34,910 research outputs found
Q & A Experiment to Search for Vacuum Dichroism, Pseudoscalar-Photon Interaction and Millicharged Fermions
A number of experiments are underway to detect vacuum birefringence and
dichroism -- PVLAS, Q & A, and BMV. Recently, PVLAS experiment has observed
optical rotation in vacuum by a magnetic field (vacuum dichroism). Theoretical
interpretations of this result include a possible pseudoscalar-photon
interaction and the existence of millicharged fermions. Here, we report the
progress and first results of Q & A (QED [quantum electrodynamics] and Axion)
experiment proposed and started in 1994. A 3.5-m high-finesse (around 30,000)
Fabry-Perot prototype detector extendable to 7-m has been built and tested. We
use X-pendulums and automatic control schemes developed by the
gravitational-wave detection community for mirror suspension and cavity
control. To polarize the vacuum, we use a 2.3-T dipole permanent magnet, with
27-mm-diameter clear borehole and 0.6-m field length,. In the experiment, the
magnet is rotated at 5-10 rev/s to generate time-dependent polarization signal
with twice the rotation frequency. Our
ellipsometer/polarization-rotation-detection-system is formed by a pair of
Glan-Taylor type polarizing prisms with extinction ratio lower than 10-8
together with a polarization modulating Faraday Cell with/without a quarter
wave plate. We made an independent calibration of our apparatus by performing a
measurement of gaseous Cotton-Mouton effect of nitrogen. We present our first
experimental results and give a brief discussion of our experimental limit on
pseudo-scalar-photon interaction and millicharged fermions.Comment: 21 pages, 13 figures, submitted to Modern Physics Letter
Capacity analysis of reservation-based random access for broadband wireless access networks
AbstractâIn this paper we propose a novel model for the capacity analysis on the reservation-based random multiple access system, which can be applied to the medium access control protocol of the emerging WiMAX technology. In such a wireless broadband access system, in order to support QoS, the channel time is divided into consecutive frames, where each frame consists of some consequent mini-slots for the transmission of requests, used for the bandwidth reservation, and consequent slots for the actual data packet transmission. Three main outcomes are obtained: first, the upper and lower bounds of the capacity are derived for the considered system. Second, we found through the mathematical analysis that the transmission rate of reservationbased multiple access protocol is maximized, when the ratio between the number of mini-slots and that of the slots per frame is equal to the reciprocal of the random multiple access algorithmâs transmission rate. Third, in the case of WiMAX networks with a large number of subscribers, our analysis takes into account both the capacity and the mean packet delay criteria and suggests to keep such a ratio constant and independent of application-level data traffic arrival rate
Amplifying ultraweak transitions in collective systems via quantum interference
We investigate laser-induced quantum interference phenomena in superradiance
processes and in an ensemble of initially excited type closely packed
three-level emitters. The lower doublet levels are pumped with a coherent laser
field. Due to constructive quantum interference effects, the superradiance
occurs on a much weaker atomic transition which is not the case in the absence
of the coherent driving. This result may be of visible relevance for enhancing
ultraweak transitions in atomic or atomic-like systems, respectively, or for
high-frequency lasing effects.Comment: 12 pages, 3 figure
WLC22-4: Efficient request mechanism usage in IEEE 802.16
IEEE 802.16 protocols for metropolitan broadband wireless access systems have been standardized recently. According to the standard, a subscriber station can deliver bandwidth request messages to a base station by numerous methods. This paper provides both the simulation and analytical models for the investigation of specified random access method, which is compared with centralized polling and station- grouping mechanisms. Based on the assumptions of Bernoulli request arrival process and ideal channel conditions, the mean delay of a request transmission is evaluated for varying number of transmission opportunities and different arrival rates
Wireless broadband access: WiMAX and beyond - Investigation of bandwidth request mechanisms under point-to-multipoint mode of WiMAX networks
The WiMAX standard specifies a metropolitan area broadband wireless access air interface. In order to support QoS for multimedia applications, various bandwidth request and scheduling mechanisms are suggested in WiMAX, in which a subscriber station can send request messages to a base station, and the base station can grant or reject the request according to the available radio resources. This article first compares two fundamental bandwidth request mechanisms specified in the standard, random access vs. polling under the point-to-multipoint mode, a mandatory transmission mode. Our results demonstrate that random access outperforms polling when the request rate is low. However, its performance degrades significantly when the channel is congested. Adaptive switching between random access and polling according to load can improve system performance. We also investigate the impact of channel noise on the random access request mechanism
An optimal gap theorem
By solving the Cauchy problem for the Hodge-Laplace heat equation for
-closed, positive -forms, we prove an optimal gap theorem for
K\"ahler manifolds with nonnegative bisectional curvature which asserts that
the manifold is flat if the average of the scalar curvature over balls of
radius centered at any fixed point is a function of .
Furthermore via a relative monotonicity estimate we obtain a stronger
statement, namely a `positive mass' type result, asserting that if is
not flat, then for any
A Practical Approach for Successive Omniscience
The system that we study in this paper contains a set of users that observe a
discrete memoryless multiple source and communicate via noise-free channels
with the aim of attaining omniscience, the state that all users recover the
entire multiple source. We adopt the concept of successive omniscience (SO),
i.e., letting the local omniscience in some user subset be attained before the
global omniscience in the entire system, and consider the problem of how to
efficiently attain omniscience in a successive manner. Based on the existing
results on SO, we propose a CompSetSO algorithm for determining a complimentary
set, a user subset in which the local omniscience can be attained first without
increasing the sum-rate, the total number of communications, for the global
omniscience. We also derive a sufficient condition for a user subset to be
complimentary so that running the CompSetSO algorithm only requires a lower
bound, instead of the exact value, of the minimum sum-rate for attaining global
omniscience. The CompSetSO algorithm returns a complimentary user subset in
polynomial time. We show by example how to recursively apply the CompSetSO
algorithm so that the global omniscience can be attained by multi-stages of SO
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