199 research outputs found
Statistical mechanics of complex networks
Complex networks describe a wide range of systems in nature and society, much
quoted examples including the cell, a network of chemicals linked by chemical
reactions, or the Internet, a network of routers and computers connected by
physical links. While traditionally these systems were modeled as random
graphs, it is increasingly recognized that the topology and evolution of real
networks is governed by robust organizing principles. Here we review the recent
advances in the field of complex networks, focusing on the statistical
mechanics of network topology and dynamics. After reviewing the empirical data
that motivated the recent interest in networks, we discuss the main models and
analytical tools, covering random graphs, small-world and scale-free networks,
as well as the interplay between topology and the network's robustness against
failures and attacks.Comment: 54 pages, submitted to Reviews of Modern Physic
Tracking interacting dust: comparison of tracking and state estimation techniques for dusty plasmas
When tracking a target particle that is interacting with nearest neighbors in
a known way, positional data of the neighbors can be used to improve the state
estimate. Effects of the accuracy of such positional data on the target track
accuracy are investigated in this paper, in the context of dusty plasmas. In
kinematic simulations, notable improvement in the target track accuracy was
found when including all nearest neighbors in the state estimation filter and
tracking algorithm, whereas the track accuracy was not significantly improved
by higher-accuracy measurement techniques. The state estimation algorithm,
involving an extended Kalman filter, was shown to either remove or
significantly reduce errors due to "pixel locking". It is concluded that the
significant extra complexity and computational expense to achieve these
relatively small improvements are likely to be unwarranted for many situations.
For the purposes of determining the precise particle locations, it is concluded
that the simplified state estimation algorithm can be a viable alternative to
using more computationally-intensive measurement techniques.Comment: 11 pages, 6 figures, Conference paper: Signal and Data Processing of
Small Targets 2010 (SPIE
New Coding/Decoding Techniques for Wireless Communication Systems
Wireless communication encompasses cellular telephony systems (mobile communication), wireless sensor networks, satellite communication systems and many other applications. Studies relevant to wireless communication deal with maintaining reliable and efficient exchange of information between the transmitter and receiver over a wireless channel. The most practical approach to facilitate reliable communication is using channel coding. In this dissertation we propose novel coding and decoding approaches for practical wireless systems. These approaches include variable-rate convolutional encoder, modified turbo decoder for local content in Single-Frequency Networks, and blind encoder parameter estimation for turbo codes. On the other hand, energy efficiency is major performance issue in wireless sensor networks. In this dissertation, we propose a novel hexagonal-tessellation based clustering and cluster-head selection scheme to maximize the lifetime of a wireless sensor network. For each proposed approach, the system performance evaluation is also provided. In this dissertation the reliability performance is expressed in terms of bit-error-rate (BER), and the energy efficiency is expressed in terms of network lifetime
Enhancing Trajectory-Based Operations for UAVs through Hexagonal Grid Indexing: A Step towards 4D Integration of UTM and ATM
Aviation is expected to face a surge in the number of manned aircraft and drones in the coming years, making it necessary to integrate Unmanned Aircraft System Traffic Management (UTM) into Air Traffic Management (ATM) to ensure safe and efficient operations. This research proposes a novel hexagonal grid-based 4D trajectory representation framework for unmanned aerial vehicle (UAV) traffic management that overcomes the limitations of existing square/cubic trajectory representation methods. The proposed model employs a hierarchical indexing structure using hexagonal cells, enabling efficient ground based strategic conflict detection and conflict free 4D trajectory planning. Additionally, the use of Hexagonal Discrete Global Grid Systems provides a more accurate representation of UAV trajectories, improved sampling efficiency and higher angular resolution. The proposed approach can be used for predeparture conflict free 4D trajectory planning, reducing computational complexity and memory requirements while improving the accuracy of strategic trajectory conflict detection. The proposed framework can also be extended for air traffic flow management trajectory planning, Air Traffic Control (ATC) workload measurement, sector capacity estimation, dynamics airspace sectorization using hexagonal sectors and traffic density calculation, contributing to the development of an efficient UTM system, and facilitating the integration of UAVs into the national airspace system with AT
Parametric 3D Convolutional Autoencoder for the Prediction of Flow Fields in a Bed Configuration of Hot Particles
The use of deep learning methods for modeling fluid flow has drawn a lot of
attention in the past few years. In situations where conventional numerical
approaches can be computationally expensive, these techniques have shown
promise in offering accurate, rapid, and practical solutions for modeling
complex fluid flow problems. The success of deep learning is often due to its
ability to extract hidden patterns and features from the data, enabling the
creation of data-driven reduced models that can capture the underlying physics
of the domain. We present a data-driven reduced model for predicting flow
fields in a bed configuration of hot particles. The reduced model consists of a
parametric 3D convolutional autoencoder. The first part resolves the spatial
and temporal dependencies present in the input sequence, while the second part
of the architecture is responsible for predicting the solution at the
subsequent timestep based on the information gathered from the preceding part.
