5,288 research outputs found
The Excess Heat Capacity in Glass-forming Liquid Systems Containing Molecules
The excess heat capacity at glass transition temperature in two types of
glass-forming systems of [xNaNO3\cdot(1-x)KNO3]60[Ca(NO3)2]40 (0 \leq x \leq 1)
and Ca(NO3)2\cdotyH2O (4 \leq y \leq 13) is studied. In the former system, with
the replacement of K+ cation with Na+ cation, the excess heat capacity is
almost invariable around 65.1 J\cdotmol-1\cdotK-1, while the excess increases
by 38.9 J\cdotmol-1\cdotK-1 with the increasing per molar H2O content in latter
system. A quantitative description of the excess heat capacity is built up with
the consideration of atomic and molecular translational motion in the
glass-forming systems. This finding might offer further understanding to the
glass transition
Base Station Cooperation in Millimeter Wave Cellular Networks: Performance Enhancement of Cell-Edge Users
Millimeter wave (mmWave) signals are much more sensitive to blockage, which
results in a significant increase of the outage probability, especially for the
users at the edge of the cells. In this paper, we exploit the technique of base
station (BS) cooperation to improve the performance of the cell-edge users in
the downlink transmission of mmWave cellular networks. We design two
cooperative schemes, which are referred to as fixed-number BS cooperation (FNC)
scheme and fixed-region BS cooperation (FRC) scheme, respectively. In FNC
scheme, the cooperative BSs consist of the M nearest BSs around the served
cell-edge users, and in FRC scheme, the cooperative BSs include all the BSs
located within a given region. We derive the expressions for the average rate
and outage probability of a typical cell-edge user located at the origin based
on the stochastic geometry framework. To reduce the computational complexity of
our analytical results for the outage probability, we further propose a Gamma
approximation based method to provide approximations with satisfying accuracy.
Our analytical results incorporate the critical characteristics of mmWave
channels, i.e., the blockage effects, the different path loss of LOS and NLOS
links and the highly directional antenna arrays. Simulation results show that
the performance of the cell-edge users is greatly improved when mmWave networks
are combined with the technique of BS cooperation.Comment: To be published in IEEE Transactions on Communication
Influence of squirt flow on fundamental guided waves propagation in borehole embedded in saturated porous media
In this paper, the reservoir is modeled by homogeneous two-phase media based
on BISQ model. We focus on the effects of the squirt flow on the fundamental
guided waves propagation in borehole embedded in saturated porous media excited
by monopole, dipole and quadrupole point sources. The full waveforms acoustic
logging in a fluid-filled borehole are simulated. The curves of velocity
dispersion, attenuation coefficients and excitation of the fundamental guided
waves have shown that velocity dispersions are almost independent of the
characteristic squirt flow length, attenuations of guided waves are enhanced
due to the squirt flow, and excitations of guided waves are decreased due to
the squirt flow. It is possible to estimate the characteristic squirt flow
length by attenuation coefficients of the guided waves from acoustical logging
data.Comment: all 18 pages 6 figure
Pilot Spoofing Attack by Multiple Eavesdroppers
In this paper, we investigate the design of a pilot spoofing attack (PSA)
carried out by multiple single-antenna eavesdroppers (Eves) in a downlink
time-division duplex (TDD) system, where a multiple antenna base station (BS)
transmits confidential information to a single-antenna legitimate user (LU).
During the uplink channel training phase, multiple Eves collaboratively impair
the channel acquisition of the legitimate link, aiming at maximizing the
wiretapping signal-to-noise ratio (SNR) in the subsequent downlink data
transmission phase. Two different scenarios are investigated: (1) the BS is
unaware of the PSA, and (2) the BS attempts to detect the presence of the PSA.
For both scenarios, we formulate wiretapping SNR maximization problems. For the
second scenario, we also investigate the probability of successful detection
and constrain it to remain below a pre-designed threshold. The two resulting
optimization problems can be unified into a more general non-convex
optimization problem, and we propose an efficient algorithm based on the
minorization-maximization (MM) method and the alternating direction method of
multipliers (ADMM) to solve it. The proposed MM-ADMM algorithm is shown to
converge to a stationary point of the general problem. In addition, we propose
a semidefinite relaxation (SDR) method as a benchmark to evaluate the
efficiency of the MM-ADMM algorithm. Numerical results show that the MM-ADMM
algorithm achieves near-optimal performance and is computationally more
efficient than the SDRbased method.Comment: Accepted by IEEE Transaction on Wireless Communication
The Tip-Induced Twisted Bilayer Graphene Superlattice on HOPG: Capillary Attraction Effect
We use the tip of the scanning tunneling microscope (STM) to manipulate
single weakly bound nanometer-sized sheets on the the highly oriented pyrolytic
graphite (HOPG) surface through artifically increasing the tip and sample
interaction in humid environment. By this means it is possible to tear apart a
graphite sheet againt a step and fold this part onto the HOPG surface and thus
generate the gaphene superlattices with hexagonal symmetry. The tip and sample
surface interactions, including the van der Waals force, eletrostatic force and
capillary attraction force originating from the Laplace pressure due to the
formation of a highly curved fluid meniscus connecting the tip and sample, are
discussed in details to understand the fromation mechnism of graphen
superlattice induced by the STM tip. Especially, the capillary force is the key
role in manipulating the graphite surface sheet in the hunmidity condition. Our
approach may provides a simple and feasible route to prepare the controllable
superlattices and graphene nanoribbons but also replenish and find down the
theory of generation of graphene superlattice on HOPG surface by the tip.Comment: 17 pages, 5 figure
Combating the Control Signal Spoofing Attack in UAV Systems
Unmanned aerial vehicle (UAV) system is vulnerable to the control signal
spoofing attack due to the openness of the wireless communications. In this
correspondence, a physical layer approach is proposed to combat the control
signal spoofing attack, i.e,. to determine whether the received control signal
packet is from the ground control station (GCS) or a potential malicious
attacker (MA), which does not need to share any secret key. We consider the
worst case where the UAV does not have any prior knowledge about the MA.
