221 research outputs found
Renormalization Group Flow of the Two-Dimensional Hierarchical Coulomb Gas
We consider a quasilinear parabolic differential equation associated with the
renormalization group transformation of the two-dimensional hierarchical
Coulomb system in the limit as the size of the block L goes to 1. We show that
the initial value problem is well defined in a suitable function space and the
solution converges, as t goes to infinity, to one of the countably infinite
equilibrium solutions. The nontrivial equilibrium solution bifurcates from the
trivial one. These solutions are fully described and we provide a complete
analysis of their local and global stability for all values of inverse
temperature. Gallavotti and Nicolo's conjecture on infinite sequence of
``phases transitions'' is also addressed. Our results rule out an intermediate
phase between the plasma and the Kosterlitz-Thouless phases, at least in the
hierarchical model we consider.Comment: 34pages,2figures, to appear in CM
Distributed power control over interference channels using ACK/NACK feedback
In this work, we consider a network composed of several single-antenna
transmitter-receiver pairs in which each pair aims at selfishly minimizing the
power required to achieve a given signal-to-interference-plus-noise ratio. This
is obtained modeling the transmitter-receiver pairs as rational agents that
engage in a non-cooperative game. Capitalizing on the well-known results on the
existence and structure of the generalized Nash equilibrium (GNE) point of the
underlying game, a low complexity, iterative and distributed algorithm is
derived to let each terminal reach the GNE using only a limited feedback in the
form of link-layer acknowledgement (ACK) or negative acknowledgement (NACK).
Numerical results are used to prove that the proposed solution is able to
achieve convergence in a scalable and adaptive manner under different operating
conditions.Comment: 5 pages, 6 figures, IEEE Global Communications Conference (GLOBECOM),
Austin, Texas, Dec. 201
An efficient receiver structure for sweep-spread-carrier underwater acoustic links
In this paper, we present an improved receiver architecture for sweep-spread-carrier modulation, a spread-spectrum technique proposed to effectively contrast the effects of time dispersion over multipath propagation channels in underwater acoustic wireless links. The proposed structure is capable of taking advantage of the energy received from all propagation paths rather than only from the strongest path, as envisaged in the pioneering paper introducing this modulation technique. A hardware version of the modem was implemented in the laboratory and its behavior was assessed and compared, using standard propagation models, to that exhibited by the traditional single-path-based scheme in terms of bit error rate. Results are presented showing that gains of a few decibels can be achieved in signal-to-noise-plus-interference ratio. Issues relevant to carrier/symbol synchronization, channel estimation, and sensitivity to Doppler distortion are also addressed
Back-Donation in High-Valent d0 Metal Complexes: Does It Exist? the Case of NbV
In the last years, some N-heterocyclic carbene (NHC) complexes of high-valent d0 transition-metal halides have been structurally characterized, showing a significant short distance between the carbene carbon and the cis-halide ligands (Clax). Some authors attributed this arrangement to a halide â\u86\u92 Ccarbene unusual "back-donation", whereas, according to others, the M-carbene bond is purely Ï\u83. More, in general, the ability of d0 metal centers to provide back-donation to suitable ligands is still debated, and detailed bond analyses for this class of systems are missing in the literature. In this contribution, we analyze in detail the NbV-L bond within neutral, cationic, and anionic derivatives of NbCl5, with L = NHC, CO, CNH, and CN-. In [NbVCl6-x(NHC)x]x-1 complexes, with NHC being either a model carbene (1,3-dimethylimidazol-2-ylidene, IMe) or a realistic one [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr], we demonstrate that the metal center is really capable of back-donation to the carbene ligand by a charge flux that involves the chloride in the trans position and, directly, the metal. In this case, a direct interaction between Clax and Ccarbene can be excluded, while if different Ï\u80-acceptor ligands, such as CO or CNH, are used (instead of NHC), the direct Clax â\u86\u92 L interligand interaction becomes predominant
Physical Layer Techniques for Wireless Communication Systems
The increasing diffusion of mobile devices requiring, everywhere and every time, reliable connections able to support the more common applications, induced in the last years the deployment of telecommunication networks based on technologies capable to respond effectively to the ever-increasing market demand, still a long way off from saturation level.
Multicarrier transmission techniques employed in standards for local networks (Wi-Fi) and metropolitan networks (WiMAX) and for many years hot research topic, have been definitely adopted beginning from the fourth generation of cellular systems (LTE). The adoption of multicarrier signaling techniques if on one hand has brought significant advantages to counteract the detrimental effects in environments with particularly harsh propagation channel, on the other hand, has imposed very strict requirements on sensitivity to recovery errors of the carrier frequency offset (CFO) due to the resulting impact on correct signal detection.
