8,136 research outputs found
From Multi-Keyholes to Measure of Correlation and Power Imbalance in MIMO Channels: Outage Capacity Analysis
An information-theoretic analysis of a multi-keyhole channel, which includes
a number of statistically independent keyholes with possibly different
correlation matrices, is given. When the number of keyholes or/and the number
of Tx/Rx antennas is large, there is an equivalent Rayleigh-fading channel such
that the outage capacities of both channels are asymptotically equal. In the
case of a large number of antennas and for a broad class of fading
distributions, the instantaneous capacity is shown to be asymptotically
Gaussian in distribution, and compact, closed-form expressions for the mean and
variance are given. Motivated by the asymptotic analysis, a simple,
full-ordering scalar measure of spatial correlation and power imbalance in MIMO
channels is introduced, which quantifies the negative impact of these two
factors on the outage capacity in a simple and well-tractable way. It does not
require the eigenvalue decomposition, and has the full-ordering property. The
size-asymptotic results are used to prove Telatar's conjecture for
semi-correlated multi-keyhole and Rayleigh channels. Since the keyhole channel
model approximates well the relay channel in the amplify-and-forward mode in
certain scenarios, these results also apply to the latterComment: accepted by IEEE IT Trans., 201
Diversity-Multiplexing Tradeoff via Asymptotic Analysis of Large MIMO Systems
Diversity-multiplexing tradeoff (DMT) presents a compact framework to compare
various MIMO systems and channels in terms of the two main advantages they
provide (i.e. high data rate and/or low error rate). This tradeoff was
characterized asymptotically (SNR-> infinity) for i.i.d. Rayleigh fading
channel by Zheng and Tse [1]. The asymptotic DMT overestimates the finite-SNR
one [2]. In this paper, using the recent results on the asymptotic (in the
number of antennas) outage capacity distribution, we derive and analyze the
finite-SNR DMT for a broad class of channels (not necessarily Rayleigh fading).
Based on this, we give the convergence conditions for the asymptotic DMT to be
approached by the finite-SNR one. The multiplexing gain definition is shown to
affect critically the convergence point: when the multiplexing gain is defined
via the mean (ergodic) capacity, the convergence takes place at realistic SNR
values. Furthermore, in this case the diversity gain can also be used to
estimate the outage probability with reasonable accuracy. The multiplexing gain
definition via the high-SNR asymptote of the mean capacity (as in [1]) results
in very slow convergence for moderate to large systems (as 1/ln(SNR)^2) and,
hence, the asymptotic DMT cannot be used at realistic SNR values. For this
definition, the high-SNR threshold increases exponentially in the number of
antennas and in the multiplexing gain. For correlated keyhole channel, the
diversity gain is shown to decrease with correlation and power imbalance of the
channel. While the SNR-asymptotic DMT of Zheng and Tse does not capture this
effect, the size-asymptotic DMT does.Comment: To appear in 2007 IEEE International Symposium on Information Theory
(ISIT 2007), Nice, June 200
Normal zone in -coated conductors
We consider the distribution of an electric field in YBCO-coated conductors
for a situation in which the DC transport current is forced into the copper
stabilizer due to a weak link -- a section of the superconducting film with a
critical current less than the transport current. The electric field in the
metal substrate is also discussed. The results are compared with recent
experiments on normal zone propagation in coated conductors for which the
substrate and stabilizer are insulated from each other. The potential
difference between the substrate and stabilizer, and the electric field in the
substrate outside the normal zone can be accounted for by a large screening
length in the substrate, comparable to the length of the sample. During a
quench, the electric field inside the interface between YBCO and stabilizer, as
well as in the buffer layer, can be several orders of magnitude greater than
the longitudinal macroscopic electric field inside the normal zone. We
speculate on the possibility of using possible microscopic electric discharges
caused by this large (kV/cm) electric field as a means to detect a
quench.Comment: 8 pages, 4 figure
Distance From the Apiary as a Factor in Alfalfa Pollination
Experiments conducted in northern Utah indicated that the distribution of honey bees (Apis mellifera L.) on an alfalfa field is modified by various influences besides distance from the colonies. Because of these other factors, generalizations concerning the effect of distance from colonies on the distribution of foraging honey bees cannot be made on the basis of experiments herein reported. A slight negative relationship between bee populations and distance from colonies in alfalfa fields was found in some of the experiments reported. In two fields where the distance was less than 600 feet, horizontal stratification of the field population was found only in the one with an average population of over two bees per square yard. The other field had less than one bee per square yard and no significant changes in population were observed up to 550 feet from the apiary. In another location bee populations decreased beyond 3,000 feet, but other variable factors on the field made it impossible to attribute these decreases solely to increasing distance from the apiary
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