WCS wireless communication by T L Singal

wireless communication

1. @ McGraw-Hill Education 1
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
1
PowerPoint Slides
Wireless Communications T L Singal
PROPRI
ETARY MATERI
AL
. ©
2010
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Graw
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2. @ McGraw-Hill Education 2
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
2
1
Evolution of Wireless Communication Systems

3. @ McGraw-Hill Education 3
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
3
Evolution of Wireless Communication Systems
‰
Brief History of Wireless Communications
‰
Advantages of Wireless Communications
‰
Disadvantages of Wireless Communications
‰
Wireless Network Generations
‰
Comparison of Wireless Systems
‰
Evolution to Next Generation Networks
‰
Applications of Wireless Communications
‰
Potential Market Areas
‰
Challenges for Research

4. @ McGraw-Hill Education 4
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
4
What is Wireless
Communications?
Wireless Communications
–
The
transmission of user information such as human voice, digital data, e-mail messages, video and other multimedia services without the use of wires

5. @ McGraw-Hill Education 5
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
5Brief History of Wireless Communications
¾
Radio and Television Communications
¾
Radar Communications
¾
Satellite Communications
¾
Wireless and Mobile Communications
¾
Cellular Communications

6. @ McGraw-Hill Education 6
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
6Transition from Analog to Digital Systems
™
System capacity
™
Quality aspects
™
Compatibility with other systems such as ISDN

7. @ McGraw-Hill Education 7
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
7
Advantages of Wireless Communications
9
Mobility
9
Increased reliability
9
Ease of installation
9
Rapid disaster recovery
9
Lower cost

8. @ McGraw-Hill Education 8
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
8
Disadvantages of Wireless Communications ‰
Radio signal interference
‰
Security
‰
Health hazards

9. @ McGraw-Hill Education 9
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
9
Wireless Network Generations
™
First Generation Analog Cellular Systems
™
Second Generation Digital Cellular Systems
™
Evolution from 2G to 3G Cellular Networks
™
Third Generation Digital Cellular Systems
™
Wireless Networking Technologies

10. @ McGraw-Hill Education 10
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
10Existing 1G Analog Cellular Systems
™
AMPS : Advanced Mobile Phone System
™
ETACS: Enhanced Total Access Communication System
™
NMT : Nordic Mobile Telephone
™
JTACS : Japanese Total Access Communication System
™
NTACS: Narrowband JTACS

11. @ McGraw-Hill Education 11
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
11
Analog Cellular Systems
Standard
Frequency Band
Multiple Access
Modulation
C
hannel
BW
FDMA
FM
AMPS
824-894 MHz
30KHz
FDMA
FM
NAMPS
824-894 MHz
10KHz
NMT-450
450-470 MHz
FDMA
FM
FDMA
FM
25KHz
NMT-900
890-960 MHz
FDMA
FM
12.5KHz
NTT
400/800 MHz
FDMA
FM
25KHz
JTACS
860-925 MHz
FDMA
FM
25KHz
NTACS
843-925 MHz
FDMA
FM
12.5KHz
ETACS
900 MHz
25KHz

12. @ McGraw-Hill Education 12
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
12
Second Generation Digital Cellular Systems ™
IS-54/IS-136 : US Digital Cellular (USDC) -
T
DMA System
™
GSM : Global System for Mobile
™
PDC : Pacific Digital Cellular -
A

Japanese TDMA Cellular Standard
™
IS-95/cdmaOne : CDMA Cellular System

13. @ McGraw-Hill Education 13
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
13
Digital Cellular Systems
Standard
Frequency Band
Multiple Access
Modulation
C
hannel
BW
π
/4-DQPSK
USDC
824-894 MHz
TDMA
30KHz
IS-95
824-894 MHz 1.8-2.0 GHz
CDMA
QPSK/ BPSK
1.25MHz
PDC
810-1501 MHz
TDMA
Π
/4-DQPSK
25KHz
GSM
890-960 MHz
TDMA
GMSK
200KHz

14. @ McGraw-Hill Education 14
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
14
An Evolution Path from GSM to 3G Network
HSCSD
GP
RS
EDGE
EGPRS
WCDM
A
WCDMA
Phase
I
9.6 kbp
s
9.6 -
2
8.8 kbps
9.6 -
5
3.6 kbps
384 kbps
144 -
384 kbps
384 -
2048 kbps
GSM Data
Data Rate
Evoluti
o
n

