32,149 research outputs found
Hamiltonian and Phase-Space Representation of Spatial Solitons
We use Hamiltonian ray tracing and phase-space representation to describe the
propagation of a single spatial soliton and soliton collisions in a Kerr
nonlinear medium. Hamiltonian ray tracing is applied using the iterative
nonlinear beam propagation method, which allows taking both wave effects and
Kerr nonlinearity into consideration. Energy evolution within a single spatial
soliton and the exchange of energy when two solitons collide are interpreted
intuitively by ray trajectories and geometrical shearing of the Wigner
distribution functions.Comment: 12 pages, 5 figure
Effects of virtual acoustics on dynamic auditory distance perception
Sound propagation encompasses various acoustic phenomena including
reverberation. Current virtual acoustic methods, ranging from parametric
filters to physically-accurate solvers, can simulate reverberation with varying
degrees of fidelity. We investigate the effects of reverberant sounds generated
using different propagation algorithms on acoustic distance perception, i.e.,
how faraway humans perceive a sound source. In particular, we evaluate two
classes of methods for real-time sound propagation in dynamic scenes based on
parametric filters and ray tracing. Our study shows that the more accurate
method shows less distance compression as compared to the approximate,
filter-based method. This suggests that accurate reverberation in VR results in
a better reproduction of acoustic distances. We also quantify the levels of
distance compression introduced by different propagation methods in a virtual
environment.Comment: 8 Pages, 7 figure
Refraction-corrected ray-based inversion for three-dimensional ultrasound tomography of the breast
Ultrasound Tomography has seen a revival of interest in the past decade,
especially for breast imaging, due to improvements in both ultrasound and
computing hardware. In particular, three-dimensional ultrasound tomography, a
fully tomographic method in which the medium to be imaged is surrounded by
ultrasound transducers, has become feasible. In this paper, a comprehensive
derivation and study of a robust framework for large-scale bent-ray ultrasound
tomography in 3D for a hemispherical detector array is presented. Two
ray-tracing approaches are derived and compared. More significantly, the
problem of linking the rays between emitters and receivers, which is
challenging in 3D due to the high number of degrees of freedom for the
trajectory of rays, is analysed both as a minimisation and as a root-finding
problem. The ray-linking problem is parameterised for a convex detection
surface and three robust, accurate, and efficient ray-linking algorithms are
formulated and demonstrated. To stabilise these methods, novel
adaptive-smoothing approaches are proposed that control the conditioning of the
update matrices to ensure accurate linking. The nonlinear UST problem of
estimating the sound speed was recast as a series of linearised subproblems,
each solved using the above algorithms and within a steepest descent scheme.
The whole imaging algorithm was demonstrated to be robust and accurate on
realistic data simulated using a full-wave acoustic model and an anatomical
breast phantom, and incorporating the errors due to time-of-flight picking that
would be present with measured data. This method can used to provide a
low-artefact, quantitatively accurate, 3D sound speed maps. In addition to
being useful in their own right, such 3D sound speed maps can be used to
initialise full-wave inversion methods, or as an input to photoacoustic
tomography reconstructions
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
Modeling the Effect of Oceanic Internal Waves on the Accuracy of Multibeam Echosounders
When ray bending corrections are applied to multibeam echosounder (MBES) data, it is assumed that the varying layers of sound speed lie along horizontally stratified planes. In many areas internal waves occur at the interface where the water’s density changes abruptly (a pycnocline), this density gradient is often associated with a strong gradient in sound speed (a velocline). The internal wave introduces uncertainty into the echo soundings through two mechanisms: (1) tilting of the velocline, and (2) vertical oscillation of the velocline’s depth. A model has been constructed in order to examine how these effects degrade the accuracy of MBES measurements. The model numerically simulates the 3D ray paths of MBES soundings for a synthetic flat seafloor, as though the soundings have been collected through a user-defined internal wave. Along with sound speed information, the ray paths are used to estimate travel times which are then utilized as inputs for a conventional 2D ray trace. The discrepancy between the 3D and 2D ray traced solutions serve as an estimate of uncertainty. The same software can be extended to model the expected anomalies associated with tidal fronts and other phenomena that result in significant tilting or oscillation of the velocline. A case study was undertaken using observed internal wave parameters on the Scotian Shelf. The case study examines how survey design parameters such as line spacing, direction of survey lines, and water column sampling density can influence the uncertainty introduced by internal waves. In particular, an examination is undertaken in which 2D ray tracing models are augmented with MBES water column imaging of the velocline. The investigation shows that internal waves have the potential to cause vertical uncertainties exceeding IHO standards and that the uncertainty can potentially be mitigated through appropriate survey design. Results from the case study also indicate that acoustic tracking of the velocline has the potential to counteract the effects of internal waves through augmentation of 2D ray tracing models. This technique is promising, however, much more research and field testing is required to ascertain the practicality, reliability and repeatability of such an approach
Modelling Uncertainty Caused by Internal Waves on the Accuracy of MBES
A 3D ray tracing model has been developed to estimate the effects of internal waves upon the accuracy of multibeam echosounders (MBES). A case study examines the variability in these effects as a function of survey line direction and also considers the case of improving 2D ray tracing models with wave parameters derived from MBES water column imagery. Results indicate that, under certain circumstances, the effects of internal waves can prove to be a significant source of uncertainty that detracts from the ability to efficiently map the seafloor with wide swath angles
The Effects of Neighboring Buildings on the Indoor Wireless Channel at 2.4 and 5.8 GHz
The effects of neighboring buildings (NBs) on the indoor
wireless channel are examined both in time and space domain
at 2.4 GHz and 5.8 GHz band using the computer
simulation of radio wave propagation based on ray-tracing
technique. The NBs in the apartment environments have a
considerable effect on the channel characteristics, such as
the exponents of path loss, mean excess delay, rms delay
spread time, coherence bandwidth, and angle dispersion of
received rays. Also the effects are shown to be different according
to the frequency band
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