3.Communication Medium

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Wireless Communication

Medium

Chapter 3 & 4

Wireless Communications principle and practiceBy- Theodore S Rappaport


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Lesson-2 & 3:

Topic Lesson Learning Outcomes Teaching-Learning

Methodology

Assessment

Method

Wireless

communication

medium,

To know the basics of radio wave propagation

To understand the various propagation model

To familiarize with scientific theories behind

radio wave propagation.

To know about different types of channel fading

Class Lecture

Question and answer

Test, exams, quiz,

etc

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The physical medium on which the communication

takes place

Also Known as communication channel

Types

Wired Medium

Metal cables

Self study

Optical fiber Will be discussed later

Wireless medium Topic of this lecture

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Communication mediums


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Radio Wave Propagation

The mechanisms behind electromagnetic wavepropagation are diverse,

Reflection, diffraction, and scattering.

Most cellular radio systems operate in urban areas, No direct line-of-sight path between the transmitter and the

receiver.

Propagation models

Predicting the average received signal strength at a given distancefrom the transmitter,

The variability of the signal strength in close spatial proximity to aparticular location.

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Wireless Channel


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Wave Propagation ModelsLarge-scale propagation models

Predict the mean signal strength for an arbitrarytransmitter-receiver (T-R) separation distance

Characterize signal strength over large T-R separationdistances (several hundreds or thousands of meters).On the other hand, propagation models

Small-scale or fading models

Characterize the rapid fluctuations of the receivedsignal strength over very short travel distances (a fewwavelengths) or short time durations (on the order ofseconds).

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Wireless Channel


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Small-scale and large-scale model

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Wireless Channel


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Free Space Propagation Model

Predict received signal strength when thetransmitter and receiver have a clear,

unobstructed line-of-sight path between them. Example, Satellite communication systems and

microwave line-of-sight radio links.

The free space power received by a receiver

antenna which is separated from a radiatingtransmitter antenna by a distance d, is given bythe Friis free space equation,

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Propagation Model


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Free Space Propagation Model

Large-scale propagation models use a close-in

distance, d0, as a known received power reference

point.

The received power at any distance d > d0 may be

related to

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Propagation Model


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Reflection

When a radio wave propagating in one mediumimpinges upon another medium,

The wave is partially reflected and partially transmitted.

In a perfect dielectric, A part of the energy is transmitted into the second medium

A part of the energy is reflected back into the first medium,

there is no loss of energy in absorption.

In a perfect conductor

All incident energy is reflected back into the first mediumwithout loss of energy.

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Propagation Characteristics


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Ground Reflection (2-ray) Model

This model has been found to be reasonably

accurate for predicting the large-scale signal

strength over distances of several kilometers formobile radio systems with tall tower

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Ground Reflection (2-ray) ModelThe received power at a distance dfrom the transmitter

can be expressed as

The path loss for the 2-ray model (with antenna gains) can

be expressed in dB as

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Diffraction The phenomenon of diffraction can be explained by Huygen's

principle,

All points on a wavefront can be considered as point sources for

the production of secondary wavelets, and that these wavelets

combine to produce a new wavefront in the direction of

propagation.

Diffraction is caused by the propagation of secondary wavelets

into a shadowed region.

The field strength of a diffracted wave in the shadowed region is

the vector sum of the electric field components of all the

secondary wavelets in the space around the obstacle.

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Diffraction

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Diffraction

Diffraction allows radio signals to propagate

Around the curved surface of the earth,

Beyond the horizon, and

Behind obstructions.

The received field strength decreases rapidly as a

receiver moves deeper into the obstructed(shadowed) region.

The diffraction field still exists and often has sufficient

strength to produce a useful signal.

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Log-distance Path Loss Model

Both theoretical and measurement-based

propagation models indicate that

Average received signal power decreaseslogarithmically with distance in outdoor or indoor radio

channels.

The average large-scale path loss for an arbitrary

T-R separation is expressed as a function of

distance by using a path loss exponent, n

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Path Loss Model


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Path-loss exponents for different environment

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Path Loss Model


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Log-normal ShadowingThe surrounding environmental clutter may be vastly

different at two different locations having the same T-Rseparation.

The long-distance model did not consider this fact

Measurements have shown that at any value of d, the pathloss at a particular location is random and distributed log-normally (normal in dB) about the mean distancedependent

X

is zero-mean Gaussian distributed random variable withstandard deviation .

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Shadowing


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Small-scale fading, or simply fading,

Describe the rapid fluctuation of the amplitude ofa radio signal over a short period of time or travel

distance, so that large-scale path loss effects maybe ignored.

Fading is caused by interference between two ormore versions of the transmitted signal which

arrive at the receiver at slightly different times. These waves, called multipath waves,

Combine signal at the receiver antenna can vary widelyin amplitude and phase.

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Fading


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Multipath Propagation

Multipath creates small-scale fading effects.

The three most important effects are: Rapid changes in signal strength over a small travel distance or

time interval

Random frequency modulation due to varying Doppler shifts ondifferent multi path signals

Time dispersion (echoes) caused by multipath propagation delays.

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Fading


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Factors Influencing Small-Scale Fading

Many physical factors in the radio propagation

channel influence small scale fading. These

include the following: Multipath propagation

Speed of the mobile

Speed of surrounding objects

Transmission band width

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Fading


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Types of Fading

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Fading


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Types of Fading

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Fading


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3.Communication Medium

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