67,842 research outputs found
Structured Dispersion Matrices From Division Algebra Codes for Space-Time Shift Keying
We propose a novel method of constructing Dispersion Matrices (DM) for Coherent Space-Time Shift Keying (CSTSK) relying on arbitrary PSK signal sets by exploiting codes from division algebras. We show that classic codes from Cyclic Division Algebras (CDA) may be interpreted as DMs conceived for PSK signal sets. Hence various benefits of CDA codes such as their ability to achieve full diversity are inherited by CSTSK. We demonstrate that the proposed CDA based DMs are capable of achieving a lower symbol error ratio than the existing DMs generated using the capacity as their optimization objective function for both perfect and imperfect channel estimation
Adversarial Detection of Flash Malware: Limitations and Open Issues
During the past four years, Flash malware has become one of the most
insidious threats to detect, with almost 600 critical vulnerabilities targeting
Adobe Flash disclosed in the wild. Research has shown that machine learning can
be successfully used to detect Flash malware by leveraging static analysis to
extract information from the structure of the file or its bytecode. However,
the robustness of Flash malware detectors against well-crafted evasion attempts
- also known as adversarial examples - has never been investigated. In this
paper, we propose a security evaluation of a novel, representative Flash
detector that embeds a combination of the prominent, static features employed
by state-of-the-art tools. In particular, we discuss how to craft adversarial
Flash malware examples, showing that it suffices to manipulate the
corresponding source malware samples slightly to evade detection. We then
empirically demonstrate that popular defense techniques proposed to mitigate
evasion attempts, including re-training on adversarial examples, may not always
be sufficient to ensure robustness. We argue that this occurs when the feature
vectors extracted from adversarial examples become indistinguishable from those
of benign data, meaning that the given feature representation is intrinsically
vulnerable. In this respect, we are the first to formally define and
quantitatively characterize this vulnerability, highlighting when an attack can
be countered by solely improving the security of the learning algorithm, or
when it requires also considering additional features. We conclude the paper by
suggesting alternative research directions to improve the security of
learning-based Flash malware detectors
Detection Strategies for Extreme Mass Ratio Inspirals
The capture of compact stellar remnants by galactic black holes provides a
unique laboratory for exploring the near horizon geometry of the Kerr
spacetime, or possible departures from general relativity if the central cores
prove not to be black holes. The gravitational radiation produced by these
Extreme Mass Ratio Inspirals (EMRIs) encodes a detailed map of the black hole
geometry, and the detection and characterization of these signals is a major
scientific goal for the LISA mission. The waveforms produced are very complex,
and the signals need to be coherently tracked for hundreds to thousands of
cycles to produce a detection, making EMRI signals one of the most challenging
data analysis problems in all of gravitational wave astronomy. Estimates for
the number of templates required to perform an exhaustive grid-based
matched-filter search for these signals are astronomically large, and far out
of reach of current computational resources. Here I describe an alternative
approach that employs a hybrid between Genetic Algorithms and Markov Chain
Monte Carlo techniques, along with several time saving techniques for computing
the likelihood function. This approach has proven effective at the blind
extraction of relatively weak EMRI signals from simulated LISA data sets.Comment: 10 pages, 4 figures, Updated for LISA 8 Symposium Proceeding
On the relaxed maximum-likelihood blind MIMO channel estimation for orthogonal space-time block codes
This paper concerns the maximum-likelihood channel estimation for MIMO
systems with orthogonal space-time block codes when the finite alphabet
constraint of the signal constellation is relaxed. We study the channel
coefficients estimation subspace generated by this method. We provide an
algebraic characterisation of this subspace which turns the optimization
problem into a purely algebraic one and more importantly, leads to several
interesting analytical proofs. We prove that with probability one, the
dimension of the estimation subspace for the channel coefficients is
deterministic and it decreases by increasing the number of receive antennas up
to a certain critical number of receive antennas, after which the dimension
remains constant. In fact, we show that beyond this critical number of receive
antennas, the estimation subspace for the channel coefficients is isometric to
a fixed deterministic invariant space which can be easily computed for every
specific OSTB code
A Three-Stage Search for Supermassive Black Hole Binaries in LISA Data
Gravitational waves from the inspiral and coalescence of supermassive
black-hole (SMBH) binaries with masses ~10^6 Msun are likely to be among the
strongest sources for the Laser Interferometer Space Antenna (LISA). We
describe a three-stage data-analysis pipeline designed to search for and
measure the parameters of SMBH binaries in LISA data. The first stage uses a
time-frequency track-search method to search for inspiral signals and provide a
coarse estimate of the black-hole masses m_1, m_2 and of the coalescence time
of the binary t_c. The second stage uses a sequence of matched-filter template
banks, seeded by the first stage, to improve the measurement accuracy of the
masses and coalescence time. Finally, a Markov Chain Monte Carlo search is used
to estimate all nine physical parameters of the binary. Using results from the
second stage substantially shortens the Markov Chain burn-in time and allows us
to determine the number of SMBH-binary signals in the data before starting
parameter estimation. We demonstrate our analysis pipeline using simulated data
from the first LISA Mock Data Challenge. We discuss our plan for improving this
pipeline and the challenges that will be faced in real LISA data analysis.Comment: 12 pages, 3 figures, submitted to Proceedings of GWDAW-11 (Berlin,
Dec. '06
Spread spectrum-based video watermarking algorithms for copyright protection
Merged with duplicate record 10026.1/2263 on 14.03.2017 by CS (TIS)Digital technologies know an unprecedented expansion in the last years. The consumer can
now benefit from hardware and software which was considered state-of-the-art several years
ago. The advantages offered by the digital technologies are major but the same digital
technology opens the door for unlimited piracy. Copying an analogue VCR tape was certainly
possible and relatively easy, in spite of various forms of protection, but due to the analogue
environment, the subsequent copies had an inherent loss in quality. This was a natural way of
limiting the multiple copying of a video material. With digital technology, this barrier
disappears, being possible to make as many copies as desired, without any loss in quality
whatsoever. Digital watermarking is one of the best available tools for fighting this threat.
The aim of the present work was to develop a digital watermarking system compliant with the
recommendations drawn by the EBU, for video broadcast monitoring. Since the watermark
can be inserted in either spatial domain or transform domain, this aspect was investigated and
led to the conclusion that wavelet transform is one of the best solutions available. Since
watermarking is not an easy task, especially considering the robustness under various attacks
several techniques were employed in order to increase the capacity/robustness of the system:
spread-spectrum and modulation techniques to cast the watermark, powerful error correction
to protect the mark, human visual models to insert a robust mark and to ensure its invisibility.
The combination of these methods led to a major improvement, but yet the system wasn't
robust to several important geometrical attacks. In order to achieve this last milestone, the
system uses two distinct watermarks: a spatial domain reference watermark and the main
watermark embedded in the wavelet domain. By using this reference watermark and techniques
specific to image registration, the system is able to determine the parameters of the attack and
revert it. Once the attack was reverted, the main watermark is recovered. The final result is a
high capacity, blind DWr-based video watermarking system, robust to a wide range of attacks.BBC Research & Developmen
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