1,648 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
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
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
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
Crossover from Conserving to Lossy Transport in Circular Random Matrix Ensembles
In a quantum dot with three leads the transmission matrix t_{12} between two
of these leads is a truncation of a unitary scattering matrix S, which we treat
as random. As the number of channels in the third lead is increased, the
constraints from the symmetry of S become less stringent and t_{12} becomes
closer to a matrix of complex Gaussian random numbers with no constraints. We
consider the distribution of the singular values of t_{12}, which is related to
a number of physical quantities. Changing the number of channels in the third
lead corresponds to increasing the amount of loss in the system (and is
distinct from prior uses of a third lead to model dephasing)
The shapes of Milky Way satellites: looking for signatures of tidal stirring
We study the shapes of Milky Way satellites in the context of the tidal
stirring scenario for the formation of dwarf spheroidal galaxies. The standard
procedures used to measure shapes involve smoothing and binning of data and
thus may not be sufficient to detect structural properties like bars, which are
usually subtle in low surface brightness systems. Taking advantage of the fact
that in nearby dwarfs photometry of individual stars is available we introduce
discrete measures of shape based on the two-dimensional inertia tensor and the
Fourier bar mode. We apply these measures of shape first to a variety of
simulated dwarf galaxies formed via tidal stirring of disks embedded in dark
matter halos and orbiting the Milky Way. In addition to strong mass loss and
randomization of stellar orbits, the disks undergo morphological transformation
that typically involves the formation of a triaxial bar after the first
pericenter passage. These tidally induced bars persist for a few Gyr before
being shortened towards a more spherical shape if the tidal force is strong
enough. We test this prediction by measuring in a similar way the shape of
nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in
Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of
eleven studied dwarfs show elongated stellar distributions in the outer parts
that may signify transition to tidal tails. We thus find the shapes of Milky
Way satellites to be consistent with the predictions of the tidal stirring
model.Comment: 14 pages, 11 figures, accepted for publication in Ap
The Sloan Lens ACS Survey. VIII. The relation between environment and internal structure of early-type galaxies
We study the relation between the internal structure of early-type galaxies
and their environment using 70 strong gravitational lenses from the Sloan ACS
Lens Survey. The Sloan database is used to determine two measures of
overdensity of galaxies around each lens: the projected number density of
galaxies inside the tenth nearest neighbor (\Sigma_{10}) and within a cone of
radius one h^{-1} Mpc (D_1). Our main results are: 1) The average overdensity
is somewhat larger than unity, consistent with lenses preferring overdense
environments as expected for massive early-type galaxies (12/70 lenses are in
known groups/clusters). 2) The distribution of overdensities is
indistinguishable from that of "twin" non-lens galaxies selected from SDSS to
have the same redshift and stellar velocity dispersion \sigma_*. Thus, within
our errors, lens galaxies are an unbiased population, and the SLACS results can
be generalized to the overall population of early-type galaxies. 3) Typical
contributions from external mass distribution are no more than a few per cent,
reaching 10-20% (~0.05-0.10 external convergence) only in the most extreme
overdensities. 4) No significant correlation between overdensity and slope of
the mass density profile of the lens is found. 5) Satellite galaxies (those
with a more luminous companion) have marginally steeper mass density profiles
than central galaxies (those without). This result suggests that tidal
stripping may affect the mass structure of early-type galaxies down to kpc
scales probed by strong lensing, when they fall into larger structures
[ABRIDGED].Comment: ApJ, in press; minor changes with respect to v
Quantum theory of dynamic multiple light scattering
We formulate a quantum theory of dynamic multiple light scattering in
fluctuating disordered media and calculate the fluctuation and the
autocorrelation function of photon number operator for light transmitted
through a disordered slab. The effect of disorder on the information capacity
of a quantum communication channel operating in a disordered environment is
estimated and the use of squeezed light in diffusing-wave spectroscopy is
discussed.Comment: Revised text, additional figur
- …