587 research outputs found
Criptografía Cuántica en Redes Clásicas
Quantum Key Distribution (QKD) is the first technology derived from Quantum Information and Quantum Computation that is being marketed. To date, the commercial applications are in the encryption of point to point links. Now, the next logical step is about to be taken. Full quantum networks able to transport the keys to be used in symmetric key cryptography with a security level previously unattained are being designed. The proof of concept of many of the ingredients needed exist and networks with a few nodes have been implemented. The first networks will cover a metropolitan area and demonstrate the feasibility of large-scale QKD integrated with classical networks. If it succeeds in the marketplace, this could signal a major shift in how cryptography is used today
Integrando la criptografía cuántica en redes clásicas
En este artículo discutimos las limitaciones y ventajas de la distribución cuántica de claves y los primeros pasos hacia su inclusión dentro de la infraestructura de seguridad en las redes de comunicaciones convencionales
Comparative Literautre: Religion and Religious Symbolism in the Tale of the Grail by Three Authors
The myth of the Grail has long been recognised as the cornerstone of Arthurian literature. Many studies have been conducted on the subject of Christian symbolism in the major Grail romances. However, the aim of the present paper is to prove that the 15th-century “Tale of the Sangrail”, found in Le Morte d’Arthur, by Thomas Malory, presents a greater degree of Christian coloration than 12th -century Chrétien de Troyes’ Perceval and Wolfram von Eschenbach’s Parzival. In order to evaluate this claim, the origin and function of the main elements at the Grail Ceremony were compared in the first place. Secondly, the main characters’ roles were examined to determine variations concerning religious beliefs and overall character development. The findings demonstrated that the main elements at the Grail Ceremony in Thomas Malory’s “The Tale of the Sangrail” are more closely linked to Christian motifs and that Perceval’s psychological development in the same work conflicts with that of a stereotypical Bildungsroman, in contrast with the previous 12th -century versions of the tale
Rate Compatible Protocol for Information Reconciliation: An application to QKD
Information Reconciliation is a mechanism that allows to weed out the
discrepancies between two correlated variables. It is an essential component in
every key agreement protocol where the key has to be transmitted through a
noisy channel. The typical case is in the satellite scenario described by
Maurer in the early 90's. Recently the need has arisen in relation with Quantum
Key Distribution (QKD) protocols, where it is very important not to reveal
unnecessary information in order to maximize the shared key length. In this
paper we present an information reconciliation protocol based on a rate
compatible construction of Low Density Parity Check codes. Our protocol
improves the efficiency of the reconciliation for the whole range of error
rates in the discrete variable QKD context. Its adaptability together with its
low interactivity makes it specially well suited for QKD reconciliation
La explotación económica del patrimonio urbano del Cabildo catedralicio de Segovia en el siglo XIV.
Sin resume
La organización de la gestión económica del cabildo catedralicio de Segovia. Siglos XIÍI - XIV.
Sin resume
Goal reasoning for autonomous agents using automated planning
Mención Internacional en el título de doctorAutomated planning deals with the task of finding a sequence of actions, namely
a plan, which achieves a goal from a given initial state. Most planning research
consider goals are provided by a external user, and agents just have to find a
plan to achieve them. However, there exist many real world domains where
agents should not only reason about their actions but also about their goals,
generating new ones or changing them according to the perceived environment.
In this thesis we aim at broadening the goal reasoning capabilities of planningbased
agents, both when acting in isolation and when operating in the same
environment as other agents.
In single-agent settings, we firstly explore a special type of planning tasks
where we aim at discovering states that fulfill certain cost-based requirements
with respect to a given set of goals. By computing these states, agents are able
to solve interesting tasks such as find escape plans that move agents in to safe
places, hide their true goal to a potential observer, or anticipate dynamically arriving
goals. We also show how learning the environment’s dynamics may help
agents to solve some of these tasks. Experimental results show that these states
can be quickly found in practice, making agents able to solve new planning
tasks and helping them in solving some existing ones.
In multi-agent settings, we study the automated generation of goals based on
other agents’ behavior. We focus on competitive scenarios, where we are interested
in computing counterplans that prevent opponents from achieving their
goals. We frame these tasks as counterplanning, providing theoretical properties
of the counterplans that solve them. We also show how agents can benefit
from computing some of the states we propose in the single-agent setting to
anticipate their opponent’s movements, thus increasing the odds of blocking
them. Experimental results show how counterplans can be found in different
environments ranging from competitive planning domains to real-time strategy
games.Programa de Doctorado en Ciencia y Tecnología Informática por la Universidad Carlos III de MadridPresidenta: Eva Onaindía de la Rivaherrera.- Secretario: Ángel García Olaya.- Vocal: Mark Robert
Fundamental limits of short-packet wireless communications
Mención Internacional en el título de doctorThis thesis concerns the maximum coding rate at which data can be transmitted
over a noncoherent, single-antenna, Rayleigh block-fading channel using an errorcorrecting
code of a given blocklength with a block-error probability not exceeding
a given value. This is an emerging problem originated by the next generation of
wireless communications, where the understanding of the fundamental limits in the
transmission of short packets is crucial. For this setting, traditional informationtheoretical
metrics of performance that rely on the transmission of long packets, such
as capacity or outage capacity, are not good benchmarks anymore, and the study
of the maximum coding rate as a function of the blocklength is needed. For the
noncoherent Rayleigh block-fading channel model, to study the maximum coding
rate as a function of the blocklength, only nonasymptotic bounds that must be
evaluated numerically were available in the literature. The principal drawback of the
nonasymptotic bounds is their high computational cost, which increases linearly with
the number of blocks (also called throughout this thesis coherence intervals) needed
to transmit a given codeword. By means of different asymptotic expansions in the
number of blocks, this thesis provides an alternative way of studying the maximum
coding rate as a function of the blocklength for the noncoherent, single-antenna,
Rayleigh block-fading channel.
