Transcript Mobile Radio Propagation : Large Scale Path Loss

Mobile Radio Propagation :
Large Scale Path Loss
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The radio
propagation
channel exhibits
many different
forms of channel
impairments, as a
result of timevarying signal
reflections,
blockage and
motion.
Mobile Radio Propagation :
Large Scale Path Loss
Free Space Propagation Loss
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Power levels :
Free Space Propagation Loss
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The free-space path loss:
Free space propagation loss
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Assumes far-field (Fraunhofer region)
 d >> D and d >>  , where
 D is the largest linear dimension of antenna
  is the carrier wavelength
No interference, no obstructions
Black board 4.2
Effective isotropic radiated power
Effective radiated power
Path loss
Fraunhofer region/far field
In log scale
Exercise
An antenna with a gain of 60 dB transmits
2kW to a satellite at 6 GHz. The satellite is at
a distance of 36000 km and receives 5nW.
Determine the satellite antenna gain. (Ans :
23 dB)
Free Space Propagation Loss
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The received power predicts to fall 6dB when
the distance to the transmitter is double (or
10dB per decade). The loss increases by 6dB
if the frequency is double.
Different from practical observation! Need to
improve the model.
Free Space Propagation Loss
Free Space Propagation Loss
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Reflection, diffraction and scattering are the
three major causes which impact propagation
in a mobile communication system.
Radio Propagation Mechanisms
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Reflection
 Conductors & Dielectric materials (refraction)
 Propagation wave impinges on an object which is large as
compared to wavelength
- e.g., the surface of the Earth, buildings, walls, etc.
Diffraction
 Fresnel zones
 Radio path between transmitter and receiver obstructed by
surface with sharp irregular edges
 Waves bend around the obstacle, even when LOS (line of
sight) does not exist
Scattering
 Objects smaller than the wavelength of the
propagation wave
- e.g. foliage, street signs, lamp posts
 “Clutter” is small relative to wavelength
Reflection
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Perfect conductors reflect with no attenuation
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Dielectrics reflect a fraction of incident energy
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Like light to the mirror
“Grazing angles” reflect max*
Steep angles transmit max*
Like light to the water
Reflection induces 180 phase shift
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Why? See yourself in the mirror
q
qr
qt
Reflection
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Reflection coefficient of ground
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(a) vertical polarization (v) or E field in the plane of
incidence.
(b) horizontal polarization (h) or E field
perpendicular to the incident plane
Reflection from smooth surface
   0 r  j '
Reflection
Reflection
Reflection
Reflection coefficients
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Equation 4.26, example 4.4, Brewster angle,
perfect conductors
Reflection coefficients
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A dielectric material is a substance that is a
poor conductor of electricity, but an efficient
supporter of electrostatic fields.
For earth, at frequency 100MHz
Propagation over smooth plane
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The received signal is the phase sum of the
direct wave and the reflected wave from the
plane (2-ray model).
Propagation over smooth plane
One
line of sight and one ground bound
Method of image
Propagation over smooth plane
Propagation over smooth plane
Propagation over smooth plane
Propagation over smooth plane
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Diffraction occurs when waves hit the edge of an obstacle
Diffraction
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“Secondary” waves propagated into the shadowed region
Water wave example
Diffraction is caused by the propagation of secondary wavelets into
a shadowed region.
Excess path length results in a phase shift
The field strength of a diffracted wave in the shadowed region is the
vector sum of the electric field components of all the secondary
wavelets in the space around the obstacle.
Huygen’s principle: all points on a wavefront can be considered as
point sources for the production of secondary wavelets, and that
these wavelets combine to produce a new wavefront in the direction
of propagation.
Diffraction
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Estimating the signal attenuation caused by
diffraction of radio waves over hills and
buildings is essential in predicting the field
strength in a given service area. It is
mathematically difficult to make very precise
estimates of the diffraction losses over
complex and irregular terrian. Some cases
have been derived, such as propagation over
a knife-edge object.
Diffraction
Diffraction geometry
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Derive of equation 4.54-4.57
Diffraction geometry
Diffraction geometry
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Fresnel-Kirchoff distraction parameters, 4.56
Diffraction
Diffraction
Fresnel Screens
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Fresnel zones relate phase shifts to the positions of obstacles
Equation 4.58
A rule of thumb used for line-of-sight microwave links 55% of the
first Fresnel zone is kept clear.
