2,681 research outputs found
Asymptotic Performance of Linear Receivers in MIMO Fading Channels
Linear receivers are an attractive low-complexity alternative to optimal
processing for multi-antenna MIMO communications. In this paper we characterize
the information-theoretic performance of MIMO linear receivers in two different
asymptotic regimes. For fixed number of antennas, we investigate the limit of
error probability in the high-SNR regime in terms of the Diversity-Multiplexing
Tradeoff (DMT). Following this, we characterize the error probability for fixed
SNR in the regime of large (but finite) number of antennas.
As far as the DMT is concerned, we report a negative result: we show that
both linear Zero-Forcing (ZF) and linear Minimum Mean-Square Error (MMSE)
receivers achieve the same DMT, which is largely suboptimal even in the case
where outer coding and decoding is performed across the antennas. We also
provide an approximate quantitative analysis of the markedly different behavior
of the MMSE and ZF receivers at finite rate and non-asymptotic SNR, and show
that while the ZF receiver achieves poor diversity at any finite rate, the MMSE
receiver error curve slope flattens out progressively, as the coding rate
increases.
When SNR is fixed and the number of antennas becomes large, we show that the
mutual information at the output of a MMSE or ZF linear receiver has
fluctuations that converge in distribution to a Gaussian random variable, whose
mean and variance can be characterized in closed form. This analysis extends to
the linear receiver case a well-known result previously obtained for the
optimal receiver. Simulations reveal that the asymptotic analysis captures
accurately the outage behavior of systems even with a moderate number of
antennas.Comment: 48 pages, Submitted to IEEE Transactions on Information Theor
Living at the Edge: A Large Deviations Approach to the Outage MIMO Capacity
Using a large deviations approach we calculate the probability distribution
of the mutual information of MIMO channels in the limit of large antenna
numbers. In contrast to previous methods that only focused at the distribution
close to its mean (thus obtaining an asymptotically Gaussian distribution), we
calculate the full distribution, including its tails which strongly deviate
from the Gaussian behavior near the mean. The resulting distribution
interpolates seamlessly between the Gaussian approximation for rates close
to the ergodic value of the mutual information and the approach of Zheng and
Tse for large signal to noise ratios . This calculation provides us with
a tool to obtain outage probabilities analytically at any point in the parameter space, as long as the number of antennas is not too
small. In addition, this method also yields the probability distribution of
eigenvalues constrained in the subspace where the mutual information per
antenna is fixed to for a given . Quite remarkably, this eigenvalue
density is of the form of the Marcenko-Pastur distribution with square-root
singularities, and it depends on the values of and .Comment: Accepted for publication, IEEE Transactions on Information Theory
(2010). Part of this work appears in the Proc. IEEE Information Theory
Workshop, June 2009, Volos, Greec
Absorption spectra of Q 0000-263 and Q 1442+101
Studying the Lyman-alpha forest allows us to trace the cosmological distribution of matter through time, and may reveal insights into important questions such as the onset of galaxy formation. An equation for determining the number of Lyman-alpha absorption lines per redshift per rest equivalent in the Lyman-alpha forest is given. For a nonevolving population of clouds gamma = 1 for q(sub 0) = 0, and gamma = 0.5 for q(sub 0) = 0.5. A detailed study of the Lyman-alpha forests of Q 1442+101 at z(sub em) = 3.54 and Q 0000-263 at z(sub em) = 4.11
Colors and K-Band Counts of Extremely Faint Field Galaxies
We combine deep K-band (Keck) with V- and I-band (NTT) observations of two
high-Galactic latitude fields, surveying a total of ~2 sq. arcmin. The K-band
galaxy counts continue to rise above K=22, reaching surface densities of few x
10^5 per sq. degree. The slope for the counts is (d log(N) per mag per sq.
degree) = 0.23 +/- 0.02 between K=18-23, consistent with other deep K surveys.
The numbers of galaxies in each mag bin is about two times greater than the
galaxy counts of Djorgovski et al. (1995).
The optical and near infrared magnitudes of all objects detected in the V+I+K
image are discussed in the context of grids of isochrone synthesis galaxy
evolutionary models (Bruzual & Charlot 1993, 1995). The colors of most of the
observed galaxies are consistent with a population drawn from a broad redshift
distribution. A few galaxies at K=19-20 are red in both colors (V-I>3; I-K>2),
consistent with being early-type galaxies having undergone a burst of star
formation at z>5 and viewed at z~1. At K>20, we find ~8 ``red outlier''
galaxies with I-K>4 and V-I<2.5, whose colors are difficult to mimic by a
single evolving or non-evolving stellar population at any redshift. They are
likely either low-metallicity, dusty dwarf galaxies, or old galaxies at high
redshift (z>1.2). Their surface density is several per square arcminute, which
is so high that they are probably common objects of low luminosity .Comment: 28 pages, 9 Figures (2 Plates), uses aaspp4.sty. The complete paper
(including large figures and the plates) are available via anonymous ftp at
ftp://coma.berkeley.edu/pub/lmoustakas/ . To appear in the Astrophysical
Journal, Feb 1 1997, vol. 47
Plasticity in foraging behaviour as a possible response to climate change
published_or_final_versio
The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6
Numerical simulations suggest that active galactic nuclei (AGNs) play an
important role in the formation of early-type galaxies by expelling gas and
dust in powerful galactic winds and quenching star formation. However, the
existence of AGN feedback capable of halting galaxy-wide star formation has yet
to be observationally confirmed. To investigate this question, we have obtained
spectra of 14 post-starburst galaxies at z~0.6 to search for evidence of
galactic winds. In 10/14 galaxies we detect Mg II 2796,2803 absorption lines
which are blueshifted by 490 - 2020 km/s with respect to the stars. The median
blueshift is 1140 km/s. We hypothesize that the outflowing gas represents a
fossil galactic wind launched near the peak of the galaxy's activity, a few 100
Myr ago. The velocities we measure are intermediate between those of luminous
starbursts and broad absorption line quasars, which suggests that feedback from
an AGN may have played a role in expelling cool gas and shutting down star
formation.Comment: 5 pages, 2 figures, accepted to ApJ Letter
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