multi channel fading description

1. FADING
PRESENTED BY
SHREEKRUPA BA
II SEM, DCN
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2. CONTENTS
 Wireless communication
 Radio wave propagation
 LOS Wireless Transmission Impairments
 Multi path propagation
 fading
 Factor influencing multipath propagation
 Types of fading
 Doppler shift
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3. Wireless communication
Transmitting signal
Received signal
satellite
Transmitting
antenna
Receiving
antenna 3

4. Radio wave propagation
Line-of-Sight Propagation
Sky Wave PropagationGround Wave Propagation
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5. LOS Wireless Transmission
Impairments
 Attenuation
 Free space loss
 Noise
 Atmospheric absorption
 Multipath propagation
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6. Multi path propagation effect
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C
A
D
B
ReceiverTransmitter
A: free space
B: reflection
C: diffraction
D: scattering

7. fading
Fading signals occur due to reflections from ground &
surrounding buildings (clutter) as well as scattered signals from
trees, people, towers, etc
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8. Factor influencing fading
 Multipath propagation
Multiple versions of the signal arrives at the receiver. Can cause signal
smearing due to inter symbol interference.
 Speed of the Mobile
Results in random frequency modulation due to Doppler shifts on each of
the multipath components. It can be positive or negative depending on the
movement of mobile receiver.
 Speed of surrounding objects
If the surrounding objects move at a greater rate than the mobile, then this
effect dominates the small-scale fading.
 Transmission bandwidth of the signal
 The bandwidth of the “multipath” channel can be quantified by the
coherence bandwidth.
 The coherence bandwidth is a measure of the maximum frequency
difference for which signals are still strongly correlated in amplitude.
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9. Practical examples of small scale multipath
fading
Common examples of multipath fading are
 temporary failure of communication due to a severe drop
in the channel signal to noise ratio (You may have also
experienced this. And you moved a steps away & noted that
reception is better. It is due to small scale fading effects. )
FM radio transmission experiencing intermittent loss of
broadcast when away from station
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10. Fading
The most common types of fading are
Small scale fading: Small scale fading or simply fading is used
to describe the rapid fluctuations of the amplitude, phases or
multipath delays of a radio signal over a short period of time
or travel distance,
Large scale fading : Large-scale fading is the result of signal
attenuation due to signal propagation over large distances and
diffraction around large objects in the propagation path.
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11. TYPES OF SMALL SCALE FADING
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12. Frequency selective fading
Frequency selective fading, where the bandwidth of the signal is greater than the
coherence bandwidth of the channel or the delay spread is greater than the symbol period.
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13. Flat fading
Flat fading is caused by absorbers between the two antenna and is countered by
antenna placement and transmit power level. Here all frequency component of
the received signal fluctuate in the same proportion simultaneously
Flat fading, where the bandwidth of the signal is less than the coherence
bandwidth of the channel or the delay spread is less than the symbol period
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14. Fast fading
Fast Fading is a kind of fading occurring with small movements of a mobile or
obstacle. Depending upon how rapidly the transmitted base band signal changes as
compared to the rate of change of the channel.
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15. Slow fading
Slow Fading is a kind of fading caused by larger movements of a mobile or
obstructions within the propagation environment. This is often modelled as log normal
distribution with a standard deviation according to the log distance path loss modelled.
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16. Small-scale fading due to movements:
Doppler shift
 How rapidly the channel fades will be affected by how fast the
receiver and/or transmitter are moving
 Motion causes Doppler shift in the received signal components
 the change in frequency of a wave for a receiver moving
relative to the transmitter
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17. Doppler shift
 Say a mobile phone moving at velocity v km/hr in the x direction and the radio
wave impinging on the receiver at an angle βk
 The motion introduces a Doppler frequency shift, fk = vcos βk/λ
 If the ray is directed opposite to the mobile’s motion (β=0), then fk=v/λ
 The frequency of the signal has increased by the Doppler spread, fk
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18. Mitigation Techniques for the Multipath
Fading Channel
 Diversity technique
 Adaptive equalization
 Forward error correction
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19. Mitigation Techniques for the Multipath Fading
Channel
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Space diversity – techniques involving physical
transmission path
Frequency diversity – techniques where the signal is spread
out over a larger frequency bandwidth or carried on multiple
frequency carriers
Time diversity – techniques aimed at spreading the data out
over time

20. THANK YOU
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