15. @ McGraw-Hill Education 15
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
15
An Evolution Path from CDMA to 3G Network
14.4 kbps
14.4 -
144 kbps
14.4 -
307 kbps
14
.4
k
bps
–
2
M
b
ps
IS-95A
IS-95B
CDMA2000 MC 1xRTT
CDMA2000 MC 3xRTT
Evoluti
o
n
D
a
ta
Rate

16. @ McGraw-Hill Education 16
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
16
Evolution of IMT-2000 standards
IMT-2000 IMT-2000
CDMA (3 modes) CDMA (3 modes)
IMT
-
SC
(Singl
e C
arrier)
TDMA
IMT
-
SC
(Singl
e C
arrier)
TDMA
I
M
T-
FT
(Frequency-Time)
FDMA/TDM
A
I
M
T-
FT
(Frequency-Time)
FDMA/TDM
A
IMT
-
MC
(M
ulti
-Carri
e
r)
IMT
-
DS
(Direct
Spread)
IMT
-
TC
(Time Code)
Cdma2000
(CDMA)
W-CDMA
(CDMA)
UTRA TDD, TD-SCDMA
(CDMA)
UWC-136
(TDMA)
DECT
(FDMA)

17. @ McGraw-Hill Education 17
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
17Convergence of Services in IMT-2000
Telephony •
Voice
•
Video
•
Fax
•
Mailbox
Internet •
Web surfi
n
g
•
Email
•
Information
•
M-
Co
mmer
c
e
Convergence
IMT-2000
Mu
ltimed
ia
•
Tel
e
visi
on
•
Radi
o
•
Infotainm
e
nt
•
Location
servi
c
es

18. @ McGraw-Hill Education 18
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
18
IMT 2000 Services
™
Voice
™
Switched Data
™
Messaging
™
Multimedia Messaging Service (MMS)
™
Immediate Messaging
™
Medium, High, and Interactive Multimedia
™
Sending multimedia postcards

19. @ McGraw-Hill Education 19
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
19
Second Generation Digital Cellular Systems ™
IS-54/IS-136 : US Digital Cellular (USDC) -
T
DMA System
™
GSM : Global System for Mobile
™
PDC : Pacific Digital Cellular -
A

Japanese TDMA Cellular Standard
™
IS-95/cdmaOne : CDMA Cellular System

20. @ McGraw-Hill Education 20
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
20
Wireless Networking Technologies
™
Wireless Local Area Network (WLAN)
™
Wireless Personal Area Network (WPAN)
™
Wireless Meteropolitan
A
rea Network
(WMAN)

21. @ McGraw-Hill Education 21
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
21
Wireless Communication Systems
Three most commonly used household
wireless communication systems are:
9
Paging System
9
Cordless Phone System
9
Cellular Telephone System

22. @ McGraw-Hill Education 22
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
22
Comparison of Wireless Communication Systems

23. @ McGraw-Hill Education 23
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
23
Cellular Communication Standards

24. @ McGraw-Hill Education 24
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
24
Vision of Next Generation Network
IP based
Next
Generation
Network
Cellular GSM
2G+

Cellular UMTS
3G
Cellular 4G
Broadcast DVB/DAB
WPAN/ WMAN
WLAN/
HiperLAN

25. @ McGraw-Hill Education 25
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
25
Next Generation Wireless Network
Functional Requirements: 9
Very high-speed and high-quality transmission
9
Open platform
9
Flexible and varied service functions

26. @ McGraw-Hill Education 26
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
26
Comparison of Cellular Network Generations
•
Analog
Tran
smissio
n
•
Mainly Speech
Communication
•
Voice Band
Data
•
Circuit Switched
•
Local Systems
•
Digital
Tran
smissio
n
•
Mainly Speech
Communication
•
Digital Data
•
Circuit
Switched
•
Glob
al R
o
aming
•
Digital
Tran
smissio
n
•
Mainly Speech
Communication
•
Increasing
Digital Data
•
Increasingly Packet
Switched
•
Glob
al R
o
aming
•
Digital
Tran
smissio
n
•
Mainly Speech and
Video
Communication
•
Mainly
Digital Data
•
Mainly Packet
Switched
•
Glob
al R
o
aming
First G
e
n
e
ration
Second Generation
Third Generation
Next Generation

