12,820 research outputs found
Optimizing Throughput Fairness of Cluster-based Cooperation in Underlay Cognitive WPCNs
In this paper, we consider a secondary wireless powered communication network
(WPCN) underlaid to a primary point-to-point communication link. The WPCN
consists of a multi-antenna hybrid access point (HAP) that transfers wireless
energy to a cluster of low-power wireless devices (WDs) and receives sensing
data from them. To tackle the inherent severe user unfairness problem in WPCN,
we consider a cluster-based cooperation where a WD acts as the cluster head
that relays the information of the other WDs. Besides, we apply energy
beamforming technique to balance the dissimilar energy consumptions of the WDs
to further improve the fairness. However, the use of energy beamforming and
cluster-based cooperation may introduce more severe interference to the primary
system than the WDs transmit independently. To guarantee the performance of
primary system, we consider an interference-temperature constraint to the
primary system and derive the throughput performance of each WD under the peak
interference-temperature constraint. To achieve maximum throughput fairness, we
jointly optimize the energy beamforming design, the transmit time allocation
among the HAP and the WDs, and the transmit power allocation of each WD to
maximize the minimum data rate achievable among the WDs (the max-min
throughput). We show that the non-convex joint optimization problem can be
transformed to a convex one and then be efficiently solved using off-the-shelf
convex algorithms. Moreover, we simulate under practical network setups and
show that the proposed method can effectively improve the throughput fairness
of the secondary WPCN, meanwhile guaranteeing the communication quality of the
primary network.Comment: The paper has been submitted for potential journal publication. arXiv
admin note: text overlap with arXiv:1707.0320
An Optimal and Progressive Approach to Online Search of Top-k Influential Communities
Community search over large graphs is a fundamental problem in graph
analysis. Recent studies propose to compute top-k influential communities,
where each reported community not only is a cohesive subgraph but also has a
high influence value. The existing approaches to the problem of top-k
influential community search can be categorized as index-based algorithms and
online search algorithms without indexes. The index-based algorithms, although
being very efficient in conducting community searches, need to pre-compute a
special-purpose index and only work for one built-in vertex weight vector. In
this paper, we investigate on-line search approaches and propose an
instance-optimal algorithm LocalSearch whose time complexity is linearly
proportional to the size of the smallest subgraph that a correct algorithm
needs to access without indexes. In addition, we also propose techniques to
make LocalSearch progressively compute and report the communities in decreasing
influence value order such that k does not need to be specified. Moreover, we
extend our framework to the general case of top-k influential community search
regarding other cohesiveness measures. Extensive empirical studies on real
graphs demonstrate that our algorithms outperform the existing online search
algorithms by several orders of magnitude
Quantitative study of the AMS-02 electron/positron spectra: implications for the pulsar and dark matter properties
The AMS-02 has just published the unprecedentedly precise measurement of the
cosmic electron and positron spectra. In this paper we try to give a
quantitative study on the AMS-02 results by a global fitting to the electron
and positron spectra, together with the updated positron fraction data. The
Markov Chain Monte Carlo algorithm is adopted to do the fitting. The primary
electron spectrum and the parameters for pulsars or dark matter which
contribute extra positrons are determined simultaneously. We find that there is
a hardening of the primary electron spectrum at GeV. With such a new
feature at the background spectrum both the pulsars and dark matter can explain
the AMS-02 results very well. The dark matter scenario shows a drop at positron
fraction at GeV, however, suffers very strong constraints from Fermi
-ray observations. The fitting results also suggest that the
propagation model with convection may be more favored by the lepton data than
the reacceleration model.Comment: 14 pages, 10 figures, 9 tables; new version includes total electron
spectrum data; version accepted by PR
Reusing Wireless Power Transfer for Backscatter-assisted Cooperation in WPCN
This paper studies a novel user cooperation method in a wireless powered
communication network (WPCN), where a pair of closely located devices first
harvest wireless energy from an energy node (EN) and then use the harvested
energy to transmit information to an access point (AP). In particular, we
consider the two energy-harvesting users exchanging their messages and then
transmitting cooperatively to the AP using space-time block codes.
Interestingly, we exploit the short distance between the two users and allow
the information exchange to be achieved by energy-conserving backscatter
technique. Meanwhile the considered backscatter-assisted method can effectively
reuse wireless power transfer for simultaneous information exchange during the
energy harvesting phase. Specifically, we maximize the common throughput
through optimizing the time allocation on energy and information transmission.
Simulation results show that the proposed user cooperation scheme can
effectively improve the throughput fairness compared to some representative
benchmark methods.Comment: The paper has been accepted for publication in MLICOM 201
Geometric, magnetic and electronic properties of folded graphene nanoribbons
Geometric and electronic properties of folded graphene nanoribbons (FGNRs)
are investigated by first-principles calculations. These properties are mainly
dominated by the competition or cooperation among stacking, curvature and edge
effects. For the zigzag FGNRs, the more stable structures are revealed to be AB
stackings, while for the armchair types, AA" stackings are more stable. The
interlayer interactions and hybridization of four orbitals lead to smaller
energy gaps, anti-crossing bands, and more band-edge states. Specifically, the
broken mirror symmetry in the odd-AB stacked zigzag FGNRs is responsible for
the spin-up and spin-down splitting subbands. All FGNRs are direct-gap
semiconductors except that the edge-edge interactions cause the even-AA stacked
zigzag FGNRs to exhibit a pair of metallic linear bands. The width-dependent
energy gaps in the armchair FGNRs can be classified into six groups.
