199,659 research outputs found
Fronthaul-Constrained Cloud Radio Access Networks: Insights and Challenges
As a promising paradigm for fifth generation (5G) wireless communication
systems, cloud radio access networks (C-RANs) have been shown to reduce both
capital and operating expenditures, as well as to provide high spectral
efficiency (SE) and energy efficiency (EE). The fronthaul in such networks,
defined as the transmission link between a baseband unit (BBU) and a remote
radio head (RRH), requires high capacity, but is often constrained. This
article comprehensively surveys recent advances in fronthaul-constrained
C-RANs, including system architectures and key techniques. In particular, key
techniques for alleviating the impact of constrained fronthaul on SE/EE and
quality of service for users, including compression and quantization,
large-scale coordinated processing and clustering, and resource allocation
optimization, are discussed. Open issues in terms of software-defined
networking, network function virtualization, and partial centralization are
also identified.Comment: 5 Figures, accepted by IEEE Wireless Communications. arXiv admin
note: text overlap with arXiv:1407.3855 by other author
Skin friction reduction by slot injection at Mach 0.8
Surface skin friction, boundary layer profiles and turbulent intensity due to axially symmetric tangential slot injection into a transonic boundary layer were measured. Effects of slot height, multiple slot injection, and injection mass flow rate on the surface skin friction downstream of the the slot have been investigated. Surface skin friction was a function of the injection mass flow rate for x/s 40. Large normal pressure gradient and relatively large turbulent intensity were found near the slot with small injection mass flow rate; the region the high turbulent intensity moved downstream with increasing injection mass flow rate. The results with two slot injections indicated that the distance between slots should be less than 30 slot heights in order to achieve some benefits from the first slot. Of significant importance in the present investigation is that the skin friction reduction obtained at transonic speed is of the same order as obtained in the hypersonic regime. Additional work is required in order to formulate a correlation between the turbulent intensity and the injection mass flow rate that may be used in future analysis
Density Evolution for Asymmetric Memoryless Channels
Density evolution is one of the most powerful analytical tools for
low-density parity-check (LDPC) codes and graph codes with message passing
decoding algorithms. With channel symmetry as one of its fundamental
assumptions, density evolution (DE) has been widely and successfully applied to
different channels, including binary erasure channels, binary symmetric
channels, binary additive white Gaussian noise channels, etc. This paper
generalizes density evolution for non-symmetric memoryless channels, which in
turn broadens the applications to general memoryless channels, e.g. z-channels,
composite white Gaussian noise channels, etc. The central theorem underpinning
this generalization is the convergence to perfect projection for any fixed size
supporting tree. A new iterative formula of the same complexity is then
presented and the necessary theorems for the performance concentration theorems
are developed. Several properties of the new density evolution method are
explored, including stability results for general asymmetric memoryless
channels. Simulations, code optimizations, and possible new applications
suggested by this new density evolution method are also provided. This result
is also used to prove the typicality of linear LDPC codes among the coset code
ensemble when the minimum check node degree is sufficiently large. It is shown
that the convergence to perfect projection is essential to the belief
propagation algorithm even when only symmetric channels are considered. Hence
the proof of the convergence to perfect projection serves also as a completion
of the theory of classical density evolution for symmetric memoryless channels.Comment: To appear in the IEEE Transactions on Information Theor
Dynamical Electron Mass in a Strong Magnetic Field
Motivated by recent interest in understanding properties of strongly
magnetized matter, we study the dynamical electron mass generated through
approximate chiral symmetry breaking in QED in a strong magnetic field. We
reliably calculate the dynamical electron mass by numerically solving the
nonperturbative Schwinger-Dyson equations in a consistent truncation within the
lowest Landau level approximation. It is shown that the generation of dynamical
electron mass in a strong magnetic field is significantly enhanced by the
perturbative electron mass that explicitly breaks chiral symmetry in the
absence of a magnetic field.Comment: 5 pages, 1 figure, published versio
Model reconstructions for the Si(337) orientation
Although unstable, the Si(337) orientation has been known to appear in
diverse experimental situations such as the nanoscale faceting of Si(112), or
in the case of miscutting a Si(113) surface. Various models for Si(337) have
been proposed over time, which motivates a comprehensive study of the structure
of this orientation. Such a study is undertaken in this article, where we
report the results of a genetic algorithm optimization of the Si(337)- surface. The algorithm is coupled with a highly optimized empirical
potential for silicon, which is used as an efficient way to build a set of
