9,232 research outputs found
Group Communication Over LTE : A Radio Access Perspective
Long Term Evolution (LTE), which has its root on commercial mobile
communications, recently becomes an influential solution to future public
safety communications. To verify the feasibility of LTE for public safety, it
is essential to investigate whether an LTE system optimized for one-to-one
communications is capable of providing group communication, which is one of the
most important service concepts in public safety. In general, a number of first
responders for public safety need to form a group for communicating with each
other or sharing the common data for collaboration on their mission. In this
article, we analyze how the current LTE system can support group communication
in a radio access aspect. Based on the requirements for group communication, we
validate whether each LTE-enabled radio access method can efficiently support
group communication. In addition, we propose a new multicast transmission
scheme, termed index-coded Hybrid Automatic Retransmission reQuest (HARQ). By
applying the index coding concept to HARQ operations, we show that the LTE
system can provide group communication more sophisticatedly in terms of radio
resource efficiency and scalability. We finally evaluate the performance of
LTE-enabled group communication using several radio access methods and show how
the proposed transmission scheme brings the performance enhancement via system
level simulations.Comment: will be published in IEEE Communications Magazin
Landau Level Quantization and Almost Flat Modes in Three-dimensional Semi-metals with Nodal Ring Spectra
We investigate novel Landau level structures of semi-metals with nodal ring
dispersions. When the magnetic field is applied parallel to the plane in which
the ring lies, there exist almost non-dispersive Landau levels at the Fermi
level (E_F = 0) as a function of the momentum along the field direction inside
the ring. We show that the Landau levels at each momentum along the field
direction can be described by the Hamiltonian for the graphene bilayer with
fictitious inter-layer couplings under a tilted magnetic field. Near the center
of the ring where the inter-layer coupling is negligible, we have Dirac Landau
levels which explain the appearance of the zero modes. Although the inter-layer
hopping amplitudes become finite at higher momenta, the splitting of zero modes
is exponentially small and they remain almost flat due to the finite artificial
in-plane component of the magnetic field. The emergence of the density of
states peak at the Fermi level would be a hallmark of the ring dispersion.Comment: 5 pages, 2 figure
Anisotropic density fluctuations, plasmons, and Friedel oscillations in nodal line semimetal
Motivated by recent experimental efforts on three-dimensional semimetals, we
investigate the static and dynamic density response of the nodal line semimetal
by computing the polarizability for both undoped and doped cases. The nodal
line semimetal in the absence of doping is characterized by a ring-shape zero
energy contour in momentum space, which may be considered as a collection of
Dirac points. In the doped case, the Fermi surface has a torus shape and two
independent processes of the momentum transfer contribute to the singular
features of the polarizability even though we only have a single Fermi surface.
In the static limit, there exist two independent singularities in the second
derivative of the static polarizability. This results in the highly anisotropic
Friedel oscillations which show the angle-dependent algebraic power law and the
beat phenomena in the oscillatory electron density near a charged impurity.
Furthermore, the dynamical polarizability has two singular lines along
and , where is
the angle between the external momentum and the plane where the nodal
ring lies. From the dynamical polarizability, we obtain the plasmon modes in
the doped case, which show anisotropic dispersions and angle-dependent plasma
frequencies. Qualitative differences between the low and high doping regimes
are discussed in light of future experiments.Comment: 11 pages, 6 figures, Minor revisions are mad
Statistical Properties of Galactic {\delta} Scuti Stars: Revisited
We present statistical characteristics of 1,578 {\delta} Scuti stars
including nearby field stars and cluster member stars within the Milky Way. We
obtained 46% of these stars (718 stars) from the works done by Rodr\'{i}guez
and collected the remaining 54% stars (860 stars) from other literatures. We
updated the entries with the latest information of sky coordinate, color,
rotational velocity, spectral type, period, amplitude and binarity. The
majority of our sample are well characterized in terms of typical period range
(0.02-0.25 days), pulsation amplitudes (<0.5 mag) and spectral types (A-F
type). Given this list of {\delta} Scuti stars, we examined relations between
their physical properties (i.e., periods, amplitudes, spectral types and
rotational velocities) for field stars and cluster members, and confirmed that
the correlations of properties are not significantly different from those
reported in the Rodr\'{i}guez's works. All the {\delta} Scuti stars are
cross-matched with several X-ray and UV catalogs, resulting in 27 X-ray and 41
UV-only counterparts. These counterparts are interesting targets for further
study because of their rarity and uniqueness in showing {\delta} Scuti-type
variability and X-ray/UV emission at the same time. The compiled catalog can be
accessed through the web interface http://stardb.yonsei.ac.kr/DeltaScutiComment: 15 pages, 8 figures, 6 tables. Accepted for publication in AJ. The
catalog is available at http://stardb.yonsei.ac.kr/DeltaScut
A Personalized Preference Learning Framework for Caching in Mobile Networks
This paper comprehensively studies a content-centric mobile network based on
a preference learning framework, where each mobile user is equipped with a
finite-size cache. We consider a practical scenario where each user requests a
content file according to its own preferences, which is motivated by the
existence of heterogeneity in file preferences among different users. Under our
model, we consider a single-hop-based device-to-device (D2D) content delivery
protocol and characterize the average hit ratio for the following two file
preference cases: the personalized file preferences and the common file
preferences. By assuming that the model parameters such as user activity
levels, user file preferences, and file popularity are unknown and thus need to
be inferred, we present a collaborative filtering (CF)-based approach to learn
these parameters. Then, we reformulate the hit ratio maximization problems into
a submodular function maximization and propose two computationally efficient
algorithms including a greedy approach to efficiently solve the cache
allocation problems. We analyze the computational complexity of each algorithm.
