100,878 research outputs found
An Accurate and Efficient Analysis of a MBSFN Network
A new accurate analysis is presented for an OFDM-based multicast-broadcast
single-frequency network (MBSFN). The topology of the network is modeled by a
constrained random spatial model involving a fixed number of base stations
placed over a finite area with a minimum separation. The analysis is driven by
a new closed-form expression for the conditional outage probability at each
location of the network, where the conditioning is with respect to the network
realization. The analysis accounts for the diversity combining of signals
transmitted by different base stations of a given MBSFN area, and also accounts
for the interference caused by the base stations of other MBSFN areas. The
analysis features a flexible channel model, accounting for path loss, Nakagami
fading, and correlated shadowing. The analysis is used to investigate the
influence of the minimum base-station separation and provides insight regarding
the optimal size of the MBSFN areas. In order to highlight the percentage of
the network that will fail to successfully receive the broadcast, the area
below an outage threshold (ABOT) is here used and defined as the fraction of
the network that provides an outage probability (averaged over the fading) that
meets a threshold.Comment: 5 pages, 4 figures, IEEE International Conference on Acoustics,
Speech and Signal Processing (ICASSP), 2014, to appea
Statistical methods for the analysis of rotation measure grids in large scale structures in the SKA era
To better understand the origin and properties of cosmological magnetic
fields, a detailed knowledge of magnetic fields in the large-scale structure of
the Universe (galaxy clusters, filaments) is crucial. We propose a new
statistical approach to study magnetic fields on large scales with the rotation
measure grid data that will be obtained with the new generation of radio
interferometers.Comment: 9 pages; to appear as part of 'Cosmic Magnetism' in Proceedings
'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)11
The Integrated Polarization of Spiral Galaxy Disks
We present integrated polarization properties of nearby spiral galaxies at
4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a
function of inclination. Spiral galaxies in our sample have observed integrated
fractional polarization in the range < 1% to 17.6%. At inclinations less than
50 degrees, the fractional polarization depends mostly on the ratio of random
to regular magnetic field strength. At higher inclinations, Faraday
depolarization associated with the regular magnetic field becomes more
important. The observed degree of polarization is lower (<4%) for more luminous
galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization
angle of the integrated emission is aligned with the apparent minor axis of the
disk for galaxies without a bar. In our axially symmetric models, the
polarization angle of the integrated emission is independent of wavelength.
Simulated distributions of fractional polarization for randomly oriented spiral
galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization
measurements, e.g. with the SKA, of unresolved spiral galaxies allow
statistical studies of the magnetic field in disk galaxies using large samples
in the local universe and at high redshift. As these galaxies behave as
idealized background sources without internal Faraday rotation, they can be
used to detect large-scale magnetic fields in the intergalactic medium.Comment: 13 pages, 6 figures; Accepted for publication in The Astrophysical
Journa
Radiative Transfer Modeling of Lyman Alpha Emitters. II. New Effects in Galaxy Clustering
We study the clustering properties of z~5.7 Lyman-alpha emitters (LAEs) in a
cosmological reionization simulation with a full Lya radiative transfer
calculation. Lya radiative transfer substantially modifies the intrinsic Lya
emission properties, compared to observed ones, depending on the density and
velocity structure environment around the Lya emitting galaxy. This
environment-dependent Lya selection introduces new features in LAE clustering,
suppressing (enhancing) the line-of-sight (transverse) density fluctuations and
giving rise to scale-dependent galaxy bias. In real space, the contours of the
three-dimensional two-point correlation function of LAEs appear to be
prominently elongated along the line of sight on large scales, an effect that
is opposite to and much stronger than the linear redshift-space distortion
effect. The projected two-point correlation function is greatly enhanced in
amplitude by a factor of up to a few, compared to the case without the
environment dependent selection effect. The new features in LAE clustering can
be understood with a simple, physically motivated model, where Lya selection
depends on matter density, velocity, and their gradients. We discuss the
