Mobile Radio Propagation

Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 1
Chapter 3
Mobile Radio Propagation

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

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

Types of Waves
Transmitter Receiver
Earth
Sky wave
Space wave
Ground wave
Troposphere
(0 - 12 km)
Stratosphere
(12 - 50 km)
Mesosphere
(50 - 80 km)
Ionosphere
(80 - 720 km)

Radio Frequency Bands
Classification
Band
Initials Frequency Range Characteristics
Extremely low ELF < 300 Hz
Ground waveInfra 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

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

Radio Propagation Effects
Transmitter
d
Receiver
hb
hm
Diffracted
Signal
Reflected Signal
Direct Signal
Building

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
PGA tte
π
=

Antenna Gain
 For a circular reflector antenna
Gain G = η ( π D / λ )2
η = net efficiency (depends on the electric field distribution over the
antenna aperture, losses, ohmic heating , typically 0.55)
D = diameter
thus, G = η (π D f /c )2
, c = λ 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

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
PGG
P trt
r =
Fast fading
Slow fading
Path loss

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 =
),(log20)(log2045.32)( 1010 kmdMHzfdBL cPF ++=

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

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)
PathLossLf(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz

Path Loss (Urban, Suburban and Open areas)
 Urban area:
where
 Suburban area:
 Open area:
[ ]
[ ] )(log)(log55.69.44
)()(log82.13)(log16.2655.69)(
1010
1010
kmdmh
mhmhMHzfdBL
b
mbcPU
−+
−−+= α
[ ]
[ ] [ ]
[ ]
[ ]








≥−
≤−
−−−
=
citymediumsmallfor
MHzfformh
MHzfformh
cityelforMHzfmhMHzf
mh
cm
cm
cmc
m
&,
400,97.4)(75.11log2.3
200,1.1)(54.1log29.8
arg,8.0)(log56.1)(7.0)(log1.1
)(
2
10
2
10
1010
α
4.5
28
)(
log2)()(
2
10 −





−=
MHzf
dBLdBL c
PUPS
[ ] 94.40)(log33.18)(log78.4)()( 10
2
10 −+−= MHzfMHzfdBLdBL ccPUPO

Path Loss
 Path loss in decreasing order:
 Urban area (large city)
 Urban area (medium and small city)
 Suburban area
 Open area

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)
PathLossLpu(dB)
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
fc=150MHz

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)
PathLossLpu(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz

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)
PathLossLps(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz

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)
PathLossLpo(dB)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz

Fading
Signal
Strength
(dB)
Distance
Path Loss
Slow Fading
(Long-term fading)
Fast Fading
(Short-term fading)

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,
σ is the standard deviation in decibels
( )
( )
,
2
1 2
2
2σ
σπ
MM
eMp
−
−
=

Log-normal Distribution
M
M
2σ
p(M)
The pdf of the received signal level

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 σ 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
>=
−
re
r
rp
r
σ
σ
.5.0)( =≤ mrrP

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
σ=1
σ=2
σ=3

Fast Fading (Continued)
 When MS far from BS, the envelope distribution of
received signal is Rician distribution. The pdf is
where
σ is the standard deviation,
I0(x) is the zero-order Bessel function of the first kind,
α is the amplitude of the direct signal
( ) 0,0
2
2
2
22
≥





=
+
−
r
r
Ie
r
rp
r
σ
α
σ
σ
α

Rician Distribution
r
Pdfp(r)
r
86420
0.6
0.5
0.4
0.3
0.2
0.1
0
α = 2
α = 1
α= 0 (Rayleigh)
σ = 1
α = 3
The pdf of the envelope variation

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.

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,
λ is the wavelength of carrier.
θ
λ
cos
v
fD =
DCR fff −=
MS
Signal
Moving
speed v


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”.

Moving Speed Effect
Time
V1 V2 V3 V4
Signalstrength

Delay Spread
Delay
SignalStrength The signals from
close by reflectors
The signals from
intermediate reflectors
The signals from
far away reflectors

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 τd.
d
R
τ2
1
<

Intersymbol Interference (ISI)
Time
Time
Time
Transmission
signal
Received signal
(short delay)
Received signal
(long delay)
1
0
1
Propagation time
Delayed signals

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.

Cochannel Interference
 Cells having the same frequency interfere with each other.
 rd is the desired signal
 ru is the interfering undesired signal
 β is the protection ratio for which
rd ≤ βru (so that the signals interfere the least)
 If P(rd ≤ βru ) is the probability that rd ≤ βru ,
Cochannel probability Pco = P(rd ≤ βru )

Mobile Radio Propagation

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Mobile Radio Propagation