BTB43403 Lecture 3b - Large Scale Path Loss

8/2/2019 BTB43403 Lecture 3b - Large Scale Path Loss

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Mobile Radio Propagation:

Large-Scale Path Loss

Source: Wireless Communications: Principles and Practice, 2nd

Edition by TS Rappaport, Chapter 4, pg 113-138

Lecture 3


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Lecture 3: Mobile Radio Propagation-


Introduction to radio wave propagation

Free space propagation model

The three basic propagation mechanisms:

Reflection

Diffraction

Scattering

Outdoor propagation models

Indoor propagation models

Source: Wireless Communications: Principles and Practice, 2nd Edition by TS Rappaport, Chapter 4


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Basic Propagation Mechanisms

Reference: Pg 114 - 125


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When a wireless signal hits an object it doesn't just stop or

bounce straight back it could turn, bend, breakup, slow down,

or even turn a corner.

There are five basic propagation mechanisms:

Reflection, Diffraction, Scattering,

Refraction, Absorption

However, only Reflection and Diffraction, will be explained

in details.

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Reflection:

Signals can be transmitted and bounced off largebuildings to a client.

Reflection can be bad of it causes signals to bleed into aunneeded area.

Diffraction:

This is often mixed up with refraction.

Diffraction occurs when your wireless signal hits a verylarge object, slows down and actually turns around thecorner.

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This is great if you want your signal to bend around

the corner but if you don't it sucks.

Plus it slows down your signal making it weaker.

Scattering:

Scattering will take place when a wireless signal hits

an object that has many irregular angels causing the

signal to shoot off into many directions.

This could happen when a signal hits a mountain side,

hills or large buildings BTB 43403



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Refraction:

Refraction is the bending of your wireless signal as it

goes through a medium of higher density than its origin.

If you signal has to travel through industrial areas with

smoke stacks valleys with warmer air pockets this will

produce refraction.

Absorption:

Shoot your signal at a larger brick building and your

signal might be sucked up and disappear.

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In Wired System

When a radio wave propagating in one medium having

different electrical properties, the wave is partially

reflected and partially transmitted

If the plane wave is incident on a prefect dielectric, part

of the energy is transmitted into the second medium and

part of the energy is reflected back into the first medium.

There loss of energy in absorption.

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In Wired System

If the second medium is a perfect conductor, then all

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

The electric field intensity of the reflected and transmitted

waves may be related to the incident wave in the mediumof origin through the Fresnel Reflection Coefficient ()

In RF practice, this is often measured in a dimensionless

ratio known as Voltage Standing Wave Ratio (VSWR).

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In Wired System

Signal reflection occurs when a signal is transmitted

along a transmission medium, such as a copper cable or

an optical fiber, some of the signal power may be

reflected back to its origin rather than being carried all

the way along the cable to the far end.

This happens because imperfections in the cable causeimpedance mismatches and non-linear changes in the

cable characteristics.

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In Wired System

These abrupt changes in characteristics cause some of the

transmitted signal to be reflected.

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Reflection

Reference: Pg 114 - 125


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Reflection happens when a propagating wave hits an object

which has very large dimensions when compared to the

wavelength of the propagating wave.

the signal just completely bounces off the object such

as the surface of the earth , buildings and walls.

This can be a good thing and a bad thing.

Reflection can be good because clients don't have to rely of

LOS (Line or Sight) service.

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Basic Propagation Mechanisms - Reflection


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Fresnel reflection coefficient

According to Fresnel formulathe angles q1of the incident

wave, q2 of the reflected wave

and q3 of the refracted wave are

given by the equation:

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q1 = q2n1sinq1 = n2sinq3



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Reflection occurs when RF energy is incident upon a

boundary between two materials (e.g. air/ground) with

different electrical characteristics

Permeability

Permittivity

Conductance Often the dielectric constant of a perfect (lossless)

dielectric is related to a relative permittivity, r

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If a dielectric material is lossy, it will absorb power and

may described by a complex dielectric constant given as

equation (4.17) (pg115)

The and are generally insensitive to operating frequency,when the material is a good conductor.

