M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 88 CHAPTER 4 Indoor and Outdoor Propagation Models Introduction A radio propagation model, also known as the radio frequency propagation model, is an empirical mathematical formulation for the characterization of radio wave propagation as a function of frequency, distance and other conditions .A single model is usually developed to predict the behavior of propagation for all similar links under similar constraints. Created with the goal of formalizing the way radio waves are propagated from one place to another, such models typically predict the path loss along a link or the effective coverage area of a transmitter. 4.1 Channel Models There are different types of channel models in underground communication. These models vary from one situation to other based on number of parameters . Modeling is different for tunnels as compared to mines . Some of the important models used in underground communication are : 4.1(a) The Geometrical Optical (GO) Model In recent years, the computational and visualization capabilities of computers have accelerated rapidly. Predicting radio signal coverage involve the use of Site Specific ( SISP ) propagation models and Graphical Information System ( GIS ) databases .SISP models support ray tracing as a means of deterministically modeling any indoor or outdoor propagation environment . Through the use of terrain databases, which may be drawn or digitized using standard graphical software packages, wireless system designers are able to include accurate representation of terrain features. Hence GO model uses geometrical optics, which traces all the paths from the transmitter to the receiver. The ray path includes LOS path, multi path reflections etc [75].
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M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 88
CHAPTER 4
Indoor and Outdoor Propagation Models
Introduction
A radio propagation model, also known as the radio frequency propagation
model, is an empirical mathematical formulation for the characterization of radio
wave propagation as a function of frequency, distance and other conditions .A single
model is usually developed to predict the behavior of propagation for all similar links
under similar constraints. Created with the goal of formalizing the way radio waves
are propagated from one place to another, such models typically predict the path loss
along a link or the effective coverage area of a transmitter.
4.1 Channel Models
There are different types of channel models in underground communication.
These models vary from one situation to other based on number of parameters .
Modeling is different for tunnels as compared to mines . Some of the important
models used in underground communication are :
4.1(a) The Geometrical Optical (GO) Model
In recent years, the computational and visualization capabilities of computers
have accelerated rapidly. Predicting radio signal coverage involve the use of Site
Specific ( SISP ) propagation models and Graphical Information System ( GIS )
databases .SISP models support ray tracing as a means of deterministically modeling
any indoor or outdoor propagation environment . Through the use of terrain databases,
which may be drawn or digitized using standard graphical software packages, wireless
system designers are able to include accurate representation of terrain features. Hence
GO model uses geometrical optics, which traces all the paths from the transmitter to
the receiver. The ray path includes LOS path, multi path reflections etc [75].
M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 89
4.1b The Waveguide Model
This is mainly used in modeling the tunnel channels. Here we consider the
underground wireless channel as an oversized wave guide with imperfect loss walls.
Due to lossy dielectric characteristic of walls, ceilings electromagnetic wave gets
strongly attenuated. Also vehicles inside the tunnel act as obstructing units for the
signal (Fig 4.1).
Fig 4.1 - Tunnel with obstructions.
Using Geometrical Optical (GO) Model, EM distribution for the excitation
plane is determined. Multiple waveguide modes propagating inside the guide is based
on excitation and under goes different levels of attenuation. The effective field
distribution will be the algebraic sum of all the modes as shown in Fig 4.2.
Fig 4.2 - Effective field distribution
Once the mode intensity is determined in the excitation plane, the propagation
of each mode is mostly governed by the tunnel itself. Hence the EM field of the rest
of the tunnel can be accurately predicted by summing the EM field of each mode.
Position of antenna determines mode intensity. Mode attenuation is determined by
tunnel size and operating frequency .Material of tunnel, humidity, pressure and
temperature determine signal propagation. In practical tunnel traffic produces
additional signal loss of a guided mode.
M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 90
4.1(c) Room-and-Pillar Model
This is used in underground mines, where there are number of paths to a given
entrance. Mine is considered as a big room with number of pillars. Structure of
mining area is important. By characterizing the room and pillars with the help of
dielectric constants modeling is done.
4.1(d) The Full Wave Model
This utilizes numerical solutions to solve the temporal and spatial Maxwell's
equations with approximate boundary conditions to model the signal propagation in
underground mines.
