8,411 research outputs found
Development and implementation of an adaptive digital beamforming network for satellite communication systems
The use of adaptive digital beamforming techniques has, until recently, been largely restricted to high performance military radar systems. Recent advances in digital technology, however, have enabled the design of single chip digital beamforming networks. This, coupled with advances in digital signal processor technology, enables complete beamforming systems to be constructed at a lower cost, thus making the application of these techniques to commercial communications systems attractive. The design and development of such an adaptative digital beamforming network are described. The system is being developed as a proof of concept laboratory based demonstrator to enable the feasibility of adaptive digital beamforming techniques for communication systems to be determined. Ultimately, digital beamforming could be used in conjunction with large array antennas for communication satellite systems. This will enable the simultaneous steering of high gain antenna beams in the direction of gr...Peer ReviewedPostprint (published version
Update rules and interevent time distributions: Slow ordering vs. no ordering in the Voter Model
We introduce a general methodology of update rules accounting for arbitrary
interevent time distributions in simulations of interacting agents. In
particular we consider update rules that depend on the state of the agent, so
that the update becomes part of the dynamical model. As an illustration we
consider the voter model in fully-connected, random and scale free networks
with an update probability inversely proportional to the persistence, that is,
the time since the last event. We find that in the thermodynamic limit, at
variance with standard updates, the system orders slowly. The approach to the
absorbing state is characterized by a power law decay of the density of
interfaces, observing that the mean time to reach the absorbing state might be
not well defined.Comment: 5pages, 4 figure
From continuous to discontinuous transitions in social diffusion
Models of social diffusion reflect processes of how new products, ideas or
behaviors are adopted in a population. These models typically lead to a
continuous or a discontinuous phase transition of the number of adopters as a
function of a control parameter. We explore a simple model of social adoption
where the agents can be in two states, either adopters or non-adopters, and can
switch between these two states interacting with other agents through a
network. The probability of an agent to switch from non-adopter to adopter
depends on the number of adopters in her network neighborhood, the adoption
threshold and the adoption coefficient , two parameters defining a Hill
function. In contrast, the transition from adopter to non-adopter is
spontaneous at a certain rate . In a mean-field approach, we derive the
governing ordinary differential equations and show that the nature of the
transition between the global non-adoption and global adoption regimes depends
mostly on the balance between the probability to adopt with one and two
adopters. The transition changes from continuous, via a transcritical
bifurcation, to discontinuous, via a combination of a saddle-node and a
transcritical bifurcation, through a supercritical pitchfork bifurcation. We
characterize the full parameter space. Finally, we compare our analytical
results with Montecarlo simulations on annealed and quenched degree regular
networks, showing a better agreement for the annealed case. Our results show
how a simple model is able to capture two seemingly very different types of
transitions, i.e., continuous and discontinuous and thus unifies underlying
dynamics for different systems. Furthermore the form of the adoption
probability used here is based on empirical measurements.Comment: 7 pages, 3 figure
Extinction-induced community reorganization in bipartite networks
We study how the community structure of bipartite mutualistic networks
changes in a dynamic context. First, we consider a real mutualistic network and
introduce extinction events according to several scenarios. We model
extinctions as node or interaction removals. For node removal, we consider
random, directed and sequential extinctions; for interaction removal, we
consider random extinctions. The bipartite network reorganizes showing an
increase of the effective modularity and a fast decrease of the persistence of
the species in the original communities with increasing number of extinction
events. Second, we compare extinctions in a real mutualistic network with the
growth of a bipartite network model. The modularity reaches a stationary value
and nodes remain in the same community after joining the network. Our results
show that perturbations and disruptive events affect the connectivity pattern
of mutualistic networks at the mesoscale level. The increase of the effective
modularity observed in some scenarios could provide some protection to the
remaining ecosystem
Rasmussen, Chenery and Watanabe, and labour-based multiplier analysis and sensitivity analysis of input coefficients for Spain in 2009
In this paper we identify the key sectors of the Spanish economy with the last input-output available data following the approaches by Rasmussen and by Chenery and Watanabe. We also calculate labour-based multipliers and sensitivity of input coefficients and we compare the results in order to present useful advice for sector policiesEn este trabajo analizamos cuáles son los sectores claves en España con los Ăşltimos datos publicados del marco input-output. Seguimos las metodologĂas de Rasmussen y de Chenery y Watanabe. TambiĂ©n hacemos uso de los multiplicadores de remuneraciones, trabajadores y asalariados asĂ como de la tĂ©cnica de sensibilidad de coeficientes. En las conclusiones se presentan los principales hallazgos que permiten diseñar una polĂtica sectorial eficaz para Españ
Is the Voter Model a model for voters?
The voter model has been studied extensively as a paradigmatic opinion
dynamics' model. However, its ability for modeling real opinion dynamics has
not been addressed. We introduce a noisy voter model (accounting for social
influence) with agents' recurrent mobility (as a proxy for social context),
where the spatial and population diversity are taken as inputs to the model. We
show that the dynamics can be described as a noisy diffusive process that
contains the proper anysotropic coupling topology given by population and
mobility heterogeneity. The model captures statistical features of the US
presidential elections as the stationary vote-share fluctuations across
counties, and the long-range spatial correlations that decay logarithmically
with the distance. Furthermore, it recovers the behavior of these properties
when a real-space renormalization is performed by coarse-graining the
geographical scale from county level through congressional districts and up to
states. Finally, we analyze the role of the mobility range and the randomness
in decision making which are consistent with the empirical observations.Comment: 13 pages, 13 figure
Local polynomial regression estimation with correlated errors
In this paper, we study the nonparametric estimation of the regression
function and its derivatives using weighted local polynomial fitting. Consider
the fixed regression model and suppose that the random observation error is
coming from a strictly stationary stochastic process. Expressions for the bias
and the variance array of the estimators of the regression function and its
derivatives are obtained and joint asymptotic normality is established. The
influence of the dependence of the data is observed in the expression of the
variance. We also propose a variable bandwidth selection procedure. A simulation
study and an analysis with real economic data illustrate the proposed
selection method.Xunta de Galicia; XUGA10501B97Xunta de Galicia; PB98-0182-c02-0
Reaction Dynamics for the Systems 7Be,8B + 208Pb at Coulomb Barrier Energies
In this contribution we describe the first results obtained for the investigation of
the elastic scattering process in the reactions induced by the Radioactive Ion Beams 7Be and
8B on a 208Pb target at Coulomb barrier energies. The experimental data were analyzed within
the framework of the optical model in order to extract the total reaction cross section. The
comparison with data available in literature for reactions induced on 208Pb by light ions in the
mass range A = 6-8 shows that the loosely-bound 8B has the largest reactivity
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