This document provides an overview of the Mobile Communications course being offered in Winter 2011. It discusses various topics related to signal propagation including transmission ranges, effects on signal propagation like fading and shadowing, propagation modes like ground-wave, sky-wave and line-of-sight. It also summarizes different types of fading, fading models and techniques for dealing with fading channels. The course is taught by Suprakash Datta and more details on the course can be found on the provided website.

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

2. 9/21/2023 CSE 4215, Winter 2011 2
Signal propagation basics
Many different effects have to be
considered

3. 9/21/2023 CSE 4215, Winter 2011 3
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

4. 9/21/2023 CSE 4215, Winter 2011 4
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

14. 9/21/2023 CSE 4215, Winter 2011 14
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


19. 9/21/2023 CSE 4215, Winter 2011 19
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







20. 9/21/2023 CSE 4215, Winter 2011 20
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