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. In
order to accelerate the time-consuming path
determination the Intelligent Ray Tracing (IRT) is based
on a preprocessing of the building data, thus combining
high accuracy with short computation time. |
 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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Short
computation time due to preprocessing
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IRT is based
on the following assumptions:
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Only a
few rays deliver the main part of energy
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The
visibility relation between walls and edges are
independent of the position of the transmitter
antenna (base station)
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Often
adjacent receiver pixels are reached by similar rays
Based on
these considerations, a (pre-)processing of the building
database is made once. In this preprocessing the
obstacles in the building database are subdivided into
small tiles. The visibility relations between these
tiles are determined and stored. During the prediction
this data can be read and has not to be determined
again. This accelerates the computation time
significantly. |
Division of
a wall during the preprocessing. Click
here to enlarge.
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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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2x2D Mode for large scenarios
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receiver propagation paths, the most dominant
ones have to be selected to obtain the total
received power with moderate computation time. A
useful acceleration to the process of ray path
finding under the consideration of the main
propagation mechanisms is the limitation to two
orthogonal planes (double 2D). Rooftop
diffracted paths are included in the vertical
plane approach, while around building diffracted
paths are modeled within the transverse plane
approach. The propagation in both the vertical
and the transverse plane is two-dimensionally
regarded. However, the determination of the
building corners in the transverse plane is not
necessarily performed in a horizontal plane.
This principle can be also considered for the
Intelligent Ray Tracing approach which leads to
the following 2 x 2D models. |
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2x2D (2D-H IRT + 2D-V IRT)
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2x2D (2D-H IRT + COST231-W-I)
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The preprocessing and as a follow on also
the determination of propagation paths is done
in two perpendicular planes. One horizontal
plane (for the wave guiding, including the
vertical wedges) and one vertical plane (for the
over rooftop propagation including the
horizontal edges). In both planes the
propagation paths are determined similar to the
3D-IRT by using ray optical methods. This
approach neglects the contributions by
reflections at the building walls which are in
most cases only relevant for the streets with
LOS to the transmitter.
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This model treats the propagation in the
horizontal plane in exactly the same way as the
previously described model, i.e. by using ray
optical methods (for the wave guiding, including
the vertical wedges). The over rooftop
propagation (vertical plane) is taken into
account by evaluating the COST
231-Walfisch-Ikegami model. By using this model
only the propagation in the horizontal plane is
determined by ray-optical methods taking into
account the vertical wedges of the buildings,
while the over rooftop propagation is modelled
by an empirical approach.
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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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