4,042 research outputs found
Multiflow Transmission in Delay Constrained Cooperative Wireless Networks
This paper considers the problem of energy-efficient transmission in
multi-flow multihop cooperative wireless networks. Although the performance
gains of cooperative approaches are well known, the combinatorial nature of
these schemes makes it difficult to design efficient polynomial-time algorithms
for joint routing, scheduling and power control. This becomes more so when
there is more than one flow in the network. It has been conjectured by many
authors, in the literature, that the multiflow problem in cooperative networks
is an NP-hard problem. In this paper, we formulate the problem, as a
combinatorial optimization problem, for a general setting of -flows, and
formally prove that the problem is not only NP-hard but it is
inapproxmiable. To our knowledge*, these results provide
the first such inapproxmiablity proof in the context of multiflow cooperative
wireless networks. We further prove that for a special case of k = 1 the
solution is a simple path, and devise a polynomial time algorithm for jointly
optimizing routing, scheduling and power control. We then use this algorithm to
establish analytical upper and lower bounds for the optimal performance for the
general case of flows. Furthermore, we propose a polynomial time heuristic
for calculating the solution for the general case and evaluate the performance
of this heuristic under different channel conditions and against the analytical
upper and lower bounds.Comment: 9 pages, 5 figure
On Capacity and Optimal Scheduling for the Half-Duplex Multiple-Relay Channel
We study the half-duplex multiple-relay channel (HD-MRC) where every node can
either transmit or listen but cannot do both at the same time. We obtain a
capacity upper bound based on a max-flow min-cut argument and achievable
transmission rates based on the decode-forward (DF) coding strategy, for both
the discrete memoryless HD-MRC and the phase-fading HD-MRC. We discover that
both the upper bound and the achievable rates are functions of the
transmit/listen state (a description of which nodes transmit and which
receive). More precisely, they are functions of the time fraction of the
different states, which we term a schedule. We formulate the optimal scheduling
problem to find an optimal schedule that maximizes the DF rate. The optimal
scheduling problem turns out to be a maximin optimization, for which we propose
an algorithmic solution. We demonstrate our approach on a four-node
multiple-relay channel, obtaining closed-form solutions in certain scenarios.
Furthermore, we show that for the received signal-to-noise ratio degraded
phase-fading HD-MRC, the optimal scheduling problem can be simplified to a max
optimization.Comment: Author's final version (to appear in IEEE Transactions on Information
Theory
Markov Decision Processes with Applications in Wireless Sensor Networks: A Survey
Wireless sensor networks (WSNs) consist of autonomous and resource-limited
devices. The devices cooperate to monitor one or more physical phenomena within
an area of interest. WSNs operate as stochastic systems because of randomness
in the monitored environments. For long service time and low maintenance cost,
WSNs require adaptive and robust methods to address data exchange, topology
formulation, resource and power optimization, sensing coverage and object
detection, and security challenges. In these problems, sensor nodes are to make
optimized decisions from a set of accessible strategies to achieve design
goals. This survey reviews numerous applications of the Markov decision process
(MDP) framework, a powerful decision-making tool to develop adaptive algorithms
and protocols for WSNs. Furthermore, various solution methods are discussed and
compared to serve as a guide for using MDPs in WSNs
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