1,858 research outputs found
From Multi-Keyholes to Measure of Correlation and Power Imbalance in MIMO Channels: Outage Capacity Analysis
An information-theoretic analysis of a multi-keyhole channel, which includes
a number of statistically independent keyholes with possibly different
correlation matrices, is given. When the number of keyholes or/and the number
of Tx/Rx antennas is large, there is an equivalent Rayleigh-fading channel such
that the outage capacities of both channels are asymptotically equal. In the
case of a large number of antennas and for a broad class of fading
distributions, the instantaneous capacity is shown to be asymptotically
Gaussian in distribution, and compact, closed-form expressions for the mean and
variance are given. Motivated by the asymptotic analysis, a simple,
full-ordering scalar measure of spatial correlation and power imbalance in MIMO
channels is introduced, which quantifies the negative impact of these two
factors on the outage capacity in a simple and well-tractable way. It does not
require the eigenvalue decomposition, and has the full-ordering property. The
size-asymptotic results are used to prove Telatar's conjecture for
semi-correlated multi-keyhole and Rayleigh channels. Since the keyhole channel
model approximates well the relay channel in the amplify-and-forward mode in
certain scenarios, these results also apply to the latterComment: accepted by IEEE IT Trans., 201
Applications of Stochastic Ordering to Wireless Communications
Stochastic orders are binary relations defined on probability distributions
which capture intuitive notions like being larger or being more variable. This
paper introduces stochastic ordering of instantaneous SNRs of fading channels
as a tool to compare the performance of communication systems over different
channels. Stochastic orders unify existing performance metrics such as ergodic
capacity, and metrics based on error rate functions for commonly used
modulation schemes through their relation with convex, and completely monotonic
(c.m.) functions. Toward this goal, performance metrics such as instantaneous
error rates of M-QAM and M-PSK modulations are shown to be c.m. functions of
the instantaneous SNR, while metrics such as the instantaneous capacity are
seen to have a completely monotonic derivative (c.m.d.). It is shown that the
commonly used parametric fading distributions for modeling line of sight (LoS),
exhibit a monotonicity in the LoS parameter with respect to the stochastic
Laplace transform order. Using stochastic orders, average performance of
systems involving multiple random variables are compared over different
channels, even when closed form expressions for such averages are not
tractable. These include diversity combining schemes, relay networks, and
signal detection over fading channels with non-Gaussian additive noise, which
are investigated herein. Simulations are also provided to corroborate our
results.Comment: 25 pages, 10 figures, Submitted to the IEEE transactions on wireless
communication
Distributed Detection and Estimation in Wireless Sensor Networks
In this article we consider the problems of distributed detection and
estimation in wireless sensor networks. In the first part, we provide a general
framework aimed to show how an efficient design of a sensor network requires a
joint organization of in-network processing and communication. Then, we recall
the basic features of consensus algorithm, which is a basic tool to reach
globally optimal decisions through a distributed approach. The main part of the
paper starts addressing the distributed estimation problem. We show first an
entirely decentralized approach, where observations and estimations are
performed without the intervention of a fusion center. Then, we consider the
case where the estimation is performed at a fusion center, showing how to
allocate quantization bits and transmit powers in the links between the nodes
and the fusion center, in order to accommodate the requirement on the maximum
estimation variance, under a constraint on the global transmit power. We extend
the approach to the detection problem. Also in this case, we consider the
distributed approach, where every node can achieve a globally optimal decision,
and the case where the decision is taken at a central node. In the latter case,
we show how to allocate coding bits and transmit power in order to maximize the
detection probability, under constraints on the false alarm rate and the global
transmit power. Then, we generalize consensus algorithms illustrating a
distributed procedure that converges to the projection of the observation
vector onto a signal subspace. We then address the issue of energy consumption
in sensor networks, thus showing how to optimize the network topology in order
to minimize the energy necessary to achieve a global consensus. Finally, we
address the problem of matching the topology of the network to the graph
describing the statistical dependencies among the observed variables.Comment: 92 pages, 24 figures. To appear in E-Reference Signal Processing, R.
Chellapa and S. Theodoridis, Eds., Elsevier, 201
On the Performance of Terrestrial Free-Space Optical (FSO) Links under the Presence of Generalized Pointing Errors
En ambos grupos se han obtenido expresiones matemáticas en forma cerrada que permiten evaluar la capacidad en todo el rango de valores de SNR (Signal-to-Noise Ratio) en algunos casos y, en otros, solo ha sido posible obtener su comportamiento asintótico debido a la dificultad matemática que presentaba el análisis. A la luz de los resultados obtenidos, podemos concluir que los sistemas MISO FSO son probablemente la solución más interesante en comparación a los sistemas SIMO y MIMO FSO. Al mismo tiempo, los resultados obtenidos en comunicaciones cooperativas permiten concluir que los sistemas cooperativos basados en retransmisión DF son capaces de aumentar la capacidad e incluso mejorar a la capacidad obtenida por un sistema basado en diversidad espacial para determinadas posiciones del nodo retransmisor.
En el caso de las contribuciones realizadas en el modelado de errores por desapuntamiento generalizado, los cuales siguen una distribución Beckmann, podemos destacar la aproximación propuesta en esta tesis que nos permite incluir de una forma eficiente y sencilla dichos errores por desapuntamiento al análisis de prestaciones de cualquier sistema de comunicaciones FSO. La herramienta propuesta es válida para analizar cualquier sistema FSO en términos de BER y probabilidad de outage y nos permite detectar qué efecto es dominante, es decir, si la turbulencia atmosférica o los errores por desapuntamiento. El efecto de la correlación también ha sido contemplado, concluyendo que no puede ser ignorado.Los sistemas de comunicaciones ópticas en espacio libre (FSO, Free-Space Optical) para aplicaciones terrestres se presentan en la actualidad como una solución muy interesante para solventar el importante reto provocado por la escasez del espectro RF (Radio-Frequency) disponible. Además, los sistemas FSO se configuran como una seria alternativa frente a otras tecnologías de acceso y transporte como los sistemas de RF debido a las altas tasas de señalización potencialmente muy superiores que se pueden conseguir. Estas ventajas, entre otras, han intensificado la investigación en estos sistemas en las últimas décadas. Por tanto, el análisis de sus prestaciones en términos de probabilidad de error de bit (BER, Bit Error-Rate), probabilidad de outage y capacidad ergódica es de interés relevante, siendo estas altamente afectadas por la turbulencia atmosférica, los errores por desapuntamiento entre transmisor y receptor así como por la niebla densa. En esta tesis, el análisis de las prestaciones de los sistemas FSO ha sido abordado, presentando novedosos resultados para la comunidad científica e investigadora. Dicho análisis de prestaciones se ha dividido en dos grandes áreas de investigación: análisis de la capacidad ergódica, y modelado de errores por desapuntamiento generalizado entre transmisor y receptor.
Las contribuciones realizadas dentro del análisis de la capacidad ergódica están divididas en dos grupos: por un lado, el análisis de la capacidad de sistemas FSO avanzados basados en diversidad espacial tales como los sistemas MISO (Multiple-Input/Single-Output), SIMO (Single-Input/Multiple-Output) y MIMO(Multiple-Input/Multiple-Output) FSO; por otro lado, el análisis de la capacidad de sistemas cooperativos basados en retransmisión DF (Detect-and-Forward)
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