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Chapt-03.ppt
- 1. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 1
Chapter 3
Mobile Radio Propagation
- 2. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 2
Outline
Speed, Wavelength, Frequency
Types of Waves
Radio Frequency Bands
Propagation Mechanisms
Radio Propagation Effects
Free-Space Propagation
Land Propagation
Path Loss
Fading: Slow Fading / Fast Fading
Delay Spread
Doppler Shift
Co-Channel Interference
The Near-Far Problem
Digital Wireless Communication System
Analog and Digital Signals
Modulation Techniques
- 3. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 3
Speed, Wavelength, Frequency
System Frequency Wavelength
AC current 60 Hz 5,000 km
FM radio 100 MHz 3 m
Cellular 800 MHz 37.5 cm
Ka band satellite 20 GHz 15 mm
Ultraviolet light 1015 Hz 10-7 m
Light speed = Wavelength x Frequency
= 3 x 108 m/s = 300,000 km/s
- 4. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 4
Types of Waves
Earth
Sky wave
Space wave
Ground wave
Troposphere
(0 - 12 km)
Stratosphere
(12 - 50 km)
Mesosphere
(50 - 80 km)
Ionosphere
(80 - 720 km)
- 5. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 5
Radio Frequency Bands
Classification Band Initials Frequency Range Characteristics
Extremely low ELF < 300 Hz
Ground wave
Infra low ILF 300 Hz - 3 kHz
Very low VLF 3 kHz - 30 kHz
Low LF 30 kHz - 300 kHz
Medium MF 300 kHz - 3 MHz Ground/Sky wave
High HF 3 MHz - 30 MHz Sky wave
Very high VHF 30 MHz - 300 MHz
Space wave
Ultra high UHF 300 MHz - 3 GHz
Super high SHF 3 GHz - 30 GHz
Extremely high EHF 30 GHz - 300 GHz
Tremendously high THF 300 GHz - 3000 GHz
- 6. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 6
Propagation Mechanisms
Reflection
Propagation wave impinges on an object which is large as
compared to wavelength
- e.g., the surface of the Earth, buildings, walls, etc.
Diffraction
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
- 7. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 7
Radio Propagation Effects
Transmitter
d
Receiver
hb
hm
Diffracted
Signal
Reflected Signal
Direct Signal
Building
- 8. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 8
Free-space Propagation
The received signal power at distance d:
where Pt is transmitting power, Ae is effective area, and Gt is the
transmitting antenna gain. Assuming that the radiated power is uniformly
distributed over the surface of the sphere.
Transmitter Distance d
Receiver
hb
hm
2
r
4
P
d
P
G
A t
t
e
- 9. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 9
Antenna Gain
For a circular reflector antenna
Gain G = ( D / )2
= net efficiency (depends on the electric field distribution over the
antenna aperture, losses, ohmic heating , typically 0.55)
D = diameter
thus, G = ( D f /c )2, c = f (c is speed of light)
Example:
Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 m
G = 39.4 dB
Frequency = 14 GHz, same diameter, wavelength = 0.021 m
G = 46.9 dB
* Higher the frequency, higher the gain for the same size antenna
- 10. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 10
Land Propagation
The received signal power:
where Gr is the receiver antenna gain,
L is the propagation loss in the channel, i.e.,
L = LP LS LF
L
P
G
G
P t
r
t
r
Fast fading
Slow fading
Path loss
- 11. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 11
Path Loss (Free-space)
Definition of path loss LP :
Path Loss in Free-space:
where fc is the carrier frequency.
This shows greater the fc , more is the loss.
,
r
t
P
P
P
L
),
(
log
20
)
(
log
20
45
.
32
)
( 10
10 km
d
MHz
f
dB
L c
PF
- 12. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 12
Path Loss (Land Propagation)
Simplest Formula:
Lp = A d-α
where
A and α: propagation constants
d : distance between transmitter and receiver
α : value of 3 ~ 4 in typical urban area
- 13. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 13
Example of Path Loss (Free-space)
Path Loss in Free-space
70
80
90
100
110
120
130
0 5 10 15 20 25 30
Distance d (km)
Path
Loss
Lf
(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
- 14. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 14
Path Loss (Urban, Suburban and Open areas)
Urban area:
where
Suburban area:
Open area:
)
(
log
)
(
log
55
.
6
9
.
44
)
(
)
(
log
82
.
13
)
(
log
16
.
26
55
.
69
)
(
10
10
10
10
km
d
m
h
m
h
m
h
MHz
f
dB
L
b
m
b
c
PU
city
medium
small
for
MHz
f
for
m
h
MHz
f
for
m
h
city
e
l
for
MHz
f
m
h
MHz
f
m
h
c
m
c
m
c
m
c
m
&
,
400
,
97
.
4
)
(
75
.
11
log
2
.
3
200
,
1
.
1
)
(
54
.
1
log
29
.
8
arg
,
8
.
0
)
(
log
56
.
1
)
(
7
.
0
)
(
log
1
.
1
)
(
2
10
2
10
10
10
4
.
5
28
)
(
log
2
)
(
)
(
2
10
MHz
f
dB
L
dB
L c
PU
PS
94
.
40
)
(
log
33
.
18
)
(
log
78
.
