3,025 research outputs found
Achieving Optimal Throughput and Near-Optimal Asymptotic Delay Performance in Multi-Channel Wireless Networks with Low Complexity: A Practical Greedy Scheduling Policy
In this paper, we focus on the scheduling problem in multi-channel wireless
networks, e.g., the downlink of a single cell in fourth generation (4G)
OFDM-based cellular networks. Our goal is to design practical scheduling
policies that can achieve provably good performance in terms of both throughput
and delay, at a low complexity. While a class of -complexity
hybrid scheduling policies are recently developed to guarantee both
rate-function delay optimality (in the many-channel many-user asymptotic
regime) and throughput optimality (in the general non-asymptotic setting),
their practical complexity is typically high. To address this issue, we develop
a simple greedy policy called Delay-based Server-Side-Greedy (D-SSG) with a
\lower complexity , and rigorously prove that D-SSG not only achieves
throughput optimality, but also guarantees near-optimal asymptotic delay
performance. Specifically, we show that the rate-function attained by D-SSG for
any delay-violation threshold , is no smaller than the maximum achievable
rate-function by any scheduling policy for threshold . Thus, we are able
to achieve a reduction in complexity (from of the hybrid
policies to ) with a minimal drop in the delay performance. More
importantly, in practice, D-SSG generally has a substantially lower complexity
than the hybrid policies that typically have a large constant factor hidden in
the notation. Finally, we conduct numerical simulations to validate
our theoretical results in various scenarios. The simulation results show that
D-SSG not only guarantees a near-optimal rate-function, but also empirically is
virtually indistinguishable from delay-optimal policies.Comment: Accepted for publication by the IEEE/ACM Transactions on Networking,
February 2014. A preliminary version of this work was presented at IEEE
INFOCOM 2013, Turin, Italy, April 201
From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
The 10th recorded outburst of the recurrent eclipsing nova USco was observed
simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9
after outburst. Two full passages of the companion in front of the nova ejecta
were observed, witnessing the reformation of the accretion disk. On day 22.9,
we observed smooth eclipses in UV and optical but deep dips in the X-ray light
curve which disappeared by day 34.9, then yielding clean eclipses in all bands.
X-ray dips can be caused by clumpy absorbing material that intersects the line
of sight while moving along highly elliptical trajectories. Cold material from
the companion could explain the absence of dips in UV and optical light. The
disappearance of X-ray dips before day 34.9 implies significant progress in the
formation of the disk. The X-ray spectra contain photospheric continuum
emission plus strong emission lines, but no clear absorption lines. Both
continuum and emission lines in the X-ray spectra indicate a temperature
increase from day 22.9 to day 34.9. We find clear evidence in the spectra and
light curves for Thompson scattering of the photospheric emission from the
white dwarf. Photospheric absorption lines can be smeared out during scattering
in a plasma of fast electrons. We also find spectral signatures of resonant
line scattering that lead to the observation of the strong emission lines.
Their dominance could be a general phenomenon in high-inclination systems such
as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure
Immunosuppression for liver transplantation in HCV-infected patients: Mechanism-based principles
We retrospectively analyzed 42 hepatitis C virus (HCV)-infected patients who underwent cadaveric liver transplantation under two strategies of immunosuppression: (1) daily tacrolimus (TAC) throughout and an initial cycle of high-dose prednisone (PRED) with subsequent gradual steroid weaning, or (2) intraoperative antithymocyte globulin (ATG) and daily TAC that was later space weaned. After 36 ± 4 months, patient and graft survival in the first group was 18/19 (94.7%) with no examples of clinically serious HCV recurrence. In the second group, the three-year patient survival was 12/23 (52%), and graft survival was 9/23 (39%); accelerated recurrent hepatitis was the principal cause of the poor results. The data were interpreted in the context of a recently proposed immunologic paradigm that is equally applicable to transplantation and viral immunity. In the framework of this paradigm, the disparate hepatitis outcomes reflected different equilibria reached under the two immunosuppression regimens between the relative kinetics of viral distribution (systemically and in the liver) and the slowly recovering HCV-specific T-cell response. As a corollary, the aims of treatment of the HCV-infected liver recipients should be to predict, monitor, and equilibrate beneficial balances between virus distribution and the absence of an immunopathologic antiviral T-cell response. In this view, favorable equilibria were accomplished in the nonweaned group of patients but not in the weaned group. In conclusion, since the anti-HCV response is unleashed when immunosuppression is weaned, treatment protocols that minimize disease recurrence in HCV-infected allograft recipients must balance the desire to reduce immunosuppression or induce allotolerance with the need to prevent antiviral immunopathology. Copyright © 2005 by the American Association for the Study of Liver Diseases
Interplanetary flow systems associated with cosmic ray modulation in 1977 - 1980
The hydromagnetic flow configurations associated with cosmic ray modulation in 1977 to 1980 were determined using solar wind plasma and magnetic field data from Voyagers 1 and 2 and Helios 1. The modulation was related to two types of large scale systems of flows: one containing a number of transients such as shocks, post shock flows and magnetic clouds; the other consisting primarily of a series of quasi-stationary flows following interaction regions containing a stream interface and often bounded by a forward reverse shock pair. Each of the three major episodes of cosmic ray modulation was characterized by the passage of the system of transient flows. Plateaus in the cosmic ray intensity time profile were associated with the passage of systems of corotating streams
Magnetic fields and flows between 1 AU and 0.3 AU during the primary mission of HELIOS 1
The recurrent flow and field patterns observed by HELIOS 1, and the relation between these patterns and coronal holes are discussed. Four types of recurrent patterns were observed: a large recurrent stream, a recurrent slow (quiet) flow, a rapidly evolving flow, and a recurrent compound stream. There recurrent streams were not stationary, for although the sources recurred at approximately the same longitudes on successive rotations, the shapes and latitudinal patterns changed from one rotation to the next. A type of magnetic field and plasma structure characterized by a low ion temperature and a high magnetic field intensity is described as well as the structures of stream boundaries between the sun at approximately 0.3 AU
Magnetic field hourly averages from the Rome-GSFC experiment aboard Helios 1 and Helio 2
Plots of all the hourly averages computed from the solar magnetic field measurements obtained during the mission are given separately for Helios 1 and Helios 2. The magnitude and the direction of the averaged field are plotted versus the number of solar rotations as seen from Helios, counted from launch
Swift X-ray Observations of Classical Novae
The new gamma-ray burst mission Swift has obtained pointed observations of
several classical novae in outburst. We analyzed all the observations of
classical novae from the Swift archive up to 30 June, 2006. We analyzed usable
observations of 12 classical novae and found 4 non-detections, 3 weak sources
and 5 strong sources. This includes detections of 2 novae exhibiting spectra
resembling those of Super Soft X-ray binary Source spectra (SSS) implying
ongoing nuclear burning on the white dwarf surface. With these new Swift data,
we add to the growing statistics of the X-ray duration and characteristics of
classical novae.Comment: Accepted for ApJ; this version contains additional material: 18
pages, 16 figure
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