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 $\ell_1$ 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$^{-1}$ 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$\alpha$ 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$\alpha$ 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$\alpha$ 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 $X^{\mathsf{T}}X=I_p$, where $X\in \mathbb{R}^{n \times p}$ is the variable and $I_p$ is the $p$-by-$p$ identity matrix. A framework of constraint preserving update schemes is proposed by decomposing each feasible point into the range space of $X$ and the null space of $X^{\mathsf{T}}$. 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 $d\times d$ matrices and $\varSigma_{K}^+$ the compact space of all one-sided infinite sequences i_{\bcdot}\colon\mathbb{N}\rightarrow\{1,...,K\}. An ergodic probability $\mu_*$ 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 $\mu_*$-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 $(i_1,...,i_n)$) if and only if for some extremal measure $\mu_*$ 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