55 pages, 22 figure
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