9,860 research outputs found
Finding community structure in networks using the eigenvectors of matrices
We consider the problem of detecting communities or modules in networks,
groups of vertices with a higher-than-average density of edges connecting them.
Previous work indicates that a robust approach to this problem is the
maximization of the benefit function known as "modularity" over possible
divisions of a network. Here we show that this maximization process can be
written in terms of the eigenspectrum of a matrix we call the modularity
matrix, which plays a role in community detection similar to that played by the
graph Laplacian in graph partitioning calculations. This result leads us to a
number of possible algorithms for detecting community structure, as well as
several other results, including a spectral measure of bipartite structure in
networks and a new centrality measure that identifies those vertices that
occupy central positions within the communities to which they belong. The
algorithms and measures proposed are illustrated with applications to a variety
of real-world complex networks.Comment: 22 pages, 8 figures, minor corrections in this versio
Achieving Low-Complexity Maximum-Likelihood Detection for the 3D MIMO Code
The 3D MIMO code is a robust and efficient space-time block code (STBC) for
the distributed MIMO broadcasting but suffers from high maximum-likelihood (ML)
decoding complexity. In this paper, we first analyze some properties of the 3D
MIMO code to show that the 3D MIMO code is fast-decodable. It is proved that
the ML decoding performance can be achieved with a complexity of O(M^{4.5})
instead of O(M^8) in quasi static channel with M-ary square QAM modulations.
Consequently, we propose a simplified ML decoder exploiting the unique
properties of 3D MIMO code. Simulation results show that the proposed
simplified ML decoder can achieve much lower processing time latency compared
to the classical sphere decoder with Schnorr-Euchner enumeration
Large-Scale MIMO Detection for 3GPP LTE: Algorithms and FPGA Implementations
Large-scale (or massive) multiple-input multiple-output (MIMO) is expected to
be one of the key technologies in next-generation multi-user cellular systems,
based on the upcoming 3GPP LTE Release 12 standard, for example. In this work,
we propose - to the best of our knowledge - the first VLSI design enabling
high-throughput data detection in single-carrier frequency-division multiple
access (SC-FDMA)-based large-scale MIMO systems. We propose a new approximate
matrix inversion algorithm relying on a Neumann series expansion, which
substantially reduces the complexity of linear data detection. We analyze the
associated error, and we compare its performance and complexity to those of an
exact linear detector. We present corresponding VLSI architectures, which
perform exact and approximate soft-output detection for large-scale MIMO
systems with various antenna/user configurations. Reference implementation
results for a Xilinx Virtex-7 XC7VX980T FPGA show that our designs are able to
achieve more than 600 Mb/s for a 128 antenna, 8 user 3GPP LTE-based large-scale
MIMO system. We finally provide a performance/complexity trade-off comparison
using the presented FPGA designs, which reveals that the detector circuit of
choice is determined by the ratio between BS antennas and users, as well as the
desired error-rate performance.Comment: To appear in the IEEE Journal of Selected Topics in Signal Processin
Developmental trajectory of Caenorhabditis elegans nervous system governs its structural organization
A central problem of neuroscience involves uncovering the principles
governing the organization of nervous systems which ensure robustness in brain
development. The nematode Caenorhabditis elegans provides us with a model
organism for studying this question. In this paper, we focus on the invariant
connection structure and spatial arrangement of the neurons comprising the
somatic neuronal network of this organism to understand the key developmental
constraints underlying its design. We observe that neurons with certain shared
characteristics - such as, neural process lengths, birth time cohort, lineage
and bilateral symmetry - exhibit a preference for connecting to each other.
Recognizing the existence of such homophily helps in connecting the physical
location and morphology of neurons with the topological organization of the
network. Further, the functional identities of neurons appear to dictate the
temporal hierarchy of their appearance during the course of development.
Providing crucial insights into principles that may be common across many
organisms, our study shows how the trajectory in the developmental landscape
constrains the eventual spatial and network topological organization of a
nervous system.Comment: 25 pages, 8 figures + 24 pages supplementary materia
Grand Design and Flocculent Spirals in the Spitzer Survey of Stellar Structure in Galaxies (S4G)
Spiral arm properties of 46 galaxies in the Spitzer Survey of Stellar
Structure in Galaxies (S4G) were measured at 3.6mu, where extinction is small
and the old stars dominate. The sample includes flocculent, multiple arm, and
grand design types with a wide range of Hubble and bar types. We find that most
optically flocculent galaxies are also flocculent in the mid-IR because of star
formation uncorrelated with stellar density waves, whereas multiple arm and
grand design galaxies have underlying stellar waves. Arm-interarm contrasts
increase from flocculent to multiple arm to grand design galaxies and with
later Hubble types. Structure can be traced further out in the disk than in
previous surveys. Some spirals peak at mid-radius while others continuously
rise or fall, depending on Hubble and bar type. We find evidence for regular
and symmetric modulations of the arm strength in NGC 4321. Bars tend to be
long, high amplitude, and flat-profiled in early type spirals, with arm
contrasts that decrease with radius beyond the end of the bar, and they tend to
be short, low amplitude, and exponential-profiled in late Hubble types, with
arm contrasts that are constant or increase with radius. Longer bars tend to
have larger amplitudes and stronger arms.Comment: 31 pages, 14 figures, ApJ in pres
Trading Goods versus Sharing Money - An Experiment Testing Wether Fairness and Efficiency are Frame Dependent
Systematic experiments with distribution games (for a survey, see Roth, 1995) have shown that participants are strongly motivated by fairness and efficiency considerations. This evidence, however, results mainly from experimental designs asking directly for sharing monetary rewards. But even when not just one kind of monetary tokens is distributed efficiency and fairness are less influential. We investigate and confirm this frame dependency more systematically by comparing net-trade-proposals and payoff-proposals for the same exchange economy with two traders, two commodities and multi-period-negotiations.
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