9,319 research outputs found
Robust rank correlation based screening
Independence screening is a variable selection method that uses a ranking
criterion to select significant variables, particularly for statistical models
with nonpolynomial dimensionality or "large p, small n" paradigms when p can be
as large as an exponential of the sample size n. In this paper we propose a
robust rank correlation screening (RRCS) method to deal with ultra-high
dimensional data. The new procedure is based on the Kendall \tau correlation
coefficient between response and predictor variables rather than the Pearson
correlation of existing methods. The new method has four desirable features
compared with existing independence screening methods. First, the sure
independence screening property can hold only under the existence of a second
order moment of predictor variables, rather than exponential tails or
alikeness, even when the number of predictor variables grows as fast as
exponentially of the sample size. Second, it can be used to deal with
semiparametric models such as transformation regression models and single-index
models under monotonic constraint to the link function without involving
nonparametric estimation even when there are nonparametric functions in the
models. Third, the procedure can be largely used against outliers and influence
points in the observations. Last, the use of indicator functions in rank
correlation screening greatly simplifies the theoretical derivation due to the
boundedness of the resulting statistics, compared with previous studies on
variable screening. Simulations are carried out for comparisons with existing
methods and a real data example is analyzed.Comment: Published in at http://dx.doi.org/10.1214/12-AOS1024 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org). arXiv admin note: text overlap with
arXiv:0903.525
Backhaul-Aware Caching Placement for Wireless Networks
As the capacity demand of mobile applications keeps increasing, the backhaul
network is becoming a bottleneck to support high quality of experience (QoE) in
next-generation wireless networks. Content caching at base stations (BSs) is a
promising approach to alleviate the backhaul burden and reduce user-perceived
latency. In this paper, we consider a wireless caching network where all the
BSs are connected to a central controller via backhaul links. In such a
network, users can obtain the required data from candidate BSs if the data are
pre-cached. Otherwise, the user data need to be first retrieved from the
central controller to local BSs, which introduces extra delay over the
backhaul. In order to reduce the download delay, the caching placement strategy
needs to be optimized. We formulate such a design problem as the minimization
of the average download delay over user requests, subject to the caching
capacity constraint of each BS. Different from existing works, our model takes
BS cooperation in the radio access into consideration and is fully aware of the
propagation delay on the backhaul links. The design problem is a mixed integer
programming problem and is highly complicated, and thus we relax the problem
and propose a low-complexity algorithm. Simulation results will show that the
proposed algorithm can effectively determine the near-optimal caching placement
and provide significant performance gains over conventional caching placement
strategies.Comment: 6 pages, 3 figures, accepted to IEEE Globecom, San Diego, CA, Dec.
201
A multi-wavelength observation and investigation of six infrared dark clouds
Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet
they play a crucial role in breeding newly-formed stars.
Aims. With the aim of further understanding the dynamics, chemistry, and
evolution of IRDCs, we carried out multi-wavelength observations on a small
sample.
Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m
telescopes, with tracers , HCN, , ,
DCO, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01,
G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05.
Results. We investigated 44 cores including 37 cores reported in previous
work and seven newly-identified cores. Toward the dense cores, we detected 6
DCO, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution
(SED) fits of the 70 to 500 m, we obtained dust
temperature and column density distributions of the IRDCs. We found that emission has a strong correlation with the dust temperature and column
density distributions, while showed the weakest correlation. It
is suggested that is indeed a good tracer in very dense
conditions, but is an unreliable one, as it has a relatively
low critical density and is vulnerable to freezing-out onto the surface of cold
dust grains. The dynamics within IRDCs are active, with infall, outflow, and
collapse; the spectra are abundant especially in deuterium species.
Conclusions. We observe many blueshifted and redshifted profiles,
respectively, with and toward the same core. This
case can be well explained by model "envelope expansion with core collapse
(EECC)".Comment: 24 pages, 11 figures, 4 tables. To be published in A&A. The
resolutions of the pictures are cut dow
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