4,197 research outputs found
Scattering by a periodic array of subwavelength slits II: surface bound state, total transmission and field enhancement in homogenization regimes
This is the second part in a series of two papers that concern with the
quantitative analysis of the electromagnetic field enhancement and anomalous
diffraction by a periodic array of subwavelength slits. In this part, we
explore the scattering problem in the homogenization regimes, where the size of
the period is much smaller than the incident wavelength. In particular, two
homogenization regimes are investigated, where the size of the pattered slits
has the same order as the size of the period in the first configuration, and
the size of the slit is much smaller than the size of the period in the second
configuration. By presenting rigorous asymptotic analysis, we demonstrate that
surface plasmonic effect mimicking that of plasmonic metals occurs in the first
homogenization regime. The corresponding dispersion curve lies below the light
line and the associated eigenmodes are surface bound sates. In addition, for
the incident plane wave, we discover and justify a novel phenomenon of total
transmission which occurs either at certain frequencies for all incident
angles, or at a special incident angle but for all frequencies. For the second
homogenization regime, the non-resonant field enhancement is investigated, and
it is shown that the fast transition of the magnetic field in the slit induces
strong electric field enhancement. Moreover, the enhancement becomes stronger
when the coupling of the slits is weaker.Comment: 41 page
Scattering by a periodic array of subwavelength slits I: field enhancement in the diffraction regime
This is the first part in a series of two papers that concern with the
quantitative analysis of the electromagnetic field enhancement and anomalous
diffraction by a periodic array of subwavelength slits. The scattering problem
in the diffraction regime is investigated in this part, for which the size of
the period is comparable to the incident wavelength. We distinguish scattering
resonances and real eigenvalues, and derive their asymptotic expansions when
they are away from the Rayleigh cut-off frequencies. Furthermore, we present
quantitative analysis of the field enhancement at resonant frequencies, by
quantifying both the enhancement order and the associated resonant modes. The
field enhancement near the Rayleigh cut-off frequencies is also investigated.
It is demonstrated that the field enhancement becomes weaker at the resonant
frequency if it is close to the Rayleigh cut-off frequencies. Finally, we also
characterize the embedded eigenvalues for the underlying periodic structure,
and point out that transmission anomaly such as Fano resonant phenomenon does
not occur for the narrow slit array.Comment: 34 pages, 3 figure
Uncoordinated Frequency Shifts based Pilot Contamination Attack Detection
Pilot contamination attack is an important kind of active eavesdropping
activity conducted by a malicious user during channel training phase. In this
paper, motivated by the fact that frequency asynchronism could introduce
divergence of the transmitted pilot signals between intended user and attacker,
we propose a new uncoordinated frequency shift (UFS) scheme for detection of
pilot contamination attack in multiple antenna system. An attack detection
algorithm is further developed based on source enumeration method. Both the
asymptotic detection performance analysis and numerical results are provided to
verify the proposed studies. The results demonstrate that the proposed UFS
scheme can achieve comparable detection performance as the existing
superimposed random sequence based scheme, without sacrifice of legitimate
channel estimation performance
A super-resolution imaging approach via subwavelength hole resonances
This work presents a new super-resolution imaging approach by using
subwavelength hole resonances. We employ a subwavelength structure in which an
array of tiny holes are etched in a metallic slab with the neighboring distance
that is smaller than half of the wavelength. By tuning the incident wave
at resonant frequencies, the subwavelength structure generates strong
illumination patterns that are able to probe both low and high spatial
frequency components of the imaging sample sitting above the structure. The
image of the sample is obtained by performing stable numerical reconstruction
from the far-field measurement of the diffracted wave. It is demonstrated that
a resolution of can be obtained for reconstructed images, thus one can
achieve super-resolution by arranging multiple holes within one wavelength.
The proposed approach may find applications in wave-based imaging such as
electromagnetic and ultrasound imaging. It attains two advantages that are
important for practical realization. It avoids the difficulty to control the
distance the between the probe and the sample surface with high precision. In
addition, the numerical reconstructed images are very stable against noise by
only using the low frequency band of the far-field data in the numerical
reconstruction
Permissive Supervisor Synthesis for Markov Decision Processes through Learning
This paper considers the permissive supervisor synthesis for probabilistic
systems modeled as Markov Decision Processes (MDP). Such systems are prevalent
in power grids, transportation networks, communication networks and robotics.
