5,147 research outputs found
An Exploration of Error-correcting Codes for use in Noise-prone Satellite Environments
Satellites are crucial for the modern world to function properly as they provide Global Navigation Satellite System (GNSS) and global communication. However, the data that is stored on these satellites can be corrupted by the radiation found in space, and its bits can be improperly flipped. In the past, Forward Error Correction (FEC) algorithms were selected based on their strength and implemented to correct these bit flips back to their original values. This thesis seeks to determine if the strength of the FEC algorithms Reed Solomon (RS) code and Reed Solomon Product Code (RSPC) directly translates to their effectiveness. These algorithms were coded and tested in Matrix Laboratory (MATLAB) and on a Field Programmable Gate Array (FPGA) under controlled parameters, including the data set sizes, number of bit flips introduced, and the distribution of the bit flips within the data set. From the experiment\u27s results, these other factors significantly influenced the effectiveness of the algorithms as well. Knowing what factors influence the algorithm\u27s effectiveness enable better decision making as to which FEC algorithm to use for a given set of circumstances. The RS codes should be used if the size of the data set is small enough for a single-instance RS code and the range of expected bit flips is narrow and lower than the code\u27s correctable limit. If the data set is large or the range of expected bit flips varies widely and surpasses the RS code\u27s correctable limit, the RSPC should be used for a higher overall success rate in exchange for a lower number of bit flips with a 100% correction rate
Electronic structure of YbB: Is it a Topological Insulator or not?
To resolve the controversial issue of the topological nature of the
electronic structure of YbB, we have made a combined study using density
functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES).
Accurate determination of the low energy band topology in DFT requires the use
of modified Becke-Johnson exchange potential incorporating the spin-orbit
coupling and the on-site Coulomb interaction of Yb electrons as large
as 7 eV. We have double-checked the DFT result with the more precise GW band
calculation. ARPES is done with the non-polar (110) surface termination to
avoid band bending and quantum well confinement that have confused ARPES
spectra taken on the polar (001) surface termination. Thereby we show
definitively that YbB has a topologically trivial B 2-Yb 5
semiconductor band gap, and hence is a non-Kondo non-topological insulator
(TI). In agreement with theory, ARPES shows pure divalency for Yb and a -
band gap of 0.3 eV, which clearly rules out both of the previous scenarios of
- band inversion Kondo TI and - band inversion non-Kondo TI. We
have also examined the pressure-dependent electronic structure of YbB,
and found that the high pressure phase is not a Kondo TI but a
\emph{p}-\emph{d} overlap semimetal.Comment: The main text is 6 pages with 4 figures, and the supplementary
information contains 6 figures. 11 pages, 10 figures in total To be appeared
in Phys. Rev. Lett. (Online publication is around March 16 if no delays.
Attack Prevention for Collaborative Spectrum Sensing in Cognitive Radio Networks
Collaborative spectrum sensing can significantly improve the detection
performance of secondary unlicensed users (SUs). However, the performance of
collaborative sensing is vulnerable to sensing data falsification attacks,
where malicious SUs (attackers) submit manipulated sensing reports to mislead
the fusion center's decision on spectrum occupancy. Moreover, attackers may not
follow the fusion center's decision regarding their spectrum access. This paper
considers a challenging attack scenario where multiple rational attackers
overhear all honest SUs' sensing reports and cooperatively maximize attackers'
aggregate spectrum utilization. We show that, without attack-prevention
mechanisms, honest SUs are unable to transmit over the licensed spectrum, and
they may further be penalized by the primary user for collisions due to
attackers' aggressive transmissions. To prevent such attacks, we propose two
novel attack-prevention mechanisms with direct and indirect punishments. The
key idea is to identify collisions to the primary user that should not happen
if all SUs follow the fusion center's decision. Unlike prior work, the proposed
simple mechanisms do not require the fusion center to identify and exclude
attackers. The direct punishment can effectively prevent all attackers from
behaving maliciously. The indirect punishment is easier to implement and can
prevent attacks when the attackers care enough about their long-term reward.Comment: 37 pages including 7 figures and 2 tables; IEEE Journal on Selected
Areas in Communications with special issue in Cooperative Networking -
Challenges and Applications (2012 expected
Effect of sintering temperature under high pressure in the uperconductivity for MgB2
We report the effect of the sintering temperature on the superconductivity of
MgB2 pellets prepared under a high pressure of 3 GPa. The superconducting
properties of the non-heated MgB2 in this high pressure were poor. However, as
the sintering temperature increased, the superconducting properties were vastly
enhanced, which was shown by the narrow transition width for the resistivity
and the low-field magnetizations. This shows that heat treatment under high
pressure is essential to improve superconducting properties. These changes were
found to be closely related to changes in the surface morphology observed using
scanning electron microscopy.Comment: 3 Pages including 3 figure
Spectroscopic Evidence for Anisotropic S-Wave Pairing Symmetry in MgB2
Scanning tunneling spectroscopy of superconducting MgB ( K)
were studied on high-density pellets and c-axis oriented films. The sample
surfaces were chemically etched to remove surface carbonates and hydroxides,
and the data were compared with calculated spectra for all symmetry-allowed
pairing channels. The pairing potential () is best described by an
anisotropic s-wave pairing model, with , where is the angle relative to the
crystalline c-axis, meV, and meV.Comment: 4 pages and 3 figures. Submitted to Physical Review Letters.
Corresponding author: Nai-Chang Yeh (e-mail: [email protected]
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