65,323 research outputs found

    Decomposition by Successive Convex Approximation: A Unifying Approach for Linear Transceiver Design in Heterogeneous Networks

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    We study the downlink linear precoder design problem in a multi-cell dense heterogeneous network (HetNet). The problem is formulated as a general sum-utility maximization (SUM) problem, which includes as special cases many practical precoder design problems such as multi-cell coordinated linear precoding, full and partial per-cell coordinated multi-point transmission, zero-forcing precoding and joint BS clustering and beamforming/precoding. The SUM problem is difficult due to its non-convexity and the tight coupling of the users' precoders. In this paper we propose a novel convex approximation technique to approximate the original problem by a series of convex subproblems, each of which decomposes across all the cells. The convexity of the subproblems allows for efficient computation, while their decomposability leads to distributed implementation. {Our approach hinges upon the identification of certain key convexity properties of the sum-utility objective, which allows us to transform the problem into a form that can be solved using a popular algorithmic framework called BSUM (Block Successive Upper-Bound Minimization).} Simulation experiments show that the proposed framework is effective for solving interference management problems in large HetNet.Comment: Accepted by IEEE Transactions on Wireless Communicatio

    Exploring open-charm decay mode ΛcΛˉc\Lambda_c\bar{\Lambda}_c of charmonium-like state Y(4630)Y(4630)

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    The newly observed X,Y,ZX, Y, Z exotic states are definitely not in the standard QQˉQ\bar Q' structures, thus their existence composes a challenge to our understanding on the fundamental principles of hadron physics. Therefore the studies on their decay patterns which are determined by the non-perturbative QCD will definitely shed light on the concerned physics. Generally the four-quark states might be in a molecular state or tetraquark or their mixture. In this work, we adopt the suggestion that Y(4630)Y(4630) is a charmonium-like tetraquark made of a diquark and an anti-diquark. If it is true, its favorable decay mode should be Y(4630)Y(4630) decaying into an open-charm baryon pair, since such a transition occurs via strong interaction and is super-OZI-allowed. In this work, we calculate the decay width of Y(4630)ΛcΛˉcY(4630)\to\Lambda_c\bar{\Lambda}_c in the framework of the quark pair creation (QPC) model. Our numerical results on the partial width computed in the tetraquark configuration coincide with the Belle data within a certain error tolerance.Comment: 8 pages, 4 figures, 1 table. Accepted by Eur. Phys. J.

    A feedback-driven bubble G24.136+00.436: a possible site of triggered star formation

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    We present a multi-wavelength study of the IR bubble G24.136+00.436. The J=1-0 observations of 12^{12}CO, 13^{13}CO and C18^{18}O were carried out with the Purple Mountain Observatory 13.7 m telescope. Molecular gas with a velocity of 94.8 km s1^{-1} is found prominently in the southeast of the bubble, shaping as a shell with a total mass of 2×104\sim2\times10^{4} MM_{\odot}. It is likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores. Their dense (a few of 10310^{3} cm3^{-3}) and massive (a few of 10310^{3} MM_{\odot}) characteristics coupled with the broad linewidths (>> 2.5 km s1^{-1}) suggest they are promising sites of forming high-mass stars or clusters. This could be further consolidated by the detection of compact HII regions in Cores A and E. We tentatively identified and classified 63 candidate YSOs based on the \emph{Spitzer} and UKIDSS data. They are found to be dominantly distributed in regions with strong emission of molecular gas, indicative of active star formation especially in the shell. The HII region inside the bubble is mainly ionized by a \simO8V star(s), of the dynamical age \sim1.6 Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as time scales involved, indicate a possible scenario of triggering star formation, signified by the "collect and collapse" process.Comment: 13 pages, 10 figures, 4 tables, accepted by Ap