13,140 research outputs found
Information-theoretic analysis of MIMO channel sounding
The large majority of commercially available multiple-input multiple-output
(MIMO) radio channel measurement devices (sounders) is based on time-division
multiplexed switching (TDMS) of a single transmit/receive radio-frequency chain
into the elements of a transmit/receive antenna array. While being
cost-effective, such a solution can cause significant measurement errors due to
phase noise and frequency offset in the local oscillators. In this paper, we
systematically analyze the resulting errors and show that, in practice,
overestimation of channel capacity by several hundred percent can occur.
Overestimation is caused by phase noise (and to a lesser extent frequency
offset) leading to an increase of the MIMO channel rank. Our analysis
furthermore reveals that the impact of phase errors is, in general, most
pronounced if the physical channel has low rank (typical for line-of-sight or
poor scattering scenarios). The extreme case of a rank-1 physical channel is
analyzed in detail. Finally, we present measurement results obtained from a
commercially employed TDMS-based MIMO channel sounder. In the light of the
findings of this paper, the results obtained through MIMO channel measurement
campaigns using TDMS-based channel sounders should be interpreted with great
care.Comment: 99 pages, 14 figures, submitted to IEEE Transactions on Information
Theor
(M-theory-)Killing spinors on symmetric spaces
We show how the theory of invariant principal bundle connections for
reductive homogeneous spaces can be applied to determine the holonomy of
generalised Killing spinor covariant derivatives of the form in a purely algebraic and algorithmic way, where is a left-invariant homomorphism. Specialising this
to the case of symmetric M-theory backgrounds (i.e. with a
symmetric space and an invariant closed 4-form), we derive several criteria
for such a background to preserve some supersymmetry and consequently find all
supersymmetric symmetric M-theory backgrounds.Comment: Updated abstract for clarity. Added missing geometries to section 6.
Main result stand
The Consistent Newtonian Limit of Einstein's Gravity with a Cosmological Constant
We derive the `exact' Newtonian limit of general relativity with a positive
cosmological constant . We point out that in contrast to the case with
, the presence of a positive in Einsteins's equations
enforces, via the condition , on the potential , a range
, within which the
Newtonian limit is valid. It also leads to the existence of a maximum mass,
. As a consequence we cannot put the boundary
condition for the solution of the Poisson equation at infinity. A boundary
condition suitably chosen now at a finite range will then get reflected in the
solution of provided the mass distribution is not spherically symmetric.Comment: Latex, 15 pages, no figures, errors correcte
Services Aren\u27t Goods: Post-Industrial Principles for Policy Design
As the United States moves from an industrial society to a post-industrial society, fewer people are engaged in the production of goods, and a majority now produce services. The processes of designing and producing goods and services are radically different. This differences calls for innovation in both the structure of the work setting and the policies which govern work in the society as a whole. The article examines differences between goods and services and proposes a new model for designing and producing services, as well as new principles for social policy for service production. The model and principles are illustrated with examples in health care
Industrial Clustering and the Returns to Inventive Activity Canadian Biotechnology Firms, 1991-2000
We examine how industrial clustering affects biotechnology firmsâ innovativeness, contrasting similar firms not located in clusters or located in clusters that are or are not focused on the firmâs technological specialization. Using detailed firm level data, we find clustered firms are eight times more innovative than geographically remote firms, with largest effects for firms located in clusters strong in their own specialization. For firms located in a cluster strong in their specialization we also find that R&D productivity is enhanced by a firmâs own R&D alliances and also by the R&D alliances of other colocated firms.Biotechnology, industrial clustering, knowledge spillovers, R&D productivity, strategic alliances
The Properties and Gaseous Environments of Powerful Classical Double Radio Galaxies
The properties of a sample of 31 very powerful classical double radio
galaxies with redshifts between zero and 1.8 are studied. The source
velocities, beam powers, ambient gas densities, total lifetimes, and total
outflow energies are presented and discussed. The rate of growth of each side
of each source were obtained using a spectral aging analysis. The beam power
and ambient gas density were obtained by applying the strong shock jump
conditions to the ends of each side of the source. The total outflow lifetime
was obtained by applying the power-law relationship between the beam power and
the total source lifetime derived elsewhere for sources of this type, and the
total outflow energy was obtained by combining the beam power and the total
source lifetime. Composite profiles were constructed by combining results
obtained from each side of each source. The composite profiles indicate that
the ambient gas density falls with distance from the central engine. The source
velocities, beam powers, total lifetimes, and total energies seem to be
independent of radio source size. This is consistent with the standard model in
which each source grows at a roughly constant rate during which time the
central engine puts out a roughly constant beam power. The fact that the total
source lifetimes and energies are independent of radio source size indicates
that the sources are being sampled at random times during their lifetimes.Comment: 7 pages, 5 figures, to appear in "Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray", eds. T. A. Rector and D. S. De Young,
ASP conference series, Replaced version has minor textual correction
Quasars, their host galaxies, and their central black holes
We present the final results from our deep HST imaging study of the hosts of
radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs).
We describe new WFPC2 R-band observations for 14 objects and model these images
in conjunction with the data already reported in McLure et al (1999). We find
that spheroidal hosts become more prevalent with increasing nuclear luminosity
such that, for nuclear luminosities M_V < -23.5, the hosts of both radio-loud
and radio-quiet AGN are virtually all massive ellipticals. Moreover we
demonstrate that the basic properties of these hosts are indistinguishable from
those of quiescent, evolved, low-redshift ellipticals of comparable mass. This
result kills any lingering notion that radio-loudness is determined by
host-galaxy morphology, and also sets severe constraints on evolutionary
schemes which attempt to link low-z ULIRGs with RQQs. Instead, we show that our
results are as expected given the relationship between black-hole and spheroid
mass established for nearby galaxies, and apply this relation to estimate the
mass of the black hole in each object. The results agree very well with
completely-independent estimates based on nuclear emission-line widths; all the
quasars in our sample have M(bh) > 5 x 10^8 solar masses, while the radio-loud
objects are confined to M(bh) > 10^9 solar masses. This apparent mass-threshold
difference, which provides a natural explanation for why RQQs outnumber RLQs by
a factor of 10, appears to reflect the existence of a minimum and maximum level
of black-hole radio output which is a strong function of black-hole mass.
Finally, we use our results to estimate the fraction of massive
spheroids/black-holes which produce quasar-level activity. This fraction is
\~0.1% at the present day, rising to > 10% at z = 2-3.Comment: Revised version accepted for publication in Monthly Notices of the
Royal Astronomical Society. 46 pages, the final 19 of which comprise an
Appendix. 15 figures in main text. A further 14 4-panel greyscale plots and
14 line plots which appear in the Appendix have been reproduced here with
reduced quality due to space limitations. A full resolution copy of the
manuscript can be obtained via ftp://ftp.roe.ac.uk/pub/jsd/dunlop2002.ps.g
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