possible Si(337) models; these structures are subsequently relaxed at the level
of ab initio density functional methods. Using this procedure, we retrieve most
of the (337) reconstructions proposed in previous works, as well as a number of
novel ones.Comment: 5 figures (low res.); to appear in J. Appl. Phy
Charge transfer via a two-strand superexchange bridge in DNA
Charge transfer in a DNA duplex chain is studied by constructing a system
with virtual electrodes connected at the ends of each DNA strand. The systeym
is described by the tight-binding model and its transport is analyzed by the
transfer matrix method. The very weak distance dependence in long
(G:C)(T:A)_M(G:C)_3 DNA chain observed in experiment [B. Giese, et al., Nature
412, 318 (2001)] is explained by a unistep two-strand superexchange bridge
without the need for the multi-step thermally-induced hopping mechanism or the
dephasing effect. The crossover number M_c of (T:A) base pairs, where crossover
between strong and weak distance dependence occurs, reflects the ratio of
intra- and inter-strand neighboring base-base couplings.Comment: accepted for publication in Phys. Rev. Let
Magnetic structure of solar flare regions producing hard X-ray pulsations
We present analysis of the magnetic field in seven solar flare regions
accompanied by the pulsations of hard X-ray (HXR) emission. These flares were
studied by Kuznetsov et al. (2016) (Paper~I), and chosen here because of the
availability of the vector magnetograms for their parent active regions (ARs)
obtained with the SDO/HMI data. In Paper~I, based on the observations only, it
was suggested that a magnetic flux rope (MFR) might play an important role in
the process of generation of the HXR pulsations. The goal of the present paper
is to test this hypothesis by using the extrapolation of magnetic field with
the non-linear force-free field (NLFFF) method. Having done this, we found that
before each flare indeed there was an MFR elongated along and above a magnetic
polarity inversion line (MPIL) on the photosphere. In two flare regions the
sources of the HXR pulsations were located at the footpoints of different
magnetic field lines wrapping around the central axis, and constituting an MFR
by themselves. In five other flares the parent field lines of the HXR
pulsations were not a part of an MFR, but surrounded it in the form of an
arcade of magnetic loops. These results show that, at least in the analyzed
cases, the "single flare loop" models do not satisfy the observations and
magnetic field modeling, while are consistent with the concept that the HXR
pulsations are a consequence of successive episodes of energy release and
electron acceleration in different magnetic flux tubes (loops) of a complex AR.
An MFR could generate HXR pulsations by triggering episodes of magnetic
reconnection in different loops in the course of its non-uniform evolution
along an MPIL. However, since three events studied here were confined flares,
actual eruptions may not be required to trigger sequential particle
acceleration episodes in the magnetic systems containing an MFR.Comment: 33 pages, 5 figures, 2 tables. Accepted for publication in Journal of
Atmospheric and Solar-Terrestrial Physics (28 April 2018
Micromachined Polycrystalline Sige-Based Thermopiles for Micropower Generation on Human Body
This paper presents a polycrystalline silicon germanium (poly-SiGe)
thermopile specially designed for thermoelectric generators used on human body.
Both the design of the single thermocouple and the arrangement of the
thermocouple array have been described. A rim structure has been introduced in
order to increase the temperature difference across the thermocouple junctions.
The modeling of the thermocouple and the thermopile has been performed
analytically and numerically. An output power of about 1 W at an output
voltage of more than 1 V is expected from the current design of thermopiles in
a watch-size generator. The key material properties of the poly-SiGe have been
measured. The thermopile has been fabricated and tested. Experimental results
clearly demonstrate the advantage of the rim structure in increasing output
voltage. In presence of forced convection, the output voltage of a non-released
thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the
rim structure is formed. A larger output voltage from the thermopile is
expected upon process completion.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
Likelihood Analysis of Repeating in the BATSE Catalogue
I describe a new likelihood technique, based on counts-in-cells statistics,
that I use to analyze repeating in the BATSE 1B and 2B catalogues. Using the 1B
data, I find that repeating is preferred over non-repeating by 4.3:1 odds, with
a well-defined peak at 5-6 repetitions per source. I find that the post-1B data
are consistent with the repeating model inferred from the 1B data, after taking
into account the lower fraction of bursts with well-determined positions.
Combining the two data sets, I find that the odds favoring repeating over
non-repeating are almost unaffected at 4:1, with a narrower peak at 5
repetitions per source. I conclude that the data sets are consistent both with
each other and with repeating, and that for these data sets the odds favor
repeating.Comment: 5 pages including 3 encapsulated figures, as a uuencoded, gzipped,
Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High
Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al.,
AIP, New Yor
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