1,738 research outputs found
Cognitive Radio from Hell: Flipping Attack on Direct-Sequence Spread Spectrum
In this paper, we introduce a strong adversarial attack, referred to as the
flipping attack, on Direct-Sequence Spread Spectrum (DSSS) systems. In this
attack, the attacker, which is appropriately positioned between the transmitter
and the receiver, instantaneously flips the transmitted symbols in the air at
50% rate, thereby driving the channel capacity to zero. Unlike the traditional
jamming attack, this attack, when perfectly executed, cannot be detected at the
receiver using signal-to-noise-ratio measurements. However, this attack
necessitates the attacker to perfectly know the realizations of all the
channels in the model. We first introduce the consequences of the flipping
attack on narrowband frequency-flat channels, and subsequently discuss its
feasibility in wideband frequency-selective channels. From the legitimate
users' perspective, we present a method to detect this attack and also propose
heuristics to improve the error-performance under the attack. We emphasize that
future cyber-physical systems that employ DSSS should design transceivers to
detect the proposed flipping attack, and then apply appropriate
countermeasures
Clustering Memes in Social Media
The increasing pervasiveness of social media creates new opportunities to
study human social behavior, while challenging our capability to analyze their
massive data streams. One of the emerging tasks is to distinguish between
different kinds of activities, for example engineered misinformation campaigns
versus spontaneous communication. Such detection problems require a formal
definition of meme, or unit of information that can spread from person to
person through the social network. Once a meme is identified, supervised
learning methods can be applied to classify different types of communication.
The appropriate granularity of a meme, however, is hardly captured from
existing entities such as tags and keywords. Here we present a framework for
the novel task of detecting memes by clustering messages from large streams of
social data. We evaluate various similarity measures that leverage content,
metadata, network features, and their combinations. We also explore the idea of
pre-clustering on the basis of existing entities. A systematic evaluation is
carried out using a manually curated dataset as ground truth. Our analysis
shows that pre-clustering and a combination of heterogeneous features yield the
best trade-off between number of clusters and their quality, demonstrating that
a simple combination based on pairwise maximization of similarity is as
effective as a non-trivial optimization of parameters. Our approach is fully
automatic, unsupervised, and scalable for real-time detection of memes in
streaming data.Comment: Proceedings of the 2013 IEEE/ACM International Conference on Advances
in Social Networks Analysis and Mining (ASONAM'13), 201
Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K
are covered by ice mantles. A nonthermal desorption mechanism is invoked to
explain the presence of gas-phase molecules in these environments, such as the
photodesorption induced by irradiation of ice due to secondary ultraviolet
photons. To quantify the effects of ice photoprocessing, an estimate of the
photon absorption in ice mantles is required. In a recent work, we reported the
vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the
solid phase. The aim was to estimate the VUV-absorption cross sections of
nonpolar molecular ice components, including CH4, CO2, N2, and O2. The column
densities of the ice samples deposited at 8 K were measured in situ by infrared
spectroscopy in transmittance. VUV spectra of the ice samples were collected in
the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged
hydrogen flow lamp. We found that, as expected, solid N2 has the lowest
VUV-absorption cross section, which about three orders of magnitude lower than
that of other species such as O2, which is also homonuclear. Methane (CH4) ice
presents a high absorption near Ly-alpha (121.6 nm) and does not absorb below
148 nm. Estimating the ice absorption cross sections is essential for models of
ice photoprocessing and allows estimating the ice photodesorption rates as the
number of photodesorbed molecules per absorbed photon in the ice.Comment: 9 pages, 6 figures, 7 table
Primordial magnetic field and non-Gaussianity of the 1-year Wilkinson Microwave Anisotropy Probe (WMAP) data
Alfven turbulence caused by statistically isotropic and homogeneous
primordial magnetic field induces correlations in the cosmic microwave
background anisotropies. The correlations are specifically between spherical
harmonic modes a_{l-1,m} and a_{l+1,m}. In this paper we approach this issue
from phase analysis of the CMB maps derived from the WMAP data sets. Using
circular statistics and return phase mapping we examine phase correlation of
\Delta l=2 for the primordial non-Gaussianity caused by the Alfven turbulence
at the epoch of recombination. Our analyses show that such specific features
from the power-law Alfven turbulence do not contribute significantly in the
phases of the maps and could not be a source of primordial non-Gaussianity of
the CMB.Comment: 8 pages, 7 figures, ApJ accepted with minor changes and the
explanation on the whitened derived CMB map
Testing Gaussian random hypothesis with the cosmic microwave background temperature anisotropies in the three-year WMAP data
We test the hypothesis that the temperature of the cosmic microwave
background is consistent with a Gaussian random field defined on the celestial
sphere, using de-biased internal linear combination (DILC) map produced from
the 3-year WMAP data. We test the phases for spherical harmonic modes with l <=
10 (which should be the cleanest) for their uniformity, randomness, and
correlation with those of the foreground templates. The phases themselves are
consistent with a uniform distribution, but not for l <= 5, and the differences
between phases are not consistent with uniformity. For l=3 and l=6, the phases
of the CMB maps cross-correlate with the foregrounds, suggestion the presence
of residual contamination in the DLC map even on these large scales. We also
use a one-dimensional Fourier representation to assemble a_lm into the \Delta
T_l(\phi) for each l mode, and test the positions of the resulting maxima and
minima for consistency with uniformity randomness on the unit circle. The
results show significant departures at the 0.5% level, with the one-dimensional
peaks being concentrated around \phi=180 degs. This strongly significant
alignment with the Galactic meridian, together with the cross-correlation of
DILC phases with the foreground maps, strongly suggests that even the lowest
spherical harmonic modes in the map are significantly contaminated with
foreground radiation.Comment: submitted to ApJL, one paragraph is added in Section 3 and some more
in the Referenc
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