27. @ McGraw-Hill Education 27
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
27Wireless Data Communications Technologies

28. @ McGraw-Hill Education 28
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
28
Wireless Data Communication Technologies
RFID/Tag
UWB &
Bluetooth
WLAN 802.11
WiMAX
8
02.16
2G/2.5G/3G Cellular Phone/Data
Satellite & GPS
0 –
1
00 m
a
few

kms
50 km
a few
tho
u
sand
kms
several tho
u
san
ds
of kms

29. @ McGraw-Hill Education 29
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
29A Typical Fixed Wireless Network

30. @ McGraw-Hill Education 30
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
30Applications of Wireless Communications
‰
Office and household environments
‰
Industrial control
‰
Education sector
‰
Health services
‰
Government and military operations
‰
Event and travel management
‰
Home entertainment
‰
Environmental and industrial research

31. @ McGraw-Hill Education 31
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
31
Potential Market Areas and Data Rates
Limited
broadcast video
FAX
Remote Office
Wirel
ess
postcard
SM
S
(text only)
Credit Card Verification
Web Clippi
ng
E-banki
n
g, E-com
m
erce
Text + image
Messagi
ng
Mu
ltimed
ia
WWW
Large File
Transfer
Mu
ltimed
ia

Messages
Interactive games and
entertainment
Mobile
Computing
10 Kb
ps 14.4 Kbps
44-64 Kb
ps
144Kbps
384 Kb
ps 2 M
bps

32. @ McGraw-Hill Education 32
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
32
Target Business Areas
‰
The Automotive Industry Market
‰
The Fleet Management
‰
Vehicle Positioning Market
‰
The Utilities Market
‰
The Security Systems Market
‰
Vending Machines

33. @ McGraw-Hill Education 33
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
33
Challenges for Research
Wireless communications –
a
major
technological areas for research as well as industrial applications.
True wireless multimedia services –
required by highly mobile subscribers seamlessly on a global arena.
Diverse IP multimedia applications.

34. @ McGraw-Hill Education 34
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
34
Summary
¾
Cellular Mobile Communications
¾
Remote Wireless Internet Connections
¾
Wireless Networks
¾
Mobility, increased network reliability, easier and less expensive installation, and support for disaster recovery

35. @ McGraw-Hill Education 35
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
35

36. @ McGraw-Hill Education 1
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
1
PowerPoint Slides
Wireless Communications T L Singal
PROPRI
ETARY MATERI
AL
. ©
2010
The Mc
Graw
-
H
ill C
o
mpanies
,
I
n
c
.

All r
ight
s
reser
v
ed.
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o

par
t
of
t
h
is
P
o
wer
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oint
s
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37. @ McGraw-Hill Education 2
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
2
2
Mobile Communication Engineering

38. @ McGraw-Hill Education 3
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
3Mobile Communication Engineering
‰
Introduction
‰
The Radio Paths
‰
The Propagation Attenuation
‰
Basic Propagation Mechanisms
‰
Mobile Radio Channel
‰
Simulation of Wireless Fading
Channels

39. @ McGraw-Hill Education 4
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
4
Introduction
The mobile radio channel is ¾
extremely random in nature
¾
difficult to analyze
¾
places fundamental limitations on the
performance of wireless communication systems

40. @ McGraw-Hill Education 5
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
5Mobile Communication Engineering?
¾
A study of signal propagation -
v
ital to
wireless communications
¾
Basic Propagation Mechanisms
¾
How wireless medium supports mobility of the users
¾
Radio propagation characteristics

41. @ McGraw-Hill Education 6
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
6
Mobile Radio Environment
™
Radio Propagation Paths
™
Propagation Attenuation
™
Multi-path Propagation
™
Basic Propagation Mechanisms
™
Mobile Radio Channel
¾
Multipath Fading
¾
Multipath Delay Spread
¾
Effect of Mobility: Doppler Shift
¾
Coherence Bandwidth and Time

42. @ McGraw-Hill Education 7
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
7
Radio Propagation Paths
™
Direct wave Path
-
a
path which is clear form
terrain contour.
™
Line of Sight (LOS) Path -
a path clear form
buildings. In the mobile radio environment, line of sight condition is generally not met.
™
Obstructive Path
-
a
path when the terrain
contour blocks the direct wave path. The signal may encounter diffraction resulting into shadow or diffraction loss.

