3,039 research outputs found
Percolation and Connectivity in the Intrinsically Secure Communications Graph
The ability to exchange secret information is critical to many commercial,
governmental, and military networks. The intrinsically secure communications
graph (iS-graph) is a random graph which describes the connections that can be
securely established over a large-scale network, by exploiting the physical
properties of the wireless medium. This paper aims to characterize the global
properties of the iS-graph in terms of: (i) percolation on the infinite plane,
and (ii) full connectivity on a finite region. First, for the Poisson iS-graph
defined on the infinite plane, the existence of a phase transition is proven,
whereby an unbounded component of connected nodes suddenly arises as the
density of legitimate nodes is increased. This shows that long-range secure
communication is still possible in the presence of eavesdroppers. Second, full
connectivity on a finite region of the Poisson iS-graph is considered. The
exact asymptotic behavior of full connectivity in the limit of a large density
of legitimate nodes is characterized. Then, simple, explicit expressions are
derived in order to closely approximate the probability of full connectivity
for a finite density of legitimate nodes. The results help clarify how the
presence of eavesdroppers can compromise long-range secure communication.Comment: Submitted for journal publicatio
Continuum Percolation in the Intrinsically Secure Communications Graph
The intrinsically secure communications graph (iS-graph) is a random graph
which captures the connections that can be securely established over a
large-scale network, in the presence of eavesdroppers. It is based on
principles of information-theoretic security, widely accepted as the strictest
notion of security. In this paper, we are interested in characterizing the
global properties of the iS-graph in terms of percolation on the infinite
plane. We prove the existence of a phase transition in the Poisson iS-graph,
whereby an unbounded component of securely connected nodes suddenly arises as
we increase the density of legitimate nodes. Our work shows that long-range
communication in a wireless network is still possible when a secrecy constraint
is present.Comment: Accepted in the IEEE International Symposium on Information Theory
and its Applications (ISITA'10), Taichung, Taiwan, Oct. 201
A Unifying Framework for Local Throughput in Wireless Networks
With the increased competition for the electromagnetic spectrum, it is
important to characterize the impact of interference in the performance of a
wireless network, which is traditionally measured by its throughput. This paper
presents a unifying framework for characterizing the local throughput in
wireless networks. We first analyze the throughput of a probe link from a
connectivity perspective, in which a packet is successfully received if it does
not collide with other packets from nodes within its reach (called the audible
interferers). We then characterize the throughput from a
signal-to-interference-plus-noise ratio (SINR) perspective, in which a packet
is successfully received if the SINR exceeds some threshold, considering the
interference from all emitting nodes in the network. Our main contribution is
to generalize and unify various results scattered throughout the literature. In
particular, the proposed framework encompasses arbitrary wireless propagation
effects (e.g, Nakagami-m fading, Rician fading, or log-normal shadowing), as
well as arbitrary traffic patterns (e.g., slotted-synchronous,
slotted-asynchronous, or exponential-interarrivals traffic), allowing us to
draw more general conclusions about network performance than previously
available in the literature.Comment: Submitted for journal publicatio
Communication in a Poisson Field of Interferers -- Part II: Channel Capacity and Interference Spectrum
In Part I of this paper, we presented a mathematical model for communication
subject to both network interference and noise, where the interferers are
scattered according to a spatial Poisson process, and are operating
asynchronously in a wireless environment subject to path loss, shadowing, and
multipath fading. We determined the distribution of the aggregate interference
and the error performance of the link. In this second part, we characterize the
capacity of the link subject to both network interference and noise. Then, we
put forth the concept of spectral outage probability (SOP), a new
characterization of the aggregate radio-frequency emission generated by
communicating nodes in a wireless network. We present some applications of the
SOP, namely the establishment of spectral regulations and the design of covert
military networks. The proposed framework captures all the essential physical
parameters that affect the aggregate network emission, yet is simple enough to
provide insights that may be of value in the design and deployment of wireless
networks.Comment: To appear in IEEE Transactions on Wireless Communication
Wireless Secrecy in Large-Scale Networks
The ability to exchange secret information is critical to many commercial,
governmental, and military networks. The intrinsically secure communications
graph (iS-graph) is a random graph which describes the connections that can be
securely established over a large-scale network, by exploiting the physical
properties of the wireless medium. This paper provides an overview of the main
properties of this new class of random graphs. We first analyze the local