Some of the important limitations of above models are:
• These models are not general but are site limited or applicable to a
particular situation .eg: Mine/tunnel with a particular geometry or terrain, at a
particular frequency.
• They don’t consider all the signal losses for modeling .Consider only
penetration & Multipath fading losses.
• Geometrical optical model & full wave models don’t consider
frequency as a parameter for modeling. Frequency is important because depth of
penetration, bending of wireless signals and most of the losses depends on frequency.
• In optical modeling ray path tracing is difficult.
• In wave guide modeling all the times they consider the geometry as
rectangular. This is not true in mines with circular and other types of complex
geometry.
In Maxwell’s equation modeling several approximations regarding boundary
conditions like smooth surface, infinite conductivity etc are assumed. This is not true
because soil conductivity changes with moisture, Mine terrain is not smooth.
M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 91
Hence we have considered a different type of modeling where we can consider
all losses, overcome some of the problems faced by the above models.
4.2 Path Loss
Path loss (or path attenuation) is the reduction in power density of an
electromagnetic wave as it propagates through space or underground. This term is
commonly used in wireless communications and signal propagation. Path loss may be
due to many effects, such as free-space loss, refraction, diffraction, reflection,
aperture-medium coupling loss, and absorption. Path loss is also influenced by terrain
contours, environment, propagation medium (dry or moist air), the distance between
the transmitter and the receiver and the height and location of antennas.
4.2(a) Causes for path loss
Path loss normally includes propagation losses caused by the natural
expansion of the radio wave front in free space (which usually takes the shape of an
ever-increasing sphere), absorption losses (sometimes called penetration losses), when
the signal passes through media not transparent to electromagnetic waves, diffraction
losses when part of the radio wave front is obstructed by an opaque obstacle, and
losses caused by other phenomena.
The signal radiated by a transmitter may also travel along many and different
paths to a receiver simultaneously; this effect is called multipath. Multipath can either
increase or decrease received signal strength, depending on whether the individual
multipath wave fronts interfere constructively or destructively. The total power of
interfering waves in a Rayleigh fading scenario vary quickly as a function of space
(which is known as small scale fading), resulting in fast fades which are very sensitive
to receiver position.
4.2(b) Prediction of path loss
Calculation of the path loss is usually called prediction. Exact prediction is
possible only for simpler cases, such as the above-mentioned free space propagation
M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 92
or the flat-earth model. For practical cases the path loss is calculated using a variety of
approximations.
Statistical methods (also called stochastic or empirical) are based on measured
and averaged losses along typical classes of radio links. Among the most commonly
used such methods are Okumura-Hata, the COST-Hata model etc. These are also
known as radio wave propagation models. For wireless communications in the VHF
and UHF frequency band, one of the most commonly used methods is that of
Okumura-Hata as refined by the COST231 project. For FM radio and TV
broadcasting the path loss is most commonly predicted using the ITU model.
Deterministic methods based on the physical laws of wave propagation are
also used; ray tracing is one such method. These methods are expected to produce
more accurate and reliable predictions of the path loss than the empirical methods;
however, they are significantly more expensive in computational effort. For these
reasons they are used predominantly for short propagation paths. Among the most
commonly used methods in the design of radio equipment such as antennas and feeds
is the finite-difference time-domain method.
The path loss in other frequency bands (MW, SW, and Microwave) is
predicted with similar methods, though the concrete algorithms and formulas may be
very different from those for VHF/UHF. Reliable prediction of the path loss in the
SW/HF band is particularly difficult, and its accuracy is comparable to weather
predictions.
Easy approximations for calculating the path loss over distances significantly
shorter than the distance to the radio horizon:
In free space the path loss increases with 20 dB per decade (one decade
is when the distance between the transmitter and the receiver increases ten times) or 6
dB per octave (one octave is when the distance between the transmitter and the
receiver doubles). This can be used as a very rough first-order approximation for SHF
(microwave) communication links.
For signals in the UHF/VHF band propagating over the surface of the
Earth the path loss increases with roughly 35 to 40 dB per decade (10 to 12 dB per
octave). This can be used in cellular networks as a first guess.