The main focus of the thesis falls in this area, investigating some aspects relating to synchronization procedures for system based on multicarrier signaling. Particular reference will be made to a network entry procedure for LTE networks and to CFO recovery for OFDM, fltered multitone modulation and direct conversion receivers.
Other contributions pertaining to physical layer issues for communication systems,
both radio and over acoustic carrier, conclude the thesis
Frequency Estimation in OFDM Direct-Conversion Receivers Using a Repeated Preamble
This paper investigates the problem of carrier frequency offset (CFO) recovery in an OFDM receiver affected by frequency-selective in-phase/quadrature (I/Q) imbalances. The analysis is based on maximum-likelihood (ML) methods and relies on the transmission of a training preamble with a repetitive structure in the time domain. After assessing the accuracy of the conventional ML (CML) scheme in a scenario characterized by I/Q impairments, we review the joint ML (JML) estimator of all unknown parameters and evaluate its theoretical performance. In order to improve the estimation accuracy, we also present a novel CFO recovery method that exploits some side-information about the signal-to-interference ratio. It turns out that both CML and JML can be derived from this scheme by properly adjusting the value of a design parameter. The accuracy of the investigated methods are compared with the relevant Cramer-Rao bound. Our results can be used to check whether conventional CFO recovery algorithms can work properly or not in the presence of I/Q imbalances and also to evaluate the potential gain attainable by more sophisticated schemes
A Systematization of Cybersecurity Regulations, Standards and Guidelines for the Healthcare Sector
The growing adoption of IT solutions in the healthcare sector is leading to a
steady increase in the number of cybersecurity incidents. As a result,
organizations worldwide have introduced regulations, standards, and best
practices to address cybersecurity and data protection issues in this sector.
However, the application of this large corpus of documents presents operational
difficulties, and operators continue to lag behind in resilience to cyber
attacks. This paper contributes a systematization of the significant
cybersecurity documents relevant to the healthcare sector. We collected the 49
most significant documents and used the NIST cybersecurity framework to
categorize key information and support the implementation of cybersecurity
measures.Comment: 14 page
Italian National Framework for Cybersecurity and Data Protection
Data breaches have been one of the most common source of concerns related to cybersecurity in the last few years for many organizations. The General Data Protection Regulation (GDPR) in Europe, strongly impacted this scenario, as organizations operating with EU citizens now have to comply with strict data protection rules.
In this paper we present the Italian National Framework for Cybersecurity and Data Protection, a framework derived from the NIST Cybersecurity Framework, that includes elements and tools to appropriately take into account data protection aspects in a way that is coherent and integrated with cybersecurity aspects. The goal of the proposed Framework is to provide organizations of different sizes and nature with a flexible and unified tool for the implementation of comprehensive cybersecurity and data protection programs
SBA‐15 modified by incorporation of alkali metals: Effect of Its structural changes on the catalytic properties of Fe/SBA‐15 system in the Fischer–tropsch synthesis
A mesoporous SBA‐15 solid is doped with Li, K, or Cs. These systems are used as support of iron nanoparticles, and each composite is utilized as a catalyst in the Fischer-Tropsch synthesis. After the activation treatment, the same Fe species are detected in all solids: Fe3O4, α‐Fe, and Fe2+ inside SBA‐15 walls. However, the species percentages and their distribution are different according to the dopant present. In all "working" catalysts, a mixture of carbides ε´‐Fe2.2C and χ‐Fe2C5, Fe3O4, and ions Fe+2 inside the SBA‐15 walls are found. Alkali cations produce different amounts and strengths of basic sites. At 1 atm and T = 703 K, the catalytic activity order is: Li > K > no doped > Cs, and the alkali metals favor the production of olefins. At 20 atm, the activity is considerably higher, even at a temperature as low as 543 K, which is attributed to structural properties of the support and to diffusional effects. All catalysts show a promising hydrocarbon production in the gasoline range. The influence of alkali metals is discussed in terms of electrostatic effects, and the limited hydrocarbon chain growth is attributed to the control over size of active iron species.Fil: Cano, Leonardo Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Cagnoli, María V.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Ciencias Aplicadas "Dr. Jorge J. Ronco". Universidad Nacional de la Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Ciencias Aplicadas; ArgentinaFil: Bengoa, José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Ciencias Aplicadas "Dr. Jorge J. Ronco". Universidad Nacional de la Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Ciencias Aplicadas; ArgentinaFil: Marchetti, Sergio Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Ciencias Aplicadas "Dr. Jorge J. Ronco". Universidad Nacional de la Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Ciencias Aplicadas; Argentin
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