Introduction
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The mobile radio channel in urban areas is characterized
by multi-path propagation. Dominant propagation
phenomena in such built-up environments are the
shadowing behind obstacles, the reflection at building
walls, wave guiding effects in street canyons and
diffractions at vertical or horizontal wedges. The
deterministic ray optical models consider these effects,
which leads to highly accurate prediction results. |
 Propagation
paths in an urban scenario.
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Path Classes
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As mentioned
above there are different types of rays (direct,
reflected, diffracted) especially when we consider the
combination of reflections and multiple diffraction. The
path loss occurring along these rays depends on the
number and the kind of interactions. Therefore we
arranged the different ray types in classes according to
the expected path loss. When doing the prediction, the
type of rays that should be considered during the
prediction is defined using these so called path
classes.
Inside a
specific class a similar interaction loss for the
different rays can be assumed and with increasing order
of the path class the interaction loss to be expected
increases. For the prediction a maximum and a minimum
number of path classes can be defined.
The maximum number defines the maximum path class which
is computed. The minimum number defines the abort
condition: The computation for an individual pixel is
canceled if at least one ray is found that is in the
minimum class or higher. |
Path Class
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Description
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1
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Direct path
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2
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Single reflection
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3
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Double reflection
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4
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Single diffraction
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5
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Triple reflection
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6
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One reflection + one diffraction
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7
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Double diffraction
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8
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Two reflections + one diffraction
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9
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Four reflections
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10
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Five reflections
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11
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Six reflections
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Consideration of Propagation Phenomena
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For the
computation of the rays, not only the free space loss
has to be considered but also the loss due to the
reflections and (multiple) diffraction. This is either
done using a physical deterministic model or using an
empirical model.
Empirical Interaction Model
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Deterministic Interaction Model
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The empirical model uses five empirical
material parameters (min. loss of incident ray,
max. loss of incident ray, loss of diffracted
ray, reflection loss, transmission loss). For
correction purposes or for the adaptation to
measurements, an offset to those material
parameters can be specified.
Herewith the empirical model has the advantage
that the needed material properties are easier
to obtain than the physical parameters required
for the deterministic model. Also the parameters
of the empirical model can more easily be
calibrated with measurements. It is therefore
easier to achieve a high accuracy with the
empirical model.
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The deterministic model uses Fresnel
Equations for the determination of the
reflection and transmission loss and the GTD/UTD
for the determination of the diffraction loss.
This model has a slightly longer computation
time and uses three physical material parameters
(permittivity, permeability and conductivity).
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Postprocessing with COST 231 Walfisch-Ikegami Model
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Ray optical
propagation models consider a maximum number of
reflections and diffractions. Due to that limited
number, not all prediction points may be reached with
the ray optical algorithms (especially far away from the
transmitter).
This
remaining part of the pixels can be computed with
empirical models, based on the direct ray between
transmitter and receiver. For urban scenarios the
COST-Walfisch-Ikegami model is implemented. A transition
function between the empirical prediction and the
ray-optical prediction leads to a smooth transition
between the two models. An example for the transition
function between the two models is shown in figure on
the right. |
Transition function between Ray Tracing and
COST-Walfisch-Ikegami
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Consideration of topography
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Recalling the influence of data base information on
prediction accuracy, the terrain profile should be
considered for the propagation modeling if the
considered urban area is not flat. The criterion taken
into account is the standard deviation of the terrain
heights in comparison to the standard deviation of the
building heights in the considered area. For large
standard deviations of the terrain heights data bases in
pixel format are required with resolutions about 20-30
m, i.e. higher resolution than for the terrain models.
The Intelligent Ray Tracing considers topography during
the computation. |
Part of a city in hilly terrain.
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