properties of the iS-graph, namely the degree distributions and their
dependence on fading, target secrecy rate, and eavesdropper collusion. To
mitigate the effect of the eavesdroppers, we propose two techniques that
improve secure connectivity. Then, we analyze the global properties of the
iS-graph, namely percolation on the infinite plane, and full connectivity on a
finite region. These results help clarify how the presence of eavesdroppers can
compromise secure communication in a large-scale network.Comment: To appear: Proc. IEEE Information Theory and Applications Workshop
(ITA'11), San Diego, CA, Feb. 2011, pp. 1-10, Invited Pape
Techniques for Enhanced Physical-Layer Security
Information-theoretic security--widely accepted as the strictest notion of
security--relies on channel coding techniques that exploit the inherent
randomness of propagation channels to strengthen the security of communications
systems. Within this paradigm, we explore strategies to improve secure
connectivity in a wireless network. We first consider the intrinsically secure
communications graph (iS-graph), a convenient representation of the links that
can be established with information-theoretic security on a large-scale
network. We then propose and characterize two techniques--sectorized
transmission and eavesdropper neutralization--which are shown to dramatically
enhance the connectivity of the iS-graph.Comment: Pre-print, IEEE Global Telecommunications Conference (GLOBECOM'10),
Miami, FL, Dec. 201
Communication in a Poisson Field of Interferers -- Part I: Interference Distribution and Error Probability
We present a mathematical model for communication subject to both network
interference and noise. We introduce a framework where the interferers are
scattered according to a spatial Poisson process, and are operating
asynchronously in a wireless environment subject to path loss, shadowing, and
multipath fading. We consider both cases of slow and fast-varying interferer
positions. The paper is comprised of two separate parts. In Part I, we
determine the distribution of the aggregate network interference at the output
of a linear receiver. We characterize the error performance of the link, in
terms of average and outage probabilities. The proposed model is valid for any
linear modulation scheme (e.g., M-ary phase shift keying or M-ary quadrature
amplitude modulation), and captures all the essential physical parameters that
affect network interference. Our work generalizes the conventional analysis of
communication in the presence of additive white Gaussian noise and fast fading,
allowing the traditional results to be extended to include the effect of
network interference. In Part II of the paper, we derive the capacity of the
link when subject to network interference and noise, and characterize the
spectrum of the aggregate interference.Comment: To appear in IEEE Transactions on Wireless Communication
Locating the Source of Diffusion in Large-Scale Networks
How can we localize the source of diffusion in a complex network? Due to the
tremendous size of many real networks--such as the Internet or the human social
graph--it is usually infeasible to observe the state of all nodes in a network.
We show that it is fundamentally possible to estimate the location of the
source from measurements collected by sparsely-placed observers. We present a
strategy that is optimal for arbitrary trees, achieving maximum probability of
correct localization. We describe efficient implementations with complexity
O(N^{\alpha}), where \alpha=1 for arbitrary trees, and \alpha=3 for arbitrary
graphs. In the context of several case studies, we determine how localization
accuracy is affected by various system parameters, including the structure of
the network, the density of observers, and the number of observed cascades.Comment: To appear in Physical Review Letters. Includes pre-print of main
paper, and supplementary materia
Natural variability of lotic Mediterranean ecosystems or wildfire perturbations: who will win?
This study evaluates the impacts of wildfires in lotic Mediterranean ecosystems. It was carried out at Monchique ridge after big wildfires occurred during 2002 and 2003. Deferential impacts were evaluated comparing historical results obtained before the wildfires (1999 and 2001), with the post fire ones (2006 and 2007). Physical and chemical parameters of the water, habitat morphology, diatoms, macrophytes, macroinvertebrates and fishes were evaluated at 10 collecting places, before and after wildfires. High recovering rates were observed to the vegetation, but it is still possible to found fire impacts over macrophytes and river morphology. Wildfires, contributed to canopy decrease and, consequently to the growth of plants that usually are controlled by shadow. As a result, vegetation biodiversity tend to increase. River banks tend also to be invaded by terrestrial plants. Higher post fires recover rates were observed to the more aquatic communities (diatoms, macroinvertebrates and fishes). For those communities, comparing spring situations before and after the fires no substantial differences were observed. Sometimes differences between consecutive years are even higher. So it can be concluded that magnitude of wildfire impacts is less than the natural inter-annual variability of Mediterranean rivers. Long-term effects of forest fires, resulting from large woody debries, were also detected by morphological alterations, like debries dams. Habitat diversity increase and impacts on aquatic communities are expected
- …