Furthermore, there exist rich features in density of states, including the
form, number, intensity and energy of the special structures
Expectations of the Cosmic Antideuteron Flux
The cosmic antideuteron is a promising probe for the dark matter annihilation
signature. In order to determine the DM signature, the background astrophysical
antideuteron flux should be carefully studied. In this work we provide a new
calculation of the secondary antideuteron flux, and pay special attention to
the uncertainties from hadronic interaction models by using several Monte Carlo
generators. The uncertainties from propagation effects are also carefully
investigated for both the astrophysical background and DM annihilation
signature in several scenarios, which are constrained by the latest B/C ratio
measured by AMS-02. Considering these uncertainties, we find that the secondary
antideuteron flux is hard to detect in the near future detectors. However, the
antideuteron signature from dark matter annihilation will be detectable even
considering the constraint from the AMS-02 observation of the
ratio.Comment: 22 pages, 9 figure
Holographic complexity of the disk subregion in (2+1)-dimensional gapped systems
Using the volume of the space enclosed by the Ryu-Takayanagi (RT) surface, we
study the complexity of the disk-shape subregion (with radius R) in various
(2+1)-dimensional gapped systems with gravity dual. These systems include a
class of toy models with singular IR and the bottom-up models for quantum
chromodynamics and fractional quantum Hall effects. Two main results are: i) in
the large-R expansion of the complexity, the R-linear term is always absent,
similar to the absence of topological entanglement entropy; ii) when the
entanglement entropy exhibits the classic `swallowtail' phase transition, the
complexity is sensitive but reacts differently.Comment: 30 pages, 7 figures, revised version accepted for publication in PR
Wetting and Diffusion of Water on Pristine and Strained Phosphorene
Phosphorene, a newly fabricated two-dimensional (2D) nanomaterial, have
exhibited promising application prospect in biology. Nonetheless, the wetting
and diffusive properties of bio-fluids on phosphorene are still elusive. In
this study, using molecular dynamics (MD) simulations, we investigated the
structural and dynamic properties of water on pristine and strained
phosphorene. The MD simulations illustrated that the diffusion of water
molecules on the phosphorene surface is anisotropic, while strain-enhanced
diffusion is clearly present which arises from strain-induced smooth of the
energy landscape. The contact angle of water droplet on phosphorene exhibited a
nonmonotonic variation with the transverse strain. The structure of water on
transverse stretched phosphorene was demonstrated to be different from that on
longitudinal stretched phosphorene. Moreover, we discovered that the contact
angle of water on strained phosphorene is proportional to the quotient of
longitudinal and transverse diffusion coefficients of interfacial water. These
findings would offer helpful insights in potential ways of manipulating the
wetting and transport of water at nanoscale, and in future bio-applications of
phosphorene.Comment: 8 pages, 6 figure
Possible Effects of Dark Energy on the Detection of Dark Matter Particles
We study in this paper the possible influence of the dark energy on the
detection of the dark matter particles. In models of dark energy described by a
dynamical scalar field such as the Quintessence, its interaction with the dark
matter will cause the dark matter particles such as the neutralino vary as a
function of space and time. Given a specific model of the Quintessence and its
interaction in this paper we calculate numerically the corrections to the
neutralino masses and the induced spectrum of the neutrinos from the
annihilation of the neutralinos pairs in the core of the Sun. This study gives
rise to a possibility of probing for dark energy in the experiments of
detecting the dark matter particles.Comment: 8 pages and 1 figur
Multi-antenna Enabled Cluster-based Cooperation in Wireless Powered Communication Networks
In this paper, we consider a wireless powered communication network (WPCN)
consisting of a multi-antenna hybrid access point (HAP) that transfers wireless
energy to and receives sensing data from a cluster of low-power wireless
devices (WDs). To enhance the throughput performance of some far-away WDs, we
allow one of the WDs to act as the cluster head (CH) that helps forward the
messages of the other cluster members (CMs). However, the performance of the
proposed cluster-based cooperation is fundamentally limited by the high energy
consumption of the CH, who needs to transmit all the WDs' messages including
its own. To tackle this issue, we exploit the capability of multi-antenna
energy beamforming (EB) at the HAP, which can focus more transferred power to
the CH to balance its energy consumption in assisting the other WDs.
Specifically, we first derive the throughput performance of each individual WD
under the proposed scheme. Then, we jointly optimize the EB design, the
transmit time allocation among the HAP and the WDs, and the transmit power
allocation of the CH to maximize the minimum data rate achievable among all the
WDs (the max-min throughput) for improved throughput fairness among the WDs. An
efficient optimal algorithm is proposed to solve the joint optimization
problem. Moreover, we simulate under practical network setups and show that the
proposed multi-antenna enabled cluster-based cooperation can effectively
improve the throughput fairness of WPCN.Comment: This paper has been accepted for publication by IEEE ACCESS journal
in July 201
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