Utilizing the channel feature of the angles of arrival, the distance-based path
loss, and the Rician- factor, we construct a generalized log-likelihood
radio (GLLR) test framework to handle the problem. Accurate approximations of
the false alarm and successful detection rate are provided to efficiently
evaluate the performance.Comment: To be published in IEEE Transactions on Vehicular Technolog
Identifying the Fake Base Station: A Location Based Approach
Fake base station (FBS) attack is a great security challenge to wireless user
equipment (UE). During the cell selection stage, the UE receives multiple
synchronization signals (SSs) from multiple nearby base stations (BSs), and
then synchronizes itself with the strongest SS. A FBS also can transmit a SS
with sufficient power to confuse the UE, which makes the UE connect to the FBS,
and may lead to the leakage of private information. In this letter,
countermeasure to the FBS attack by utilizing the location information is
investigated. Two location awareness based FBS-resistance schemes are proposed
by checking the received signal strength according to the position of the UE
and a legitimate BS map. The successful cheating rate (SCR) definded as the
probability that the UE will connect to the FBS is investigated. Numeric
results show that with the two proposed schemes, the SCR can be greatly reduced
especially when the transmit power of the FBS is large. Beyond that, a
cooperation aided method is further proposed to improve the performance, and we
show that the cooperation aided method can further suppress the SCR when the
signal strength from the FBS is similar to that from the legitimate BS.Comment: To be published in IEEE communications letter
Passive Beamforming for IRS Aided Wireless Networks
In this letter, we design passive beamforming in an intelligent reflecting
surface (IRS) assisted multiple-user wireless network. Two different scenarios
are considered, namely, multicasting and multi-user downlink transmission. We
optimize the passive beamforming vector of the IRS to maximize the smallest
signal-to-noise ratio of the users in both scenarios. Based on the alternating
direction method of multipliers algorithm, a low complexity method is designed
to iteratively solve the established problem. In each iteration of the proposed
method, the solution is in closed form, and thus the computation complexity is
low. Numerical results are presented to show the efficiency of the proposed
method.Comment: Accepted by IEEE WC
Cooperative Transmission for Physical Layer Security by Exploring Social Awareness
Social awareness and social ties are becoming increasingly fashionable with
emerging mobile and handheld devices. Social trust degree describing the
strength of the social ties has drawn lots of research interests in many fields
including secure cooperative communications. Such trust degree reflects the
users' willingness for cooperation, which impacts the selection of the
cooperative users in the practical networks. In this paper, we propose a
cooperative relay and jamming selection scheme to secure communication based on
the social trust degree under a stochastic geometry framework. We aim to
analyze the involved secrecy outage probability (SOP) of the system's
performance. To achieve this target, we propose a double Gamma ratio (DGR)
approach through Gamma approximation. Based on this, the SOP is tractably
obtained in closed form. The simulation results verify our theoretical
findings, and validate that the social trust degree has dramatic influences on
the network's secrecy performance.Comment: 6 pages, 4 figures, IEEE GLOBECOM 2017 Conferenc
The tunnelling spectra of quasi-free-standing graphene monolayer
With considering the great success of scanning tunnelling microscopy (STM)
studies of graphene in the past few years, it is quite surprising to notice
that there is still a fundamental contradiction about the reported tunnelling
spectra of quasi-free-standing graphene monolayer. Many groups observed V-shape
spectra with linearly vanishing density-of-state (DOS) at the Dirac point,
whereas, the others reported spectra with a gap of 60 meV pinned to the Fermi
level in the quasi-free-standing graphene monolayer. Here we systematically
studied the two contradicted tunnelling spectra of the quasi-free-standing
graphene monolayer on several different substrates and provided a consistent
interpretation about the result. The gap in the spectra arises from the
out-of-plane phonons in graphene, which mix the Dirac electrons at the
Brillouin zone corners with the nearly free-electron states at the zone center.
Our experiment indicated that interactions with substrates could effectively
suppress effects of the out-of-plane phonons in graphene and enable us to
detect only the DOS of the Dirac electrons in the spectra. We also show that it
is possible to switch on and off the out-of-plane phonons of graphene at the
nanoscale, i.e., the tunnelling spectra show switching between the two distinct
features, through voltage pulses applied to the STM tip.Comment: 4 Figure
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