447,022 research outputs found
Wireless Networks with Energy Harvesting and Power Transfer: Joint Power and Time Allocation
In this paper, we consider wireless powered communication networks which
could operate perpetually, as the base station (BS) broadcasts energy to the
multiple energy harvesting (EH) information transmitters. These employ "harvest
then transmit" mechanism, as they spend all of their energy harvested during
the previous BS energy broadcast to transmit the information towards the BS.
Assuming time division multiple access (TDMA), we propose a novel transmission
scheme for jointly optimal allocation of the BS broadcasting power and time
sharing among the wireless nodes, which maximizes the overall network
throughput, under the constraint of average transmit power and maximum transmit
power at the BS. The proposed scheme significantly outperforms "state of the
art" schemes that employ only the optimal time allocation. If a single EH
transmitter is considered, we generalize the optimal solutions for the case of
fixed circuit power consumption, which refers to a much more practical
scenario.Comment: 5 pages, 2 figures in IEEE Signal Processing Letters, vol. 23, no. 1,
January 201
On the Approximability of Digraph Ordering
Given an n-vertex digraph D = (V, A) the Max-k-Ordering problem is to compute
a labeling maximizing the number of forward edges, i.e.
edges (u,v) such that (u) < (v). For different values of k, this
reduces to Maximum Acyclic Subgraph (k=n), and Max-Dicut (k=2). This work
studies the approximability of Max-k-Ordering and its generalizations,
motivated by their applications to job scheduling with soft precedence
constraints. We give an LP rounding based 2-approximation algorithm for
Max-k-Ordering for any k={2,..., n}, improving on the known
2k/(k-1)-approximation obtained via random assignment. The tightness of this
rounding is shown by proving that for any k={2,..., n} and constant
, Max-k-Ordering has an LP integrality gap of 2 -
for rounds of the
Sherali-Adams hierarchy.
A further generalization of Max-k-Ordering is the restricted maximum acyclic
subgraph problem or RMAS, where each vertex v has a finite set of allowable
labels . We prove an LP rounding based
approximation for it, improving on the
approximation recently given by Grandoni et al.
(Information Processing Letters, Vol. 115(2), Pages 182-185, 2015). In fact,
our approximation algorithm also works for a general version where the
objective counts the edges which go forward by at least a positive offset
specific to each edge.
The minimization formulation of digraph ordering is DAG edge deletion or
DED(k), which requires deleting the minimum number of edges from an n-vertex
directed acyclic graph (DAG) to remove all paths of length k. We show that
both, the LP relaxation and a local ratio approach for DED(k) yield
k-approximation for any .Comment: 21 pages, Conference version to appear in ESA 201
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