4,543 research outputs found
Movement-Efficient Sensor Deployment in Wireless Sensor Networks With Limited Communication Range.
We study a mobile wireless sensor network (MWSN) consisting of multiple
mobile sensors or robots. Three key factors in MWSNs, sensing quality, energy
consumption, and connectivity, have attracted plenty of attention, but the
interaction of these factors is not well studied. To take all the three factors
into consideration, we model the sensor deployment problem as a constrained
source coding problem. %, which can be applied to different coverage tasks,
such as area coverage, target coverage, and barrier coverage. Our goal is to
find an optimal sensor deployment (or relocation) to optimize the sensing
quality with a limited communication range and a specific network lifetime
constraint. We derive necessary conditions for the optimal sensor deployment in
both homogeneous and heterogeneous MWSNs. According to our derivation, some
sensors are idle in the optimal deployment of heterogeneous MWSNs. Using these
necessary conditions, we design both centralized and distributed algorithms to
provide a flexible and explicit trade-off between sensing uncertainty and
network lifetime. The proposed algorithms are successfully extended to more
applications, such as area coverage and target coverage, via properly selected
density functions. Simulation results show that our algorithms outperform the
existing relocation algorithms
Continuous Spatial Query Processing:A Survey of Safe Region Based Techniques
In the past decade, positioning system-enabled devices such as smartphones have become most prevalent. This functionality brings the increasing popularity of
location-based services
in business as well as daily applications such as navigation, targeted advertising, and location-based social networking.
Continuous spatial queries
serve as a building block for location-based services. As an example, an Uber driver may want to be kept aware of the nearest customers or service stations. Continuous spatial queries require updates to the query result as the query or data objects are moving. This poses challenges to the query efficiency, which is crucial to the user experience of a service. A large number of approaches address this efficiency issue using the concept of
safe region
. A safe region is a region within which arbitrary movement of an object leaves the query result unchanged. Such a region helps reduce the frequency of query result update and hence improves query efficiency. As a result, safe region-based approaches have been popular for processing various types of continuous spatial queries. Safe regions have interesting theoretical properties and are worth in-depth analysis. We provide a comparative study of safe region-based approaches. We describe how safe regions are computed for different types of continuous spatial queries, showing how they improve query efficiency. We compare the different safe region-based approaches and discuss possible further improvements
Theory of random packings
We review a recently proposed theory of random packings. We describe the
volume fluctuations in jammed matter through a volume function, amenable to
analytical and numerical calculations. We combine an extended statistical
mechanics approach 'a la Edwards' (where the role traditionally played by the
energy and temperature in thermal systems is substituted by the volume and
compactivity) with a constraint on mechanical stability imposed by the
isostatic condition. We show how such approaches can bring results that can be
compared to experiments and allow for an exploitation of the statistical
mechanics framework. The key result is the use of a relation between the local
Voronoi volume of the constituent grains and the number of neighbors in contact
that permits a simple combination of the two approaches to develop a theory of
random packings. We predict the density of random loose packing (RLP) and
random close packing (RCP) in close agreement with experiments and develop a
phase diagram of jammed matter that provides a unifying view of the disordered
hard sphere packing problem and further shedding light on a diverse spectrum of
data, including the RLP state. Theoretical results are well reproduced by
numerical simulations that confirm the essential role played by friction in
determining both the RLP and RCP limits. Finally we present an extended
discussion on the existence of geometrical and mechanical coordination numbers
and how to measure both quantities in experiments and computer simulations.Comment: 9 pages, 5 figures. arXiv admin note: text overlap with
arXiv:0808.219
Jamming II: Edwards' statistical mechanics of random packings of hard spheres
The problem of finding the most efficient way to pack spheres has an
illustrious history, dating back to the crystalline arrays conjectured by
Kepler and the random geometries explored by Bernal in the 60's. This problem
finds applications spanning from the mathematician's pencil, the processing of
granular materials, the jamming and glass transitions, all the way to fruit
packing in every grocery. There are presently numerous experiments showing that
the loosest way to pack spheres gives a density of ~55% (RLP) while filling all
the loose voids results in a maximum density of ~63-64% (RCP). While those
values seem robustly true, to this date there is no physical explanation or
theoretical prediction for them. Here we show that random packings of
monodisperse hard spheres in 3d can pack between the densities 4/(4 + 2 \sqrt
3) or 53.6% and 6/(6 + 2 \sqrt 3) or 63.4%, defining RLP and RCP, respectively.
The reason for these limits arises from a statistical picture of jammed states
in which the RCP can be interpreted as the ground state of the ensemble of
jammed matter with zero compactivity, while the RLP arises in the infinite
compactivity limit. We combine an extended statistical mechanics approach 'a la
Edwards' (where the role traditionally played by the energy and temperature in
thermal systems is substituted by the volume and compactivity) with a
constraint on mechanical stability imposed by the isostatic condition.
Ultimately, our results lead to a phase diagram that provides a unifying view
of the disordered hard sphere packing problem.Comment: 55 pages, 19 figures, C. Song, P. Wang, H. A. Makse, A phase diagram
for jammed matter, Nature 453, 629-632 (2008
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