2,132 research outputs found
Infrared behavior and gauge artifacts in de Sitter spacetime: The photon field
We study the infrared (long distance) behavior of the free photon field in de
Sitter spacetime. Using a two-parameter family of gauge fixing terms, we show
that the infrared (IR) behavior of the two-point function is highly
gauge-dependent and ranges from vanishing to growing. This situation is in
disagreement with its counterpart in flat spacetime, where the two-point
function vanishes in the IR for any choice of the gauge fixing parameters. A
criterion to isolate the "physical" part of the two-point function is given and
is shown to lead to a well-behaved two-point function in the IR.Comment: 4 pages, published versio
Self-organization without heat: the geometric ratchet effect
We point out a surprising feature of diffusion in inhomogeneous media: under
suitable conditions, the rectification of the Brownian paths by a diffusivity
gradient can result in initially spread tracers spontaneously concentrating.
This "geometric ratchet effect" demonstrates that, in violation of the
classical statements of the second law of (non-equilibrium) thermodynamics,
self-organization can take place in thermodynamic systems at local equilibrium
without heat being produced or exchanged with the environment. We stress the
role of Bayesian priors in a suitable reformulation of the second law
accommodating this geometric ratchet effect.Comment: 8 pages, 4 figure
Earthquake resistance of composite beam-columns.
Imperial Users onl
AROMA: Automatic Generation of Radio Maps for Localization Systems
WLAN localization has become an active research field recently. Due to the
wide WLAN deployment, WLAN localization provides ubiquitous coverage and adds
to the value of the wireless network by providing the location of its users
without using any additional hardware. However, WLAN localization systems
usually require constructing a radio map, which is a major barrier of WLAN
localization systems' deployment. The radio map stores information about the
signal strength from different signal strength streams at selected locations in
the site of interest. Typical construction of a radio map involves measurements
and calibrations making it a tedious and time-consuming operation. In this
paper, we present the AROMA system that automatically constructs accurate
active and passive radio maps for both device-based and device-free WLAN
localization systems. AROMA has three main goals: high accuracy, low
computational requirements, and minimum user overhead. To achieve high
accuracy, AROMA uses 3D ray tracing enhanced with the uniform theory of
diffraction (UTD) to model the electric field behavior and the human shadowing
effect. AROMA also automates a number of routine tasks, such as importing
building models and automatic sampling of the area of interest, to reduce the
user's overhead. Finally, AROMA uses a number of optimization techniques to
reduce the computational requirements. We present our system architecture and
describe the details of its different components that allow AROMA to achieve
its goals. We evaluate AROMA in two different testbeds. Our experiments show
that the predicted signal strength differs from the measurements by a maximum
average absolute error of 3.18 dBm achieving a maximum localization error of
2.44m for both the device-based and device-free cases.Comment: 14 pages, 17 figure
Novel Antenna Design Suitable For Evaporation Duct
This project investigates a design of antenna array that might be suitable for implementation in the evaporation duct. The design investigates a novel horn antenna array that can produce high gain over a narrow beam width. The novel design is to operate in duct environment via having marine grade protection. This design will be explore the physical unique properties of the evaporation duct and utilize it to establish long distance communication over the evaporation duct
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