Unlike centralized planning and optimization based planning, we propose a novel
supervisor synthesis framework based on learning and compositional model
checking to generate permissive local supervisors in a distributed manner. With
the recent advance in assume-guarantee reasoning verification for probabilistic
systems, building the composed system can be avoided to alleviate the state
space explosion and our framework learn the supervisors iteratively based on
the counterexamples from verification. Our approach is guaranteed to terminate
in finite steps and to be correct
A mathematical theory for Fano resonance in a periodic array of narrow slits
This work concerns resonant scattering by a perfectly conducting slab with
periodically arranged subwavelength slits, with two slits per period. There are
two classes of resonances, corresponding to poles of a scattering problem. A
sequence of resonances has an imaginary part that is nonzero and on the order
of the width of the slits; these are associated with Fabry-Perot
resonance, with field enhancement of order in the slits. The
focus of this study is another class of resonances which become real valued at
normal incidence, when the Bloch wavenumber is zero. These are
embedded eigenvalues of the scattering operator restricted to a period cell,
and the associated eigenfunctions extend to surface waves of the slab that lie
within the radiation continuum. When , the real embedded
eigenvalues will be perturbed as complex-valued resonances, which induce the
Fano resonance phenomenon. We derive the asymptotic expansions of embedded
eigenvalues and their perturbations as resonances when the Bloch wavenumber
becomes nonzero. Based on the quantitative analysis of the diffracted field, we
prove that the Fano-type anomalies occurs for the transmission of energy
through the slab, and show that the field enhancement is of order
, which is stronger than Fabry-Perot resonance
Frequency Synchronization for Uplink Massive MIMO Systems
In this paper, we propose a frequency synchronization scheme for multiuser
orthogonal frequency division multiplexing (OFDM) uplink with a large-scale
uniform linear array (ULA) at base station (BS) by exploiting the angle
information of users. Considering that the incident signal at BS from each user
can be restricted within a certain angular spread, the proposed scheme could
perform carrier frequency offset (CFO) estimation for each user individually
through a \textit{joint spatial-frequency alignment} procedure and can be
completed efficiently with the aided of fast Fourier transform (FFT). A
multi-branch receive beamforming is further designed to yield an equivalent
single user transmission model for which the conventional single-user channel
estimation and data detection can be carried out. To make the study complete,
the theoretical performance analysis of the CFO estimation is also conducted.
We further develop a user grouping scheme to deal with the unexpected scenarios
that some users may not be separated well from the spatial domain. Finally,
various numerical results are provided to verify the proposed studies
Symbol Detection of Ambient Backscatter Systems with Manchester Coding
Ambient backscatter communication is a newly emerged paradigm, which utilizes
the ambient radio frequency (RF) signal as the carrier to reduce the system
battery requirement, and is regarded as a promising solution for enabling large
scale deployment of future Internet of Things (IoT) networks. The key issue of
ambient backscatter communication systems is how to perform reliable detection.
In this paper, we propose novel encoding methods at the information tag, and
devise the corresponding symbol detection methods at the reader. In particular,
Manchester coding and differential Manchester coding are adopted at the
information tag, and the corresponding semi-coherent Manchester (SeCoMC) and
non-coherent Manchester (NoCoMC) detectors are developed. In addition,
analytical bit error rate (BER) expressions are characterized for both
detectors assuming either complex Gaussian or unknown deterministic ambient
signal. Simulation results show that the BER performance of unknown
deterministic ambient signal is better, and the SeCoMC detector outperforms the
NoCoMC detector. Finally, compared with the prior detectors for ambient
backscatter communications, the proposed detectors have the advantages of
achieving superior BER performance with lower communication delay.Comment: accepted by IEEE transaction on wireless communicatio
Magnetization of potassium doped p-terphenyl and p-quaterphenyl by high pressure synthesis
By using high pressure synthesis method, we have fabricated the potassium
doped para-terphenyl. The temperature dependence of magnetization measured in
both zero-field-cooled and field-cooled processes shows step like transitions
at about 125 K. This confirms earlier report about the possible
superconductivity like transition in the same system. However, the
magnetization hysteresis loop exhibits a weak ferromagnetic background. After
removing this ferromagnetic background, a Meissner effect like magnetic
shielding can be found. A simple estimate on the diamagnetization of this step
tells that the diamagnetic volume is only about 0.0427% at low temperatures, if
we assume the penetration depth is much smaller than the size of possible
superconducting grains. This magnetization transition does not shift with
magnetic field but is suppressed and becomes almost invisible above 1.0 T. The
resistivity measurements are failed because of an extremely large resistance.
By using the same method, we also fabricated the potassium doped
para-quaterphenyl. A similar step like transition at about 125 K was also
observed by magnetization measurement. Since there is an unknown positive
background and the diamagnetic volume is too small, it is insufficient to
conclude that this step is derived from superconductivity although it looks
like.Comment: 4 pages, 3 figure
Magnetism and Superconductivity in Iron-based Superconductors Decided by Condensed Particle-hole Excitations away from the Fermi Level
The origin of magnetism and superconductivity in iron-based superconductors
is still unclear. Here, by investigating the momentum-dependent particle-hole
excitations which quantify the tendency of itinerant electrons towards various
magnetic states or superconducting phases, we unravel a novel origin to account
for the variety of physical properties of iron-based compounds. We show that
condensation of particle-hole excitations away from the Fermi surface in
momentum space is the underlying mechanism in deciding the magnetic and
superconducting properties of iron-based materials. The applicability of this
scenario to the whole family of iron-based superconductors suggests that
inclusion of the orbital degrees of freedom, which may lead to competing
tendencies towards different magnetically ordered states, is more crucial than
taking into account the strong correlations. Our findings further indicate that
in order to properly model these materials, the electronic states away from the
Fermi level have to be considered.Comment: 10 pages, 6 figure
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