11,164 research outputs found
The Impact of Channel Feedback on Opportunistic Relay Selection for Hybrid-ARQ in Wireless Networks
This paper presents a decentralized relay selection protocol for a dense
wireless network and describes channel feedback strategies that improve its
performance. The proposed selection protocol supports hybrid
automatic-repeat-request transmission where relays forward parity information
to the destination in the event of a decoding error. Channel feedback is
employed for refining the relay selection process and for selecting an
appropriate transmission mode in a proposed adaptive modulation transmission
framework. An approximation of the throughput of the proposed adaptive
modulation strategy is presented, and the dependence of the throughput on
system parameters such as the relay contention probability and the adaptive
modulation switching point is illustrated via maximization of this
approximation. Simulations show that the throughput of the proposed selection
strategy is comparable to that yielded by a centralized selection approach that
relies on geographic information.Comment: 30 pages, 9 figures, submitted to the IEEE Transactions on Vehicular
Technology, revised March 200
Opportunistic Relay Selection with Limited Feedback
It has been shown that a decentralized relay selection protocol based on
opportunistic feedback from the relays yields good throughput performance in
dense wireless networks. This selection strategy supports a hybrid-ARQ
transmission approach where relays forward parity information to the
destination in the event of a decoding error. Such an approach, however,
suffers a loss compared to centralized strategies that select relays with the
best channel gain to the destination. This paper closes the performance gap by
adding another level of channel feedback to the decentralized relay selection
problem. It is demonstrated that only one additional bit of feedback is
necessary for good throughput performance. The performance impact of varying
key parameters such as the number of relays and the channel feedback threshold
is discussed. An accompanying bit error rate analysis demonstrates the
importance of relay selection.Comment: 5 pages, 6 figures, to appear in Proceedings of 2007 IEEE Vehicular
Technology Conference-Spring in Dublin, Irelan
Relay-Assisted User Scheduling in Wireless Networks with Hybrid-ARQ
This paper studies the problem of relay-assisted user scheduling for downlink
wireless transmission. The base station or access point employs hybrid
automatic-repeat-request (HARQ) with the assistance of a set of fixed relays to
serve a set of mobile users. By minimizing a cost function of the queue lengths
at the base station and the number of retransmissions of the head-of-line
packet for each user, the base station can schedule an appropriate user in each
time slot and an appropriate transmitter to serve it. It is shown that a
priority-index policy is optimal for a linear cost function with packets
arriving according to a Poisson process and for an increasing convex cost
function where packets must be drained from the queues at the base station.Comment: 14 pages, 5 figures, submitted to the IEEE Transactions on Vehicular
Technology in October 2008, revised in March 2009 and May 200
Ranking Economics Journals, Economics Departments, and Economists Using Teaching-Focused Research Productivity
This paper constructs new rankings of economics journals, economics departments, and economists that employ a measure of teaching-focused research productivity, an area of growing importance in recent years. The ranking methodologies presented here use information from articles that were published from 1991 through the early part of 2005 within the Journal of Economic Literature\u27 s economic education classifications (A200-A290). The Journal of Economic Literature tops the list of journals, followed by the Review of Economics and Statistics and the American Economic Review . Among the top institutions are Vanderbilt University, Indiana University, and the University of Wisconsin. Others that rank high here, such as Oberlin College and Denison University, do not often fare as well using methodologies that evaluate more traditional types of economics research. Finally, among the economists we find that John Siegfried, William Becker, and Michael Watts are ranked above other economists
An Energy-Based Comparison of Long-Hop and Short-Hop Routing in MIMO Networks
This paper considers the problem of selecting either routes that consist of
long hops or routes that consist of short hops in a network of multiple-antenna
nodes, where each transmitting node employs spatial multiplexing. This
distance-dependent route selection problem is approached from the viewpoint of