Fresnel diffraction geometry
Knife-edge diffraction
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Fresnel integral, 4.59
Knife-edge diffraction loss
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Gain
Exam. 4.7
Exam. 4.8
Multiple knife-edge diffraction
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Multiple Knife-edge diffraction : For the presence of two
knife edges, replace it by an equivalent knife edge.
Scattering
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Rough surfaces
 Lamp posts and trees, scatter all directions
Critical height for bumps is f(,incident angle), 4.62
 Smooth if its minimum to maximum protuberance h is less
than critical height.
 Scattering loss factor modeled with Gaussian distribution,
4.63, 4.64.
Nearby metal objects (street signs, etc.)
 Usually modeled statistically
Large distant objects
 Analytical model: Radar Cross Section (RCS)
 Bistatic radar equation, 4.66
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It is therefore expected that the received
signal is stronger than predicted from
reflection and diffraction models alone.
Measured results
Measured results
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Propagation Models
Large scale models predict behavior averaged
over distances >> 
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Function of distance & significant environmental
features, roughly frequency independent
Breaks down as distance decreases
Useful for modeling the range of a radio system and
rough capacity planning,
Experimental rather than the theoretical for previous
three models
Path loss models, Outdoor models, Indoor models
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Small scale (fading) models describe signal
variability on a scale of 
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Multipath effects (phase cancellation) dominate,
path attenuation considered constant
Frequency and bandwidth dependent
Focus is on modeling “Fading”: rapid change in
signal over a short distance or length of time.
Log Distance Path Loss Models
Log Distance Path Loss Models
Log Distance Path Loss Models
Log Distance Path Loss Models
Typical large-scale path loss
For Example
Sometime
different values are used for n
depending on the distance from the transmitter.
For Example
For Example
n does not directly reflect the strength of the received power
Log-Normal Shadowing Model
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Shadowing occurs when objects block LOS
between transmitter and receiver
A simple statistical model can account for
unpredictable “shadowing”
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PL(d)(dB)=PL(d)+X0,
Add a 0-mean Gaussian RV to Log-Distance PL
Variance  is usually from 3 to 12.
Reason for Gaussian
Measured large-scale path loss
Determine n and  by mean and variance
 Equ. 4.70
 Equ. 4.72
 Basic of Gaussian
distribution
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Area versus Distance coverage model with
shadowing model
Percentage for
SNR larger than
a threshold
 Equ. 4.79
 Exam. 4.9
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Okumura
Model
The major disadvantage with the model is its low response to rapid
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changes in terrain, therefore the model is fairly good in urban areas,
but not as good in rural areas.
Common standard deviations between predicted and measured path
loss values are around 10 to 14 dB.
G(hre)：
 hte 
G (hte )  20 log 
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 200 
1000m  hte  30 m
 hre 
G (hre )  10 log  
 3 
hre  3 m
 hre 
G (hre )  20 log  
 3 
10m  hre  3 m
Okumura and Hata’s model
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Example 4.10
Hata
Model
Empirical formulation of the graphical data in the Okamura model.
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Valid 150MHz to 1500MHz, Used for cellular systems
The following classification was used by Hata:
LdB  A  B log d  E
■Urban area
■Suburban area
LdB  A  B log d  C
■Open area
L  A  B log d  D
dB
A  69.55  26.16 log f  13.82hb
B  44.9  6.55 log hb
C  2(log( f / 28)) 2  5.4
D  4.78 log( f / 28) 2  18.33 log f  40.94
E  3.2(log( 11.75hm )) 2  4.97
for large cities, f  300MHz
E  8.29(log( 1.54hm )) 2  1.1
for large cities, f  300MHz
E  (1.11log f  0.7)hm  (1.56 log f  0.8) for medium to small cities
PCS Extension of Hata Model
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COST-231 Hata Model, European standard
Higher frequencies: up to 2GHz
Smaller cell sizes
Lower antenna heights
LdB  F  B log d  E  G
F  46.3  33.9 log f  13.82 log hb
f >1500MHz
3 Metropolitan centers
G  Medium sized city and suburban areas
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EE 552/452 Spring 2007