35,444 research outputs found
Energy Efficiency of Downlink Transmission Strategies for Cloud Radio Access Networks
This paper studies the energy efficiency of the cloud radio access network
(C-RAN), specifically focusing on two fundamental and different downlink
transmission strategies, namely the data-sharing strategy and the compression
strategy. In the data-sharing strategy, the backhaul links connecting the
central processor (CP) and the base-stations (BSs) are used to carry user
messages -- each user's messages are sent to multiple BSs; the BSs locally form
the beamforming vectors then cooperatively transmit the messages to the user.
In the compression strategy, the user messages are precoded centrally at the
CP, which forwards a compressed version of the analog beamformed signals to the
BSs for cooperative transmission. This paper compares the energy efficiencies
of the two strategies by formulating an optimization problem of minimizing the
total network power consumption subject to user target rate constraints, where
the total network power includes the BS transmission power, BS activation
power, and load-dependent backhaul power. To tackle the discrete and nonconvex
nature of the optimization problems, we utilize the techniques of reweighted
minimization and successive convex approximation to devise provably
convergent algorithms. Our main finding is that both the optimized data-sharing
and compression strategies in C-RAN achieve much higher energy efficiency as
compared to the non-optimized coordinated multi-point transmission, but their
comparative effectiveness in energy saving depends on the user target rate. At
low user target rate, data-sharing consumes less total power than compression,
however, as the user target rate increases, the backhaul power consumption for
data-sharing increases significantly leading to better energy efficiency of
compression at the high user rate regime.Comment: 14 pages, 8 figures, accepted by JSAC Energy-Efficient Techniques for
5G Wireless Communication Systems special issu
Coronal and Chromospheric Signatures of Large-Scale Disturbances Associated with a Major Solar Eruption
We present both coronal and chromospheric observations of large-scale
disturbances associated with a major solar eruption on 2005 September 7. In
GOES/SXI, arclike coronal brightenings are recorded propagating in the southern
hemisphere. The SXI front shows an initially constant speed of 730 km s
and decelerates later on, and its center is near the central position angle of
the associated coronal mass ejection (CME) but away from flare site.
Chromospheric signatures of the disturbances are observed in both MLSO/PICS
H and MLSO/CHIP He I 10830 {\AA}, and can be divided into two parts.
The southern signatures occur in regions where the SXI front sweeps over, with
the H bright front coincident with the SXI front while the He I dark
front lagging the SXI front but showing a similar kinematics. Ahead of the path
of the southern signatures, oscillations of a filament are observed. The
northern signatures occur near the equator, with the H and He I fronts
coincident with each other. They first propagate westward, and then deflect to
the north at the boundary of an equatorial coronal hole (CH). Based on these
observational facts, we suggest that the global disturbances are associated
with the CME lift-off, and show a hybrid nature: a mainly non-wave CME flank
nature for the SXI signatures and the corresponding southern chromospheric
signatures, and a shocked fast-mode coronal magnetohydrodynamics (MHD) wave
nature for the northern chromospheric signatures.Comment: 9 pages, 7 figures, accepted for publication in Ap
Sparse Beamforming and User-Centric Clustering for Downlink Cloud Radio Access Network
This paper considers a downlink cloud radio access network (C-RAN) in which
all the base-stations (BSs) are connected to a central computing cloud via
digital backhaul links with finite capacities. Each user is associated with a
user-centric cluster of BSs; the central processor shares the user's data with
the BSs in the cluster, which then cooperatively serve the user through joint
beamforming. Under this setup, this paper investigates the user scheduling, BS
clustering and beamforming design problem from a network utility maximization
perspective. Differing from previous works, this paper explicitly considers the
per-BS backhaul capacity constraints. We formulate the network utility
maximization problem for the downlink C-RAN under two different models
depending on whether the BS clustering for each user is dynamic or static over
different user scheduling time slots. In the former case, the user-centric BS
cluster is dynamically optimized for each scheduled user along with the
beamforming vector in each time-frequency slot, while in the latter case the
user-centric BS cluster is fixed for each user and we jointly optimize the user
scheduling and the beamforming vector to account for the backhaul constraints.
