Lecture Note about Propagation wave in Mobile Communication.

1. 9/21/2023 CSE 4215, Winter 2011 1
CSE 4215/5431:
Mobile Communications
Winter 2011
Suprakash Datta
datta@cs.yorku.ca
Office: CSEB 3043
Phone: 416-736-2100 ext 77875
Course page: http://www.cs.yorku.ca/course/4215
Some slides are adapted from the book website

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Signal propagation basics
Many different effects have to be
considered

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Signal propagation ranges
• Transmission range
– communication possible
– low error rate
• Detection range
– detection of the signal
possible
– no communication
possible
• Interference range
– signal may not be
detected
– signal adds to the
background noise
distance
sender
transmission
detection
interference

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Signal propagation
• Propagation in free space always like light (straight line)
• Receiving power proportional to 1/d² in vacuum – much more in real
environments
(d = distance between sender and receiver)
• Receiving power additionally influenced by
• fading (frequency dependent)
• shadowing
• reflection at large obstacles
• refraction depending on the density of a medium
• scattering at small obstacles
• diffraction at edges
reflection scattering diffraction
shadowing refraction

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Real world example

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Propagation Modes
• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight propagation

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Ground Wave Propagation

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Ground Wave Propagation
• Follows contour of the earth
• Can Propagate considerable distances
• Frequencies up to 2 MHz
• Example
– AM radio

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Sky Wave Propagation

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Sky Wave Propagation
• Signal reflected from ionized layer of
atmosphere back down to earth
• Signal can travel a number of hops,
back and forth between ionosphere and
earth’s surface
• Reflection effect caused by refraction
• Examples
– Amateur radio
– CB radio

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Line-of-Sight Propagation

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Line-of-Sight Propagation
• Transmitting and receiving antennas must be
within line of sight
– Satellite communication – signal above 30 MHz
not reflected by ionosphere
– Ground communication – antennas within effective
line of site due to refraction
• Refraction – bending of microwaves by the
atmosphere
– Velocity of electromagnetic wave is a function of
the density of the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums

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Line-of-Sight Equations
• Optical line of sight
• Effective, or radio, line of sight
• d = distance between antenna and horizon
(km)
• h = antenna height (m)
• K = adjustment factor to account for
refraction, rule of thumb K = 4/3
h
d 57
.
3

h
d 
 57
.
3

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Line-of-Sight Equations
• Maximum distance between two
antennas for LOS propagation:
• h1 = height of antenna one
• h2 = height of antenna two
 
2
1
57
.
3 h
h 



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LOS Wireless Transmission
Impairments
• Attenuation and attenuation distortion
• Free space loss
• Atmospheric absorption
• Multipath (diffraction, reflection,
refraction…)
• Noise
• Thermal noise

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Attenuation
• Strength of signal falls off with distance over
transmission medium
• Attenuation factors for unguided media:
– Received signal must have sufficient strength so
that circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher
than noise to be received without error
– Attenuation is greater at higher frequencies,
causing distortion

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Free Space Loss
• Free space loss, ideal isotropic antenna
• Pt = signal power at transmitting antenna
• Pr = signal power at receiving antenna
•  = carrier wavelength
• d = propagation distance between antennas
• c = speed of light ( 3  10 8 m/s)
where d and  are in the same units (e.g.,
meters)
   
2
2
2
2
4
4
c
fd
d
P
P
r
t 





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Free Space Loss
• Free space loss equation can be recast:









d
P
P
L
r
t
dB
4
log
20
log
10
    dB
98
.
21
log
20
log
20 


 d

    dB
56
.
147
log
20
log
20
4
log
20 








 d
f
c
fd


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Free Space Loss
• Free space loss accounting for gain of
other antennas
• Gt = gain of transmitting antenna
• Gr = gain of receiving antenna
• At = effective area of transmitting antenna
• Ar = effective area of receiving antenna
       
t
r
t
r
t
r
r
t
A
A
f
cd
A
A
d
G
G
d
P
P
2
2
2
2
2
2
4







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Free Space Loss
• Free space loss accounting for gain of
other antennas can be recast as
     
r
t
dB A
A
d
L log
10
log
20
log
20 

 
      dB
54
.
169
log
10
log
20
log
20 



 r
t A
A
d
f

21. Multipath Propagation

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Multipath propagation
• Signal can take many different paths between sender
and receiver due to reflection, scattering, diffraction
• Time dispersion: signal is dispersed over time
– interference with “neighbor” symbols, Inter Symbol
Interference (ISI)
• The signal reaches a receiver directly and phase
shifted
– distorted signal depending on the phases of the different
parts
signal at sender
signal at receiver
LOS pulses
multipath
pulses

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Atmospheric absorption
• Water vapor and oxygen contribute
most
• Water vapor: peak attenuation near
22GHz, low below 15Ghz
• Oxygen: absorption peak near 60GHz,
lower below 30 GHz.
• Rain and fog may scatter (thus
attenuate) radio waves.
• Low frequency band usage helps…

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Effects of mobility
• Channel characteristics change over time and
location
– signal paths change
– different delay variations of different signal parts
– different phases of signal parts
–  quick changes in the power received (short term
fading)
• Additional changes in
– distance to sender
– obstacles further away
–  slow changes in the average
power received (long term fading)
short term fading
long term
fading
t
power

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Fading channels
• Fading: Time variation of received
signal power
• Mobility makes the problem of modeling
fading difficult
• Multipath propagation is a key reason
• Most challenging technical problem for
Mobile Communications

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Types of Fading
• Short term (fast) fading
• Long term (slow) fading
• Flat fading – across all frequencies
• Selective fading – only in some frequencies
• Rayleigh fading – no LOS path, many other
paths
• Rician fading – LOS path plus many other
paths

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Fading models

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Dealing with fading channels
• Error correction
• Adaptive equalization
– attempts to increase signal power as needed
– can be done with analog circuits or DSP