4
)
(
)
( 10
2
10
MHz
f
MHz
f
dB
L
dB
L c
c
PU
PO
- 15. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 15
Path Loss
Path loss in decreasing order:
Urban area (large city)
Urban area (medium and small city)
Suburban area
Open area
- 16. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 16
Example of Path Loss (Urban Area: Large City)
Path Loss in Urban Area in Large City
100
110
120
130
140
150
160
170
180
0 10 20 30
Distance d (km)
Path
Loss
Lpu
(dB)
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
fc=150MHz
- 17. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 17
Example of Path Loss
(Urban Area: Medium and Small Cities)
Path Loss in Urban Area for Small & Medium Cities
100
110
120
130
140
150
160
170
180
0 10 20 30
Distance d (km)
Path
Loss
Lpu
(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
- 18. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 18
Example of Path Loss (Suburban Area)
Path Loss in Suburban Area
90
100
110
120
130
140
150
160
170
0 5 10 15 20 25 30
Distance d (km)
Path
Loss
Lps
(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
- 19. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 19
Example of Path Loss (Open Area)
Path Loss in Open Area
80
90
100
110
120
130
140
150
0 5 10 15 20 25 30
Distance d (km)
Path
Loss
Lpo
(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
- 20. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 20
Fading
Signal
Strength
(dB)
Distance
Path Loss
Slow Fading
(Long-term fading)
Fast Fading
(Short-term fading)
- 21. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 21
Slow Fading
The long-term variation in the mean level is known as slow fading
(shadowing or log-normal fading). This fading caused by shadowing.
Log-normal distribution:
- The pdf of the received signal level is given in decibels by
where M is the true received signal level m in decibels, i.e., 10log10m,
M is the area average signal level, i.e., the mean of M,
is the standard deviation in decibels
,
2
1 2
2
2
M
M
e
M
p
- 22. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 22
Log-normal Distribution
M
M
2
p(M)
The pdf of the received signal level
- 23. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 23
Fast Fading
The signal from the transmitter may be reflected from
objects such as hills, buildings, or vehicles.
When MS far from BS, the envelope distribution of received signal is
Rayleigh distribution. The pdf is
where is the standard deviation.
Middle value rm of envelope signal within sample range to be
satisfied by
We have rm = 1.777
0
,
2
2
2
2
r
e
r
r
p
r
.
5
.
0
)
(
m
r
r
P
- 24. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 24
Rayleigh Distribution
The pdf of the envelope variation
r
2 4 6 8 10
P(r)
0
0.2
0.4
0.6
0.8
1.0
=1
=2
=3
- 25. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 25
Fast Fading (Continued)
When MS far from BS, the envelope distribution of
received signal is Rician distribution. The pdf is
where
is the standard deviation,
I0(x) is the zero-order Bessel function of the first kind,
is the amplitude of the direct signal
0
,
0
2
2
2
2
2
r
r
I
e
r
r
p
r
- 26. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 26
Rician Distribution
r
Pdf
p(r)
r
8
6
4
2
0
0.6
0.5
0.4
0.3
0.2
0.1
0
= 2
= 1
= 0 (Rayleigh)
= 1
= 3
The pdf of the envelope variation
- 27. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 27
Characteristics of Instantaneous Amplitude
Level Crossing Rate:
Average number of times per second that the signal
envelope crosses the level in positive going direction.
Fading Rate:
Number of times signal envelope crosses middle
value in positive going direction per unit time.
Depth of Fading:
Ratio of mean square value and minimum value of
fading signal.
Fading Duration:
Time for which signal is below given threshold.
- 28. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 28
Doppler Shift
Doppler Effect: When a wave source and a receiver are moving towards
each other, the frequency of the received signal will not be the same as the
source.
When they are moving toward each other, the frequency of the received signal
is higher than the source.
When they are opposing each other, the frequency decreases.
Thus, the frequency of the received signal is
where fC is the frequency of source carrier,
fD is the Doppler frequency.
Doppler Shift in frequency:
where v is the moving speed,
is the wavelength of carrier.
cos
v
fD
D
C
R f
f
f
MS
Signal
Moving
speed v
- 29. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 29
Delay Spread
When a signal propagates from a transmitter to a
receiver, signal suffers one or more reflections.
This forces signal to follow different paths.
Each path has different path length, so the time of
arrival for each path is different.
This effect which spreads out the signal is called
“Delay Spread”.
- 30. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 30
Moving Speed Effect
Time
V1 V2 V3 V4
Signal
strength
- 31. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 31
Delay Spread
Delay
Signal
Strength
The signals from
close by reflectors
The signals from
intermediate reflectors
The signals from
far away reflectors
- 32. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 32
Intersymbol Interference (ISI)
Caused by time delayed multipath signals
Has impact on burst error rate of channel
Second multipath is delayed and is received
during next symbol
For low bit-error-rate (BER)
R (digital transmission rate) limited by delay
spread d.
d
R
2
1
- 33. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 33
Intersymbol Interference (ISI)
Time
Time
Time
Transmission
signal
Received signal
(short delay)
Received signal
(long delay)
1
0
1
Propagation time
Delayed signals
- 34. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 34
Coherence Bandwidth
Coherence bandwidth Bc:
Represents correlation between 2 fading signal
envelopes at frequencies f1 and f2.
Is a function of delay spread.
Two frequencies that are larger than coherence
bandwidth fade independently.
Concept useful in diversity reception
Multiple copies of same message are sent using
different frequencies.
- 35. Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 35
Cochannel Interference
Cells having the same frequency interfere with each other.
rd is the desired signal
ru is the interfering undesired signal
is the protection ratio for which
rd ru (so that the signals interfere the least)
If P(rd ru ) is the probability that rd ru ,
Cochannel probability Pco = P(rd ru )