In both cases, the nonconvex per-BS backhaul constraints are approximated using
the reweighted l1-norm technique. This approximation allows us to reformulate
the per-BS backhaul constraints into weighted per-BS power constraints and
solve the weighted sum rate maximization problem through a generalized weighted
minimum mean square error approach. This paper shows that the proposed dynamic
clustering algorithm can achieve significant performance gain over existing
naive clustering schemes. This paper also proposes two heuristic static
clustering schemes that can already achieve a substantial portion of the gain.Comment: 14 pages, 9 figures, to appear in IEEE Access, Special Issue on
Recent Advances in Cloud Radio Access Networks, 201
750 GeV Di-photon Excess and Strongly First-Order Electroweak Phase Transition
A new scalar particle, coupled to photons and gluons via loops of vector-like
quarks, provides a simple theoretical interpretation of the 750 GeV diphoton
excess reported by the experiments at the Large Hadron Collider (LHC). In this
paper, we show that this model contains a large, phenomenologically viable
parameter space region in which the electroweak phase transition (EWPT) is
strongly first-order, opening the possibility that electroweak baryogenesis
mechanism can be realized in this context. A large coupling between the Higgs
doublet and the heavy scalar, required for a strongly first-order EWPT, can
arise naturally in composite Higgs models. The scenario makes robust
predictions that will be tested in near-future experiments. The cross section
of resonant di-Higgs production at the 13 TeV LHC is predicted to be at least
20 fb, while the Higgs cubic self-coupling is enhanced by 40% or more with
respect to its Standard Model (SM) value.Comment: 7 pages, 4 figures, final version published in Physical Review
A Framework of Constraint Preserving Update Schemes for Optimization on Stiefel Manifold
This paper considers optimization problems on the Stiefel manifold
, where is the variable and
is the -by- identity matrix. A framework of constraint preserving
update schemes is proposed by decomposing each feasible point into the range
space of and the null space of . While this general
framework can unify many existing schemes, a new update scheme with low
complexity cost is also discovered. Then we study a feasible
Barzilai-Borwein-like method under the new update scheme. The global
convergence of the method is established with an adaptive nonmonotone line
search. The numerical tests on the nearest low-rank correlation matrix problem,
the Kohn-Sham total energy minimization and a specific problem from statistics
demonstrate the efficiency of the new method. In particular, the new method
performs remarkably well for the nearest low-rank correlation matrix problem in
terms of speed and solution quality and is considerably competitive with the
widely used SCF iteration for the Kohn-Sham total energy minimization.Comment: 29 pages, 1 figur
Multicolor Graphene Nanoribbon/Semiconductor Nanowire Heterojunction Light-Emitting Diodes
We report novel graphene nanoribbon (GNR)/semiconductor nanowire (SNW)
heterojunction light-emitting diodes (LEDs) for the first time. The GNR and SNW
have a face-to-face contact structure, which has the merit of bigger active
region. ZnO, CdS, and CdSe NWs were employed in our case. At forward biases,
the GNR/SNW heterjunction LEDs could emit light with wavelengths varying from
ultraviolet (380 nm) to green (513 nm) to red (705 nm), which were determined
by the band-gaps of the involved SNWs. The mechanism of light emitting for the
GNR/SNW heterojunction LED was discussed. Our approach can easily be extended
to other semiconductor nano-materials. Moreover, our achievement opens the door
to next-generation display technologies, including portable, "see-through", and
conformable products.Comment: 12 pages, 4 figure
Extremal ergodic measures and the finiteness property of matrix semigroups
Let \bS=\{S_1,...,S_K\} be a finite set of complex matrices and
the compact space of all one-sided infinite sequences
i_{\bcdot}\colon\mathbb{N}\rightarrow\{1,...,K\}. An ergodic probability
of the Markov shift
\theta\colon\varSigma_{K}^+\rightarrow\varSigma_{K}^+;\ i_{\bcdot}\mapsto
i_{\bcdot+1}, is called "extremal" for \bS, if
{\rho}(\bS)=\lim_{n\to\infty}\sqrt[n]{\norm{S_{i_1}...S_{i_n}}} holds for
-a.e. i_{\bcdot}\in\varSigma_{K}^+, where \rho(\bS) denotes the
generalized/joint spectral radius of \bS. Using extremal norm and Kingman
subadditive ergodic theorem, it is shown that \bS has the spectral finiteness
property (i.e. \rho(\bS)=\sqrt[n]{\rho(S_{i_1}...S_{i_n})} for some
finite-length word ) if and only if for some extremal measure
of \bS, it has at least one periodic density point
i_{\bcdot}\in\varSigma_{K}^+.Comment: 9 pages; accepted by Proceedings of the AM
Are optical and X-ray AGN mostly disjoint?
The relationship between the populations of optically and X-ray selected
Active Galactic Nuclei (AGN) has been unclear due to divergent results from
different studies. Arnold et al. (2009) claim that X-ray AGN are almost
entirely disjoint from optical AGN, while the Swift-BAT 70-month hard X-ray
survey reported that 553 of their 711 X-ray AGN are optical. In this work, we
set out to understand this difference by cross-checking between these studies
and examining their sampling and AGN-selection criteria. We also re-analyze the
X-ray and optical AGN in 16 groups and clusters reported by Arnold et al. using
our own optical spectrum fitting techniques. We find that 6 of the 8 X-ray AGN
in the Arnold et al. sample are also optical AGN, contrary to Arnold et al.'s
report that only 1 of the 8 X-ray AGN is also an optical AGN, thereby falsifies
their conclusion that optical and X-ray AGN are nearly disjoint sets
Existence and Uniqueness of Tronqu\'ee Solutions of the Third and Fourth Painlev\'e Equations
It is well-known that the first and second Painlev\'e equations admit
solutions characterised by divergent asymptotic expansions near infinity in
specified sectors of the complex plane. Such solutions are pole-free in these
sectors and called tronqu\'ee solutions by Boutroux. In this paper, we show
that similar solutions exist for the third and fourth Painlev\'e equations as
well.Comment: 20 page
Topic2Vec: Learning Distributed Representations of Topics
Latent Dirichlet Allocation (LDA) mining thematic structure of documents
plays an important role in nature language processing and machine learning
areas. However, the probability distribution from LDA only describes the
statistical relationship of occurrences in the corpus and usually in practice,
probability is not the best choice for feature representations. Recently,
embedding methods have been proposed to represent words and documents by
learning essential concepts and representations, such as Word2Vec and Doc2Vec.
The embedded representations have shown more effectiveness than LDA-style
representations in many tasks. In this paper, we propose the Topic2Vec approach
which can learn topic representations in the same semantic vector space with
words, as an alternative to probability. The experimental results show that
Topic2Vec achieves interesting and meaningful results.Comment: 6 pages, 3 figure
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