43. @ McGraw-Hill Education 8
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
8
The Propagation Attenuation
™
In general, the propagation path loss increases with frequency of transmission, f
c
and the
distance between cell site and mobile, R.
™
In a real mobile radio environment, the propagation path-loss varies as L
p
∞
R
γ
,
where
γ
is path-loss exponent which varies
between 2 and 6, depending on the actual conditions.
™
γ
= 2 is free-space condition and
γ
= 4 is
typical
value for mobile radio environment.

44. @ McGraw-Hill Education 9
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
9
Path Loss Exponent Values

45. @ McGraw-Hill Education 10
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
10
Signal Attenuation Rate
™
Free-Space conditions:
Signal strength
decays at the rate of 6 dB/octave
or 20
dB/decade
™
Mobile Radio Propagation
Environment condition:
Signal strength
decays at the rate of 12 dB/octave
or 40
dB/decade

46. @ McGraw-Hill Education 11
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
11Radio Propagation Mechanisms
(
1) Direct Signal
(
2) Grou
nd Reflected Signal
(
3) Reflected Signal
(
4) Scattered Sign
al
(
5) Diffracted Signal
Building
h
t
hr
r
(1)
(3)
(2)
(5)
(4)
Cell-site Tx
Mobile Rx

47. @ McGraw-Hill Education 12
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
12
Propagation Mechanisms
¾
Reflection
: Propagating wave impinges on an
object which is large compared to wavelength, such as the surface of the Earth, buildings, walls, etc.
¾
Diffraction
: Radio path between transmitter
and receiver obstructed by surface with sharp irregular edges. Waves bend around the obstacle, even when LOS does not exist
¾
Scattering
: Objects smaller than the
wavelength of the propagating wave, e.g. foliage, street signs, lamp posts.

48. @ McGraw-Hill Education 13
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
13
Effects of Reflection on Signal Propagation
BS -
T
x
)
MS -
R
x
)
Building size >10m
120
o
Direct p
a
th
Reflected path

49. @ McGraw-Hill Education 14
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
14
Diffraction
™
A change in wave pattern caused by interference between waves that have been reflected from a surface or a point
™
Causes regions of waves strengthening and weakening
™
Results in bending of the wave
™
Can occur in different situations when waves
√
Pass through a narrow slit
√
Pass the edge of a reflector
√
Reflec
t off
two different surfaces approx
imately one
wavelength apart

50. @ McGraw-Hill Education 15
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
15
Diffraction of Radio Signal
BS -
T
x
)
Object
≈
33 cm
MS -
R
x
)
Direct p
a
th
Diffract
ed path

51. @ McGraw-Hill Education 16
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
16
Scattering
of Radio Signal
BS -
T
x
Object
≈
33 cm
Scattering of
signals
MS -
R
x

52. @ McGraw-Hill Education 17
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
17
Blocking and Absorption
™
Some substances like trees and shrubs, clouds, mist and other atmospheric moisture and dust, metal screen, human body near a hand held absorb
radio waves
™
Higher frequency radio waves are absorbed more
than
lower frequency radio waves

53. @ McGraw-Hill Education 18
T L SINGAL : Wireless Communications
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18
Refraction
™
Refraction is the bending of electromagnetic waves as they pass from medium of one density into medium of another density.
™
Radio waves typically bend due to changes in density of air caused by changes in humidity, temperature or pressure.
™
Dielectric constant describes how the wave will propagate through the material.

54. @ McGraw-Hill Education 19
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
19
Refraction
of Radio Signal
tr
o
p
o
s
p
h
e
r
e
o
r
io
n
o
s
p
h
e
r
e
Earth
re
fl
e
c
te
d
ra
d
io
s
ig
n
a
l
d
i
r
e
c
t

r
a
d
i
o

s
i
g
n
a
l
r
e
f
r
a
c
t
e
d

r
a
d
i
o

s
i
g
n
a
l

55. @ McGraw-Hill Education 20
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
20
Mobile Radio Channel
™
Mobile radio channels introduce noise, fading, interference, and other distortions into the signals that they transmit.
™
In mobile communication system, a signal experiences multipath propagation which causes rapid signal level fluctuations of the amplitude of a radio signal in a short time over a short distance called
fading
.

