6,664 research outputs found
Optimization in Geometric Graphs: Complexity and Approximation
We consider several related problems arising in geometric graphs. In particular,
we investigate the computational complexity and approximability properties of several optimization problems in unit ball graphs and develop algorithms to find exact
and approximate solutions. In addition, we establish complexity-based theoretical
justifications for several greedy heuristics.
Unit ball graphs, which are defined in the three dimensional Euclidian space, have
several application areas such as computational geometry, facility location and, particularly, wireless communication networks. Efficient operation of wireless networks
involves several decision problems that can be reduced to well known optimization
problems in graph theory. For instance, the notion of a \virtual backbone" in a wire-
less network is strongly related to a minimum connected dominating set in its graph
theoretic representation.
Motivated by the vastness of application areas, we study several problems including maximum independent set, minimum vertex coloring, minimum clique partition,
max-cut and min-bisection. Although these problems have been widely studied in
the context of unit disk graphs, which are the two dimensional version of unit ball
graphs, there is no established result on the complexity and approximation status
for some of them in unit ball graphs. Furthermore, unit ball graphs can provide a
better representation of real networks since the nodes are deployed in the three dimensional space. We prove complexity results and propose solution procedures for
several problems using geometrical properties of these graphs.
We outline a matching-based branch and bound solution procedure for the maximum k-clique problem in unit disk graphs and demonstrate its effectiveness through
computational tests. We propose using minimum bottleneck connected dominating
set problem in order to determine the optimal transmission range of a wireless network that will ensure a certain size of "virtual backbone". We prove that this problem
is NP-hard in general graphs but solvable in polynomial time in unit disk and unit
ball graphs.
We also demonstrate work on theoretical foundations for simple greedy heuristics.
Particularly, similar to the notion of "best" approximation algorithms with respect to
their approximation ratios, we prove that several simple greedy heuristics are "best"
in the sense that it is NP-hard to recognize the gap between the greedy solution
and the optimal solution. We show results for several well known problems such as
maximum clique, maximum independent set, minimum vertex coloring and discuss
extensions of these results to a more general class of problems.
In addition, we propose a "worst-out" heuristic based on edge contractions for
the max-cut problem and provide analytical and experimental comparisons with a
well known "best-in" approach and its modified versions
Heuristics for Network Coding in Wireless Networks
Multicast is a central challenge for emerging multi-hop wireless
architectures such as wireless mesh networks, because of its substantial cost
in terms of bandwidth. In this report, we study one specific case of multicast:
broadcasting, sending data from one source to all nodes, in a multi-hop
wireless network. The broadcast we focus on is based on network coding, a
promising avenue for reducing cost; previous work of ours showed that the
performance of network coding with simple heuristics is asymptotically optimal:
each transmission is beneficial to nearly every receiver. This is for
homogenous and large networks of the plan. But for small, sparse or for
inhomogeneous networks, some additional heuristics are required. This report
proposes such additional new heuristics (for selecting rates) for broadcasting
with network coding. Our heuristics are intended to use only simple local
topology information. We detail the logic of the heuristics, and with
experimental results, we illustrate the behavior of the heuristics, and
demonstrate their excellent performance
An Order-based Algorithm for Minimum Dominating Set with Application in Graph Mining
Dominating set is a set of vertices of a graph such that all other vertices
have a neighbour in the dominating set. We propose a new order-based randomised
local search (RLS) algorithm to solve minimum dominating set problem in
large graphs. Experimental evaluation is presented for multiple types of
problem instances. These instances include unit disk graphs, which represent a
model of wireless networks, random scale-free networks, as well as samples from
two social networks and real-world graphs studied in network science. Our
experiments indicate that RLS performs better than both a classical greedy
approximation algorithm and two metaheuristic algorithms based on ant colony
optimisation and local search. The order-based algorithm is able to find small
dominating sets for graphs with tens of thousands of vertices. In addition, we
propose a multi-start variant of RLS that is suitable for solving the
minimum weight dominating set problem. The application of RLS in graph
mining is also briefly demonstrated
Maximizing the Probability of Delivery of Multipoint Relay Broadcast Protocol in Wireless Ad Hoc Networks with a Realistic Physical Layer
It is now commonly accepted that the unit disk graph used to model the
physical layer in wireless networks does not reflect real radio transmissions,
and that the lognormal shadowing model better suits to experimental
simulations. Previous work on realistic scenarios focused on unicast, while
broadcast requirements are fundamentally different and cannot be derived from
unicast case. Therefore, broadcast protocols must be adapted in order to still
be efficient under realistic assumptions. In this paper, we study the
well-known multipoint relay protocol (MPR). In the latter, each node has to
choose a set of neighbors to act as relays in order to cover the whole 2-hop
neighborhood. We give experimental results showing that the original method
provided to select the set of relays does not give good results with the
realistic model. We also provide three new heuristics in replacement and their
performances which demonstrate that they better suit to the considered model.
The first one maximizes the probability of correct reception between the node
and the considered relays multiplied by their coverage in the 2-hop
neighborhood. The second one replaces the coverage by the average of the
probabilities of correct reception between the considered neighbor and the
2-hop neighbors it covers. Finally, the third heuristic keeps the same concept
as the second one, but tries to maximize the coverage level of the 2-hop
neighborhood: 2-hop neighbors are still being considered as uncovered while
their coverage level is not higher than a given coverage threshold, many
neighbors may thus be selected to cover the same 2-hop neighbors
GraphCombEx: A Software Tool for Exploration of Combinatorial Optimisation Properties of Large Graphs
We present a prototype of a software tool for exploration of multiple
combinatorial optimisation problems in large real-world and synthetic complex
networks. Our tool, called GraphCombEx (an acronym of Graph Combinatorial
Explorer), provides a unified framework for scalable computation and
presentation of high-quality suboptimal solutions and bounds for a number of
widely studied combinatorial optimisation problems. Efficient representation
and applicability to large-scale graphs and complex networks are particularly
considered in its design. The problems currently supported include maximum
clique, graph colouring, maximum independent set, minimum vertex clique
covering, minimum dominating set, as well as the longest simple cycle problem.
Suboptimal solutions and intervals for optimal objective values are estimated
using scalable heuristics. The tool is designed with extensibility in mind,
with the view of further problems and both new fast and high-performance
heuristics to be added in the future. GraphCombEx has already been successfully
used as a support tool in a number of recent research studies using
combinatorial optimisation to analyse complex networks, indicating its promise
as a research software tool
Efficient Algorithms for Distributed Detection of Holes and Boundaries in Wireless Networks
We propose two novel algorithms for distributed and location-free boundary
recognition in wireless sensor networks. Both approaches enable a node to
decide autonomously whether it is a boundary node, based solely on connectivity
information of a small neighborhood. This makes our algorithms highly
applicable for dynamic networks where nodes can move or become inoperative.
We compare our algorithms qualitatively and quantitatively with several
previous approaches. In extensive simulations, we consider various models and
scenarios. Although our algorithms use less information than most other
approaches, they produce significantly better results. They are very robust
against variations in node degree and do not rely on simplified assumptions of
the communication model. Moreover, they are much easier to implement on real
sensor nodes than most existing approaches.Comment: extended version of accepted submission to SEA 201
A PTAS for the minimum dominating set problem in unit disk graphs
We present a polynomial-time approximation scheme (PTAS) for the minimum dominating set problem in unit disk graphs. In contrast to previously known approximation schemes for the minimum dominating set problem on unit disk graphs, our approach does not assume a geometric representation of the vertices (specifying the positions of the disks in the plane) to be given as part of the input. \u
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