FADING

PRESENTED BY

SHREEKRUPA BAII SEM, DCN

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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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Wireless communication

Transmitting signalReceived signal

satellite

Transmitting antenna

Receivingantenna

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Radio wave propagation

Line-of-Sight Propagation

Sky Wave PropagationGround Wave Propagation

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LOS Wireless Transmission Impairments

Attenuation

Free space loss

Noise

Atmospheric absorption

Multipath propagation

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Multi path propagation effect

CA

D

BReceiverTransmitter

A: free spaceB: reflectionC: diffractionD: scattering

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fadingFading signals occur due to reflections from ground & surrounding buildings (clutter) as well as scattered signals from trees, people, towers, etc

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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 objectsIf 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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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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FadingThe 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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TYPES OF SMALL SCALE FADING

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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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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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Fast fadingFast 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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Slow fadingSlow 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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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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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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Mitigation Techniques for the Multipath Fading Channel

Diversity technique

Adaptive equalization

Forward error correction

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Mitigation Techniques for the Multipath Fading Channel

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

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THANK YOU