We also propose the utilization of a post-processing non-trainable output layer
following the decoding path to incorporate the physical knowledge, e.g.,
no-slip condition, into the prediction. The evaluation of the reduced model for
a bed configuration with variable particle temperature showed accurate results
at a fraction of the computational cost required by traditional numerical
simulation methods
Contamination source inference in water distribution networks
We study the inference of the origin and the pattern of contamination in
water distribution networks. We assume a simplified model for the dyanmics of
the contamination spread inside a water distribution network, and assume that
at some random location a sensor detects the presence of contaminants. We
transform the source location problem into an optimization problem by
considering discrete times and a binary contaminated/not contaminated state for
the nodes of the network. The resulting problem is solved by Mixed Integer
Linear Programming. We test our results on random networks as well as in the
Modena city network
Lorentzian and Euclidean Quantum Gravity - Analytical and Numerical Results
We review some recent attempts to extract information about the nature of
quantum gravity, with and without matter, by quantum field theoretical methods.
More specifically, we work within a covariant lattice approach where the
individual space-time geometries are constructed from fundamental simplicial
building blocks, and the path integral over geometries is approximated by
summing over a class of piece-wise linear geometries. This method of
``dynamical triangulations'' is very powerful in 2d, where the regularized
theory can be solved explicitly, and gives us more insights into the quantum
nature of 2d space-time than continuum methods are presently able to provide.
It also allows us to establish an explicit relation between the Lorentzian- and
Euclidean-signature quantum theories. Analogous regularized gravitational
models can be set up in higher dimensions. Some analytic tools exist to study
their state sums, but, unlike in 2d, no complete analytic solutions have yet
been constructed. However, a great advantage of our approach is the fact that
it is well-suited for numerical simulations. In the second part of this review
we describe the relevant Monte Carlo techniques, as well as some of the
physical results that have been obtained from the simulations of Euclidean
gravity. We also explain why the Lorentzian version of dynamical triangulations
is a promising candidate for a non-perturbative theory of quantum gravity.Comment: 69 pages, 16 figures, references adde
Hofstadter butterflies and magnetically induced band-gap quenching in graphene antidot lattices
We study graphene antidot lattices (GALs) in magnetic fields. Using a
tight-binding model and a recursive Green's function technique that we extend
to deal with periodic structures, we calculate Hofstadter butterflies of GALs.
We compare the results to those obtained in a simpler gapped graphene model. A
crucial difference emerges in the behaviour of the lowest Landau level, which
in a gapped graphene model is independent of magnetic field. In stark contrast
to this picture, we find that in GALs the band gap can be completely closed by
applying a magnetic field. While our numerical simulations can only be
performed on structures much smaller than can be experimentally realized, we
find that the critical magnetic field for which the gap closes can be directly
related to the ratio between the cyclotron radius and the neck width of the
GAL. In this way, we obtain a simple scaling law for extrapolation of our
results to more realistically sized structures and find resulting quenching
magnetic fields that should be well within reach of experiments.Comment: 8 pages, 8 figure
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