The first approximation on the maximum coding rate derived in this thesis is a
high-SNR normal approximation. This central-limit-theorem-based approximation
becomes accurate as the signal-to-noise ratio (SNR) and the number of coherence
intervals L of size T tend to infinity. We show that the high-SNR normal approximation
is roughly equal to the normal approximation one obtains by transmitting
one pilot symbol per coherence block to estimate the fading coefficient, and by then
transmitting T−1 symbols per coherence block over a coherent fading channel. This
suggests that, at high SNR, one pilot symbol per coherence block suffices to achieve
both the capacity and the channel dispersion. While the approximation was derived
under the assumption that the number of coherence intervals and the SNR tend to
infinity, numerical analyses suggest that it becomes accurate already at SNR values of
15 dB, for 10 coherence intervals or more, and probabilities of error of 10−3 or more. The derived normal approximation is not only useful because it complements
the nonasymptotic bounds available in the literature, but also because it lays the
foundation for analytical studies that analyze the behavior of the maximum coding
rate as a function of system parameters such as SNR, number of coherence intervals,
or blocklength. An example of such a study concerns the optimal design of a simple
slotted-ALOHA protocol, which is also given in this thesis.
Since a big amount of services and applications in the next generation of wireless
communication systems will require to operate at low SNRs and small probabilities
of error (for instance, SNR values of 0 dB and probabilities of error of 10−6), the
second half of this thesis presents saddlepoint approximations of upper and lower
nonasymptotic bounds on the maximum coding rate that are accurate in that regime.
Similar to the normal approximation, these approximations become accurate as the
number of coherence intervals L increases, and they can be calculated efficiently.
Indeed, compared to the nonasymptotic bounds, which require the evaluation of
L-dimensional integrals, the saddlepoint approximations only require the evaluation
of four one-dimensional integrals. Although developed under the assumption of
large L, the saddlepoint approximations are shown to be accurate even for L = 1 and
SNR values of 0 dB or more. The small computational cost of these approximations
can be further avoided by performing high-SNR saddlepoint approximations that
can be evaluated in closed form. These approximations can be applied when some
conditions of convergence are satisfied and are shown to be accurate for 10 dB or
more.
In our analysis, the saddlepoint method is applied to the tail probabilities appearing
in the nonasymptotic bounds. These probabilities often depend on a set
of parameters, such as the SNR. Existing saddlepoint expansions do not consider
such dependencies. Hence, they can only characterize the behavior of the expansion
error in function of the number of coherence intervals L, but not in terms of the
remaining parameters. In contrast, we derive a saddlepoint expansion for random
variables whose distribution depends on an extra parameter, carefully analyze the
error terms, and demonstrate that they are uniform in such an extra parameter. We
then apply the expansion to the Rayleigh block-fading channel and obtain approximations
in which the error terms depend only on the blocklength and are uniform in
the remaining parameters.
Furthermore, the proposed approximations are shown to recover the normal approximation and the reliability function of the channel, thus providing a unifying
tool for the two regimes, which are usually considered separately in the literature.
Specifically, we show that the high-SNR normal approximation can be recovered from
the normal approximation derived from the saddlepoint approximations. By means
of the error exponent analysis that recovers the reliability function of the channel,
we also obtain easier-to-evaluate approximations of the saddlepoint approximations
consisting of the error exponent of the channel multiplied by a subexponential
factor. Numerical evidence suggests that these approximations are as accurate as
the saddlepoint approximations.
Finally, this thesis includes a practical case study where we analyze the benefit of
cooperation in optical wireless communications, a promising technology that can play
an important role in the next generation of wireless communications due to the high
data rates it can achieve. Specifically, a cooperative multipoint transmission and
reception scheme is evaluated for visible light communication (VLC) in an indoor
scenario. The proposed scheme is shown to provide SNR improvements of 3 dB or
more compared to a noncooperative scheme, especially when there is non-line-of-sight
(NLOS) between the access point and the receiver.Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Joerg Widmer.- Secretario: Matilde Pilar Sánchez Fernández.- Vocal: Petar Popovsk
Protocolos de reconciliación de información óptimos y no interactivos para distribución cuántica de claves
Propuesta de protocolos de reconciliación de información basados en códigos LDPC no interactivos aplicados a la distribución cuántica de claves
Thin Client Technology in the Academic Environment
From research groups at the universities of developed countries there is a growing interest in providing solutions to problems of developing countries. In this context we have studied typical problems in many (educational) institutions, such as the lack of technicians who repair the computers, the administration of the machines, and also the difficulty to maintain and configure the old hardware available due to the variety of characteristics of the different machines and the amount of hardware breakdowns and software issues (viruses, administration issues) that the local staff has to face up to with their equipments.
We propose a thin client approach that takes into account the human, hardware and software characteristics of developing institutions to provide a complete service for a computer network. The network administration is reduced to the administration of one server only. The maintenance of the machines is simplified and old computers can simulate the running of a powerful computer. Our proposal results in a cheap, simple (from the support point of view) and powerful (in terms of achieved functionalities) design
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