4,945 research outputs found
Millimeter Wave MIMO Channel Estimation Based on Adaptive Compressed Sensing
Multiple-input multiple-output (MIMO) systems are well suited for
millimeter-wave (mmWave) wireless communications where large antenna arrays can
be integrated in small form factors due to tiny wavelengths, thereby providing
high array gains while supporting spatial multiplexing, beamforming, or antenna
diversity. It has been shown that mmWave channels exhibit sparsity due to the
limited number of dominant propagation paths, thus compressed sensing
techniques can be leveraged to conduct channel estimation at mmWave
frequencies. This paper presents a novel approach of constructing beamforming
dictionary matrices for sparse channel estimation using the continuous basis
pursuit (CBP) concept, and proposes two novel low-complexity algorithms to
exploit channel sparsity for adaptively estimating multipath channel parameters
in mmWave channels. We verify the performance of the proposed CBP-based
beamforming dictionary and the two algorithms using a simulator built upon a
three-dimensional mmWave statistical spatial channel model, NYUSIM, that is
based on real-world propagation measurements. Simulation results show that the
CBP-based dictionary offers substantially higher estimation accuracy and
greater spectral efficiency than the grid-based counterpart introduced by
previous researchers, and the algorithms proposed here render better
performance but require less computational effort compared with existing
algorithms.Comment: 7 pages, 5 figures, in 2017 IEEE International Conference on
Communications Workshop (ICCW), Paris, May 201
73 GHz Wideband Millimeter-Wave Foliage and Ground Reflection Measurements and Models
This paper presents 73 GHz wideband outdoor foliage and ground reflection
measurements. Propagation measurements were made with a 400 Megachip-per-second
sliding correlator channel sounder, with rotatable 27 dBi (7 degrees half-
power beamwidth) horn antennas at both the transmitter and receiver, to study
foliage-induced scattering and de-polarization effects, to assist in developing
future wireless systems that will use adaptive array antennas. Signal
attenuation through foliage was measured to be 0.4 dB/m for both co- and
cross-polarized antenna configurations. Measured ground reflection coefficients
for dirt and gravel ranged from 0.02 to 0.34, for incident angles ranging from
60 degrees to 81 degrees (with respect to the normal incidence of the surface).
These data are useful for link budget design and site-specific (ray-tracing)
models for future millimeter-wave communication systems.Comment: 6 pages, 4 figures, 2015 IEEE International Conference on
Communications (ICC), ICC Workshop
3-D Statistical Channel Model for Millimeter-Wave Outdoor Mobile Broadband Communications
This paper presents an omnidirectional spatial and temporal 3-dimensional
statistical channel model for 28 GHz dense urban non-line of sight
environments. The channel model is developed from 28 GHz ultrawideband
propagation measurements obtained with a 400 megachips per second broadband
sliding correlator channel sounder and highly directional, steerable horn
antennas in New York City. A 3GPP-like statistical channel model that is easy
to implement in software or hardware is developed from measured power delay
profiles and a synthesized method for providing absolute propagation delays
recovered from 3-D ray-tracing, as well as measured angle of departure and
angle of arrival power spectra. The extracted statistics are used to implement
a MATLAB-based statistical simulator that generates 3-D millimeter-wave
temporal and spatial channel coefficients that reproduce realistic impulse
responses of measured urban channels. The methods and model presented here can
be used for millimeter-wave system-wide simulations, and air interface design
and capacity analyses.Comment: 7 pages, 6 figures, ICC 2015 (London, UK, to appear
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Innovations and Trends in Pension Plan Coverage, Pension Type and Plan Design
In this paper, we outline recent trends in employer pension pIan structure in the United States, focusing on plan coverage, plan type and pension plan design. We then identify the key factors that we believe will shape company-sponsored pension design in the future, drawing conclusions from a review of recent research and practice. Finally, we offer a cautious prognosis about the future of pension pIan coverage, pIan type and pIan design, focusing on the role of labor force aging, as well as anticipated developments in the business environment and anticipated changes in public policy
Cataclysmic Variables with Evolved Secondaries and the Progenitors of AM CVn Stars
We present the results of a systematic study of cataclysmic variables (CVs)
and related systems, combining detailed binary-population synthesis (BPS)
models with a grid of 120 binary evolution sequences calculated with a
Henyey-type stellar evolution code. In these sequences, we used 3 masses for
the white dwarf (0.6, 0.8, 1.0 Msun) and seven masses for the donor star in the
range of 0.6-1.4 Msun. The shortest orbital periods were chosen to have
initially unevolved secondaries, and the longest orbital period for each
secondary mass was taken to be just longer than the bifurcation period (16 - 22
hr), beyond which systems evolve towards long orbital periods. These
calculations show that systems which start with evolved secondaries near the
end or just after their main-sequence phase become ultra-compact systems with
periods as short as 7 min. These systems are excellent candidates for AM CVn
stars. Using a standard BPS code, we show how the properties of CVs at the
beginning of mass transfer depend on the efficiency for common-envelope (CE)
ejection and the efficiency of magnetic braking. In our standard model, where
CE ejection is efficient, some 10 per cent of all CVs have initially evolved
secondaries (with a central hydrogen abundance X_c < 0.4) and ultimately become
ultra-compact systems (implying a Galactic birthrate for AM CVn-like stars of
10^{-3} yr^{-1}). Almost all CVs with orbital periods longer than 5 hr are
found to have initially evolved or relatively massive secondaries. We show that
their distribution of effective temperatures is in good agreement with the
distribution of spectral types obtained by Beuermann et al. (1998).Comment: 16 pages, 6 figures (Fig. 4 in reduced format). Submitted to MNRA
Upgrading and testing program for narrow band high resolution planetary IR imaging spectrometer
An imaging spectrometer, intended primarily for observations of the outer planets, which utilizes an acoustically tuned optical filter (ATOF) and a charge coupled device (CCD) television camera was modified to improve spatial resolution and sensitivity. The upgraded instrument was a spatial resolving power of approximately 1 arc second, as defined by an f/7 beam at the CCD position and it has this resolution over the 50 arc second field of view. Less vignetting occurs and sensitivity is four times greater. The spectral resolution of 15 A over the wavelength interval 6500 A - 11,000 A is unchanged. Mechanical utility has been increased by the use of a honeycomb optical table, mechanically rigid yet adjustable optical component mounts, and a camera focus translation stage. The upgraded instrument was used to observe Venus and Saturn
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