implications and consequences of the effects on galaxy clustering from Lya
selection in interpreting clustering measurements and in constraining cosmology
and reionization from LAEs.Comment: 31 pages, 26 figures, revised according to the referee's comments,
more discussions and tests, published in Ap
A Measurement Based Shadow Fading Model for Vehicle-to-Vehicle Network Simulations
The vehicle-to-vehicle (V2V) propagation channel has significant implications
on the design and performance of novel communication protocols for vehicular ad
hoc networks (VANETs). Extensive research efforts have been made to develop V2V
channel models to be implemented in advanced VANET system simulators for
performance evaluation. The impact of shadowing caused by other vehicles has,
however, largely been neglected in most of the models, as well as in the system
simulations. In this paper we present a shadow fading model targeting system
simulations based on real measurements performed in urban and highway
scenarios. The measurement data is separated into three categories,
line-of-sight (LOS), obstructed line-of-sight (OLOS) by vehicles, and non
line-of-sight due to buildings, with the help of video information recorded
during the measurements. It is observed that vehicles obstructing the LOS
induce an additional average attenuation of about 10 dB in the received signal
power. An approach to incorporate the LOS/OLOS model into existing VANET
simulators is also provided. Finally, system level VANET simulation results are
presented, showing the difference between the LOS/OLOS model and a channel
model based on Nakagami-m fading.Comment: 10 pages, 12 figures, submitted to Hindawi International Journal of
Antennas and Propagatio
Pulsar timing in extreme mass ratio binaries: a general relativistic approach
The detection of a pulsar (PSR) in a tight, relativistic orbit around a
supermassive or intermediate mass black hole - such as those in the Galactic
centre or in the centre of Globular clusters - would allow for precision tests
of general relativity (GR) in the strong-field, non-linear regime. We present a
framework for calculating the theoretical time-frequency signal from a PSR in
such an Extreme Mass Ratio Binary (EMRB). This framework is entirely
relativistic with no weak-field approximations and so able to account for all
higher-order strong-field gravitational effects, relativistic spin dynamics,
the convolution with astrophysical effects and the combined impact on the PSR
timing signal. Specifically we calculate both the spacetime path of the pulsar
radio signal and the complex orbital and spin dynamics of a spinning pulsar
around a Kerr black hole, accounting for spacetime curvature and frame
dragging, relativistic and gravitational time delay, gravitational light
bending, temporal and spatial dispersion induced by the presence of plasma
along the line of sight and relativistic aberration. This then allows for a
consistent time-frequency solution to be generated. Such a framework is key for
assessing the use of PSR as probes of strong field GR, helping to inform the
detection of an EMRB system hosting a PSR and, most essentially, for providing
an accurate theoretical basis to then compare with observations to test
fundamental physics.Comment: 19 pages, 15 Figures. Accepted for publication in MNRA
Key Generation in Wireless Sensor Networks Based on Frequency-selective Channels - Design, Implementation, and Analysis
Key management in wireless sensor networks faces several new challenges. The
scale, resource limitations, and new threats such as node capture necessitate
the use of an on-line key generation by the nodes themselves. However, the cost
of such schemes is high since their secrecy is based on computational
complexity. Recently, several research contributions justified that the
wireless channel itself can be used to generate information-theoretic secure
keys. By exchanging sampling messages during movement, a bit string can be
derived that is only known to the involved entities. Yet, movement is not the
only possibility to generate randomness. The channel response is also strongly
dependent on the frequency of the transmitted signal. In our work, we introduce
a protocol for key generation based on the frequency-selectivity of channel
fading. The practical advantage of this approach is that we do not require node
movement. Thus, the frequent case of a sensor network with static motes is
supported. Furthermore, the error correction property of the protocol mitigates
the effects of measurement errors and other temporal effects, giving rise to an
agreement rate of over 97%. We show the applicability of our protocol by
implementing it on MICAz motes, and evaluate its robustness and secrecy through
experiments and analysis.Comment: Submitted to IEEE Transactions on Dependable and Secure Computin
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