Chanel Fading

EEE 435:

Advanced Digital Communications

Lecture 6:

Fading Channels and Diversity Techniques

Refik Caglar Kizilirmak

Nazarbayev University

Block diagram of a typical digital

communication systems

Fading Channels

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•

•

•

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The Challenge

Characterizing Mobile Radio Propagation

Signal Time Spreading

Time variance of the channel

Mitigating fading (diversity techniques)

The Challenge

• AWGN channel model is the usual starting

point for developing basic performance

results.

• If radio channel’s propagating characteristics

are not defined, a simple free space model

can be used to model attenuation

=

d: distance, λ: wavelength

4π

λ

2

2

Path loss

The Challenge

• In mobile communications, a signal can travel

from transmitter to receiver over multiple

reflective paths. This phenomenon called

multipath propagation, giving rise to

multipath fading.

• The effects of fading is usually more

challenging than AWGN.

Fading Channels

•

•

•

•

•

The Challenge

Characterizing Mobile Radio Propagation

Signal Time Spreading

Time variance of the channel

Mitigating fading (diversity techniques)

Reflection, Diffraction, Scattering

• Reflection: a wave impinges upon a smooth

surface.

• Diffraction: waves bended around the corners

or obstacles.

• Scattering: waves scattered in all directions.

Reflection, Diffraction, Scattering

Fading

• Large scale fading: It includes mean-path loss and

a log-normally distributed variation about the

mean. Not much to do about this kind of fading.

• Small scale fading: It refers to dramatic changes in

the signal amplitude and phase that can be

experienced as a result of small changes in the

spatial changes (less than λ/2). Diversity

techniques are applied.

Fading

Rayleigh Fading

• If there is sufficiently

large scatterers, the

channel impulse

response will be

modeled as Gaussian

process.

All scatterings arrive at the same time,

superimposed to...