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large-Scale Propagation Models

Used to model variations due to path loss and shadowing occur that over

relatively large distances

# Variation due to path loss occurs over very large distances (100-1000 meters).

# variation due to shadowing occurs over distances proportional to the length of

the obstructing object (10-100 meters in outdoor environments and less in indoorenvironments).

this variation is sometimes referred to as large-scale propagation effects.

Propagation ModelsPropagation models are used to predict the received signal strength at the

receiver. They may be classified in two categories-

# Large-scale propagation models # Small-scale propagation models

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Propagation Characteristics

# Path Loss # Shadowing # Multipath Fading

In wireless communication systems transmission path between the

transmitter and receiver varies randomly because of the following

propagation characteristics-

Path Loss

Path loss is caused by dissipation of the power radiated by the transmitter.

Shadowing

Shadowing is caused by random variation in the received power due to

blockage from objects in the signal path.

Multipath Fading

Multipath Fading is caused by constructive and destructive addition of

multipath signal components.

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Fig. - Path loss, Shadowing and Fading

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Shadowing

A signal transmitted through a wireless channel will typically

experience random variation due to blockage from objects in

the signal path, giving rise to random variations of the

received power at a given distance. This is known asshadowing.

Shadowing is also caused by changes in reflecting surfacesand scattering objects.

Statistical models must be used to characterize the

shadowing. The log-normal shadowing is most common

model used for the modeling of this additional random

attenuation.

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Small-Scale Propagation Model

Used to model variation due to the constructive and destructive addition of

multipath signal components.# Variation due to multipath occurs over very short distances, on the order of

the signal wavelength,

Therefore these variations are sometimes referred to as small-scalepropagation effects.

Fig - Received power in large

signal & small signal propagation

Model.

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Factors influencing small scale fadingFactors influencing small scale fading

Multipath propagation. Speed of the mobile.

Speed of the surrounding

objects.

Transmission bandwidth of the

signal.

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Parameters of Mobile Multipath

Channels:

In order to compare different multipath channels

we need parameters which quantify the multipathchannel, they are:

1. Delay spread

2. Coherence bandwidth

3. Doppler spread4. Coherence time

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Delay Spread

Mean excess delay

RMS delay spread Excess delay spread

Mean excess delay is the first moment of the power

delay profile and is defined by the equation

==

h

k

h

kk

k

k

k

kk

P

P

a

a

)(

)(

2

2

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RMS delay spread is the square root of the second

central moment of the power delay profile and is

defined by the equation:

where

22 )(

=

==

h

k

h

kk

k

k

k

kk

P

P

a

a

)(

)(2

2

22

2

Maximum excess delay is defined as the - , where ,is the first arriving signal and is the maximum delay atwhich a multipath component is within X dB of the

strongest arriving multipath signal.

x 0x

0

Delay Spread contd.

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Indoor Power Delay Profile

Delay Spread contd.

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Coherence bandwidth

Coherence bandwidth (Bc): is statistical measure

of range of frequencies over which channelcan be considered as flat.

Flat means channel passes all spectralcomponents with approximately equal gain

and linear phase.

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Coherence bandwidth contd.

It is the range of frequencies over which two

frequency components have a strong potentialfor amplitude correlation.

If two sinusoids with a frequency separation ofgreater than Bc are propagating in the same

channel, they are affected quite differently by

the channel.

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If frequency correlation function is above 0.9

If frequency correlation function is above 0.5

50

1Bc

51Bc

Coherence bandwidth contd.

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Types of Small-Scale Fading

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Flat-fading (non-freq. Selective)

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Frequency selective fading