energy efficiency, where a route is selected with the objective of minimizing
the transmission energy consumed while satisfying a target outage criterion at
the final destination. Deterministic line networks and two-dimensional random
networks are considered. It is shown that when 1) the number of hops traversed
between the source and destination grows large or 2) when the target success
probability approaches one or 3) when the number of transmit and/or receive
antennas grows large, short-hop routing requires less energy than long-hop
routing. It is also shown that if both routing strategies are subject to the
same delay constraint, long-hop routing requires less energy than short-hop
routing as the target success probability approaches one. In addition,
numerical analysis indicates that given loose outage constraints, only a small
number of transmit antennas are needed for short-hop routing to have its
maximum advantage over long-hop routing, while given stringent outage
constraints, the advantage of short-hop over long-hop routing always increases
with additional transmit antennas.Comment: 27 pages, 12 figures, submitted to IEEE Transactions on Vehicular
Technology in March 2009, revised in July 200
The bacterial transposon Tn7 causes premature polyadenylation of mRNA in eukaryotic organisms: TAGKO mutagenesis in filamentous fungi
TAGKO is a Tn7-based transposition system for genome wide mutagenesis in filamentous fungi. The effects of transposon insertion on the expression of TAGKO alleles were examined in Magnaporthe grisea and Mycosphaerella graminicola. Northern analysis showed that stable, truncated transcripts were expressed in the TAGKO mutants. Mapping of the 3′-ends of TAGKO cDNAs revealed that they all contain Tn7 end sequences, regardless of the transposon orientation. Polyadenylation signals characteristic of eukaryotic genes, preceded by stop codons in all frames, are located in both ends of the bacterial transposon. Thus, TAGKO transcripts are prematurely polyadenylated, and truncated proteins are predicted to be translated in the fungal mutants. Depending on the extent of protein truncation, TAGKO mutations in HPD4 (encoding p-hydroxyphenylpyruvate dioxygenase) resulted in tyrosine sensitivity in the two fungi. Similarly, a particular M.grisea CBS1 (encoding cystathionine β-synthase) TAGKO cDNA failed to complement cysteine auxotrophy in a yeast CBS mutant. TAGKO, therefore, represents a useful tool for in vivo study of truncated gene products in filamentous fungi.postprin
Heisenberg-picture approach to the exact quantum motion of a time-dependent forced harmonic oscillator
In the Heisenberg picture, the generalized invariant and exact quantum
motions are found for a time-dependent forced harmonic oscillator. We find the
eigenstate and the coherent state of the invariant and show that the
dispersions of these quantum states do not depend on the external force. Our
formalism is applied to several interesting cases.Comment: 15 pages, two eps files, to appear in Phys. Rev. A 53 (6) (1996
GENFIRE: A generalized Fourier iterative reconstruction algorithm for high-resolution 3D imaging
Tomography has made a radical impact on diverse fields ranging from the study
of 3D atomic arrangements in matter to the study of human health in medicine.
Despite its very diverse applications, the core of tomography remains the same,
that is, a mathematical method must be implemented to reconstruct the 3D
structure of an object from a number of 2D projections. In many scientific
applications, however, the number of projections that can be measured is
limited due to geometric constraints, tolerable radiation dose and/or
acquisition speed. Thus it becomes an important problem to obtain the
best-possible reconstruction from a limited number of projections. Here, we
present the mathematical implementation of a tomographic algorithm, termed
GENeralized Fourier Iterative REconstruction (GENFIRE). By iterating between
real and reciprocal space, GENFIRE searches for a global solution that is
concurrently consistent with the measured data and general physical
constraints. The algorithm requires minimal human intervention and also
incorporates angular refinement to reduce the tilt angle error. We demonstrate
that GENFIRE can produce superior results relative to several other popular
tomographic reconstruction techniques by numerical simulations, and by
experimentally by reconstructing the 3D structure of a porous material and a
frozen-hydrated marine cyanobacterium. Equipped with a graphical user
interface, GENFIRE is freely available from our website and is expected to find
broad applications across different disciplines.Comment: 18 pages, 6 figure
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