56. @ McGraw-Hill Education 21
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
21
Impairment to Radio Channel -
F
ading
™
Multipath waves are generated because the antenna height of mobile is lower than its typical surroundings, and the operating wave length is much less than the sizes of the surrounding structures at mobile.
™
The sum of multipath waves causes a signal fading phenomenon.
™
The signal may fade in range of about 40 dB (10 dB above and 30 dB below the average signal). If the mobile moves fast, the rate of signal fluctuations is fast.

57. @ McGraw-Hill Education 22
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
22
Multipath Fading in a Mobile Radio
Environment
Multipath fading
~10
0
λ
Wirel
ess
Medi
um
Radio path
Tx
Antenna
Cell Sit
e
Cell Sit
e

58. @ McGraw-Hill Education 23
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
23
Types of Fading
™
Fading effects due to multipath time delay spread
¾
Flat (non-frequency selective) fading
¾
Frequency selective fading
™
Fading effects due to Doppler spread
¾
Fast fading (Rayleigh fading)
¾
Slow fading (Rician
f
ading)

59. @ McGraw-Hill Education 24
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
24Flat (Non-frequency Selective) Fading
™
When radio channel has a constant gain and linear phase response but its bandwidth is greater
than that of the transmitted signal
™
All
frequency components of the received
signal fluctuates in the
same
proportions
simultaneously
™
Described by
Rayleigh distribution
™
Typical flat fading channels cause
deep
fades
, (20 or 30 dB)

60. @ McGraw-Hill Education 25
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
25
Frequency Selective Fading
™
When radio channel has a constant gain and linear phase response but its bandwidth is
less
than that of the transmitted signal
™
Affects the different spectral components of a radio signal
unequally
™
Due to time dispersion of the transmitted symbols within the channel, the channel induces intersymbol
interference
™
Frequency selective fading channels are also known as wideband channels

61. @ McGraw-Hill Education 26
T L SINGAL : Wireless Communications
McGraw-Hill Education ©2010
26
Fast Fading (Rayleigh fading)
™
Rapid fluctuations in received signal strength occur over distances of about one-half a wavelength.
™
The channel impulse response changes rapidly within the symbol duration.
™
The coherence time of the channel is smaller than symbol period of the transmitted signal.
™
This causes frequency dispersion, also called time selective fading, due to Doppler spreading.

62. @ McGraw-Hill Education 27
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27
Slow Fading (Rician
fading)
™
Rapid fluctuations in received signal strength occur over distances of about one-half a wavelength.
™
The channel impulse response changes rapidly within the symbol duration.
™
The coherence time of the channel is smaller than symbol period of the transmitted signal.
™
This causes frequency dispersion, also called time selective fading, due to Doppler spreading.

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28A Typical Fading Signal Received
0
51
0
1
5
2
0
2
5
3
0
-130-120-110-100
-90-80
Relative Position (m)
Signal Level (dBm)

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Shadow Fading
¾
The variation of the signal strength due to location
¾
Similar to slow fading
¾
Typically modeled by attenuation in signal amplitude that follows a log-normal distribution
¾
The variation in shadow fading is specified by the standard deviation of the logarithm of this attenuation

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Effects of Multipath Fading
(as noticed by the listener)
9
Rapid change in volume
9
Random frequency modulation
9
Echoes
9
Distortion
9
Dropped call

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Multipath Delay Spread
™
Multipath propagation yields signal paths of different paths with different times of arrival at the receiver.
™
Spreads/smears the signal, could cause inter- symbol interference, limits maximum symbol rate
™
Delay Spread also occurs due to Rayleigh fading which results from the signal’s amplitude and phase being altered by reflections.

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32Delay spread of a received signal

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33
Typical Delay Spread Values

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Doppler Shift
™
The relative motion between the cell site and mobile results in random frequency change due to different Doppler shifts on each of the multipath components.
™
The Doppler shift, f
d
is given by
f
d
= (1/
λ
c
) V
m
cos
θ
where
λ
c
is the wavelength of the carrier signal, V
m
is
the relative velocity of the mobile, the angle
θ
is
between the motion of the mobile and direction of arrival of the scattered waves.

