15,489 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
Current at a distance and resonant transparency in Weyl semimetals
Surface Fermi arcs are the most prominent manifestation of the topological
nature of Weyl semimetals. In the presence of a static magnetic field oriented
perpendicular to the sample surface, their existence leads to unique
inter-surface cyclotron orbits. We propose two experiments which directly probe
the Fermi arcs: a magnetic field dependent non-local DC voltage and sharp
resonances in the transmission of electromagnetic waves at frequencies
controlled by the field. We show that these experiments do not rely on quantum
mechanical phase coherence, which renders them far more robust and
experimentally accessible than quantum effects. We also comment on the
applicability of these ideas to Dirac semimetals.Comment: 10 pages, 8 figure
Testing and Optimization of an Automated Air Sampling System for the Future Air Sampling Network of the Integrated Carbon Observation System (ICOS)
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 firms’ innovativeness, contrasting similar firms not located in clusters or located in clusters that are or are not focused on the firm’s 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 firm’s 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
Expectation adjustment in the housing market: insights from the Scottish auction system
This paper examines price expectation adjustment of house buyers and sellers to rapid changes in the housing market using data from Scotland where houses are sold through 'first-price sealed-bid' auctions. These auctions provide more information on market signals, incentives and the behaviour of market participants than private treaty sales. This paper therefore provides a theoretical framework for analysing revealed preference data generated from these auctions. We specifically focus on the analysis of the selling to asking price difference, the 'bid-premium'. The bid-premium is shown to be affected by expectations of future price movements, market duration and high bidding frequency. The bid-premium reflects consumers' expectations, adapting to market conditions more promptly than asking price setting behaviour and final sale prices. The volatile conditions of the recent housing market bubble are fully reflected in the bid-premium, whereas the asking and sale prices are much less prone to rapid movements
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