Moreover, we analyze the corresponding level of the approximation that our
greedy algorithm can achieve compared to the optimal solution. Using a
real-world dataset, we demonstrate that the proposed framework employing the
personalized file preferences brings substantial gains over its counterpart for
various system parameters.Comment: 21 pages, 10 figures, 1 table, to appear in the IEEE Transactions on
Mobile Computin
Interference-Aware Opportunistic Random Access in Dense IoT Networks
It is a challenging task to design a random access protocol that achieves the
optimal throughput in multi-cell random access with decentralized transmission
due to the difficulty of coordination. In this paper, we present a
decentralized interference-aware opportunistic random access (IA-ORA) protocol
that enables us to obtain the optimal throughput scaling in an ultra-dense
multi-cell random access network with one access point (AP) and a number of
users. In sharp contrast to opportunistic scheduling for cellular multiple
access where users are selected by base stations, under the IA-ORA protocol,
each user opportunistically transmits with a predefined physical layer (PHY)
data rate in a decentralized manner if not only the desired signal power to the
serving AP is sufficiently large but also the generating interference leakage
power to the other APs is sufficiently small (i.e., two threshold conditions
are fulfilled). As a main result, it is shown that the optimal aggregate
throughput scaling (i.e., the MAC throughput of in a cell and the
power gain) is achieved in a high signal-to-noise ratio regime if the number of
per-cell users exceeds some level. Additionally, it is numerically demonstrated
via computer simulations that under practical settings, the proposed IA-ORA
protocol outperforms conventional opportunistic random access protocols in
terms of aggregate throughput.Comment: 20 pages, 10 figures, 3 tables. Published in the IEEE Acces
Entropy of black holes in topologically massive gravity
We study the issue of black hole entropy in the topologically massive
gravity. Assuming that the presence of gravitational Chern-Simons term with the
coupling does modify the horizon radius , we propose
as the Bekenstein-Hawking entropy. This
entropy of CS-BTZ black hole satisfies the first-law of thermodynamics and the
area-law but it is slightly different from the shifted-entropy based on the BTZ black hole with outer
and inner horizon . In the case of , represents
the entropy of non-rotating BTZ black hole with the Chern-Simons term
(NBTZ-CS), while reduces to the entropy of NBTZ black hole. It shows that
may be a candidate for the entropy of the CS-BTZ black hole.Comment: 11 pages, an expanded version to discuss thermodynamic aspects of
BTZ-CS and CS-BTZ black hole
Entropic force versus temperature force
We introduce the cavity enclosing a source mass to define the temperature
force. Starting with the Tolman temperature in the stationary spacetime, we
find a non-relativistic temperature with the
Newtonian potential . This temperature could be also derived from the
Tolman-Ehrenfest effect, satisfying a relation of
with the local temperature . Finally, we derive the temperature force
which leads to the Newtonian force
law without introducing the holographic screen defined by holographic principle
and equipartition law for entropic force.Comment: 10 pages, 2 figure
Critical spin superflow in a spinor Bose-Einstein condensate
We investigate the critical dynamics of spin superflow in an easy-plane
antiferromagnetic spinor Bose-Einstein condensate. Spin-dipole oscillations are
induced in a trapped condensate by applying a linear magnetic field gradient
and we observe that the damping rate increases rapidly as the field gradient
increases above a certain critical value. The onset of dissipation is found to
be associated with the generation of dark-bright solitons due to the modulation
instability of the counterflow of two spin components. Spin turbulence emerges
as the solitons decay because of their snake instability. We identify another
critical point for spin superflow, in which transverse magnon excitations are
dynamically generated via spin-exchanging collisions, which leads to the
transient formation of axial polar spin domains.Comment: 5 pages, 5 figure
Symmetry breaking and effective photon mass
We suggest a possibility that the photon can acquire a finite mass in a
medium when the external interaction and symmetry is broken on the basis of
Chern-Simons gauge theory
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