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Doppler Spread
¾
Doppler shift will be positive or negative depending
on whether the mobile receiver is moving towards or away from the cell site. ¾
In mobile radio applications, the Doppler
spectrum or Doppler spread for a Rayleigh fading channel is usually modeled by D(
λ
) = (0.16/f
dm
) x [1-(
λ
c
/ f
dm
)
2
]
-0.5
for -
f
dm
≤λ
c
≤
f
dm
where f
dm
is the maximum Doppler frequency possible
f
dm
= V
m
/
λ
c

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Level Crossing Rate
¾
It is possible to relate the time rate of change of the
received signal to the signal level and velocity of the mobile. ¾
The level crossing rate,
N
L
is defined as the
expected rate at which the Rayleigh fading envelope, normalized to the local RMS signal level, crosses a specified threshold level in a positive- going direction.
N
L
= 2.5 f
dm
ρ
e
-
ρ
2
ρ
is the value of the specified l
evel
L, normalized to the local
rms
a
mplitude of the fading envelope, that is, L/Lrms.

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Fade Rate and Fade Duration
¾
Fade rate
is defined as the number of times
that the signal envelope crosses the threshold value in a positive going direction per unit time.
Average fade rate = 2 V
m
/
λ
c
¾
The
average fade duration
is defined as
the average period of time for which the received signal is below a specified level L.
Average fade duration,
Ť
= 0.4(e
ρ
2
-1)/(f
dm
ρ
)

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Coherence Bandwidth
™
The coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered flat.
™
The coherence bandwidth, B
c
represents
the correlation between two fading signal envelopes at frequencies f
1
and f
2
and is a
function of delay spread
Ŧ
d
.
B
c
≈
1 / (2
πŦ
d
)
Where
Ŧ
d
is the delay spread.

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Coherence Time
™
Coherence time is the time duration over which two received signals have a strong potential for amplitude correlation.
™
It is used to characterize the time varying nature of the frequency dispersiveness
of the channel in the
time domain.
™
Coherence time,
Ŧ
c
is inversely proportional of
Doppler spread.
Ŧ
c
≈
0.423 / f
dm
Where f
dm
is the maximum Doppler shift given by V
m
/
λ
c
.

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Wireless Fading Channels
™
Simulating a wireless communication system involves modeling a mobile radio channel based on mathematical descriptions of the channel.
™
Even when a mobile receiver is stationary, the received signal may fade due to movement of surrounding objects in the radio channel.
™
Rayleigh and Rician
fading channels are useful
models of real-world phenomena in wireless communications.

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Impulse Response of
Channel
™
The wireless channels can be characterized by a parameter U, defined as ratio of the power in the dominant path to the power in the scattered path.
9
When U = 0 (that is, power in the dominant path is zero), the channel is
Rayle
i
gh channel.
9
When U is equal to infinity (that is, power in the scattered path is zero), the channel is AWGN.
™
The impulse response is a wideband channel characterization and contains all information necessary to simulate any type of radio transmission through the channel.

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Simulating a Fading Channel
™
The BERTool
of Communications ToolBox
o
f technical
computing simulation software MATLAB implements a baseband channel model for multipath propagation conditions.
™
A mobile radio channel may be modeled as a linear filter with a time varying impulse response, where the time variation is due to receiver motion in space.
™
The Communications Toolbox models a fading channel as a linear Finite Impulse Response (FIR) filter.

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BER Plot for Fading Channels
0
5
10
15
20
25
30
35
10
-1
1
(1)
(2)
(3)
(4)
(5)
(1) Frequency
-select
ive or Fast fading
(2) Flat and slow fadi
ng Ray
l
eigh limit U=0
(3) Rician
fading U=4
(4) Rician
fading U=16
(5) Additive white gaussian
noise U=
∞
(1)
Bit Error Rate (BER)
10
-2
10
-3
10
-4
E
b
/N
0
(dB)

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Summary
™
Impairments to propagation include reflection, diffraction, scattering, and many other similar phenomena.
™
Channel impairments cause multipath propagation and mobile signal fading.
™
Multipath propagation results in delay spread, which causes intersymbol
interference limiting
the bandwidth of the channel, and irreducible error rates.

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