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Ground Reflection (Two-Ray) Model

In a mobile radio channel , a single direct path between the

base station and a mobile is seldom the only physical means

of propagation

Hence, the free space propagation model equation 4.5 is in

most cases inaccurate when used alone.

The two-ray ground reflection model shown in Figure 4.7 is

a useful propagation model, which considers both the direct

path and a ground reflected path.

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This model is the most accurate for predicting the large-scale

fading.

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In mobile communications systems, the maximum T-R

separation distance is at most only a few tens ofkilometer and the earth may be assumed to be flat.

Good for systems that use tall towers (over 50 m tall)

Good for line-of-sight microcell systems in urban

Referring to Figure 4.7, ht is the height of the transmitter

and hr is the height of the receiver.



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If E0 is the free space received Energy at a reference distance ,

d0 from the transmitter. Then for d >>d0 , the received energyis given as:

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000 cos, dd

c

dt

d

dEtdE c

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Where E(d,t)= E0d0/d represents the envelope of theenergy at d-meters from transmitter.



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The total received energy, ETOT is the results from a

combination of a direct line-of-sight path , ELOS and a reflected

path, Eg

The direct line-of-sight path , ELOS is given as:


c

dt

d

dEtdE cLOS

'

'

00' cos, 4.34


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And the reflected path, Eg, is given as:

The total received energy, ETOT is the results from a

combination of a direct line-of-sight path, ELOS and a reflected

path, Eg..

c

dt

d

dEtdE cg

''

''

00'' cos, 4.35




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c

d

td

dE

c

d

td

dE

tdE ccTOT

''

''

00

'

'

00

cos)1(cos,

gLOSTOT EEE 4.38

4.39

For the direct path let d = d; for the reflected path d = d



The Resultant total energy-field is given as:


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for large TR separation : igoes to 0 (angle of incidence to

the ground of the reflected wave) and = 1 Phase difference can occur depending on the phase difference

between direct and reflected E fields

The phase difference is due to Path difference, =d

d, between

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ReflectionGround Reflection (Two-Ray) Model


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From two triangles with sides dand (ht+ h

r) or (h

th

r)

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Basic Propagation Mechanisms Reflection



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can be expanded using a Taylor series expansion

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For d0=100meter,E0=1, fc=1 GHz, ht=50 meters, hr=1.5 meters,at t=0

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note that the magnitude is with respect to a reference ofE0=1

at d0=100 meters, so near 100 meters the signal can be stronger

thanE0=1 the second ray adds in energy that would have been lost

otherwise

for large distances d satisfies (4.50):

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rtrt

hhhhd

20

3

20

4.50




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the received Energy-field can be approximated as :

mVd

k

d

hh

d

dEdE

rt

TOT/

22

2

00

4.51




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rtrt hhGGddBPl log20log20log10log10(log40

4.52

The free space power received at dis related to the square of

the electric field through equation 4.15.

Combining Equations (4.2), (4.15) and (4.51) the receivedpower at distance dfrom the transmitter for the two-ray

ground bounce model can be expressed as:

4

22

d

hhGGPP ttrttr or

4.53

Basic Propagation MechanismsReflectionGround Reflection (Two-Ray) Model

L t 3 M bil R di P ti


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Example 4.6:

A mobile is located 5km away from a base station and uses a vertical

/4 monopole antenna with a gain of 2.25 dB to receive cellular radio

signals. The E-field at 1km from the transmitter is measured to be 10 -3

v/m. The carrier frequency used for this system is 900MHz.

a) Find the length and the effective aperture of the receiving antenna.

b) Find the received power at the mobile using the two-ray groundreflection model assuming the height of the transmitting antenna is

50m and the receiving antenna is 1.5m above ground

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Basic Propagation MechanismsReflectionGround Reflection (Two-Ray) Model

BTB43403 Lecture 3b - Large Scale Path Loss

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