Alcatel-Lucent GSM Basic Part-1 PPT

8/10/2019 Alcatel-Lucent GSM Basic Part-1 PPT

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All Rights Reserved Alcatel-Lucent 2009

Basic RF OverviewBasic RF Overview

Created by

Pulok Sinha (Trainer, ALUMS)

09320096525


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ObjectivesObjectives

In this session basic RF aspects will be covered:

Decibel(dB)

Decibel with respect to mW (dBm)

Propagation Mechanisms

Transmission Problems in RF Propagation

Antennas

Antenna Types

Radiation Pattern

Antenna Properties

Azimuth and requirementAntenna tilt

Diversity

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Basic DefinitionBasic Definition


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DecibelDecibel (dB)(dB)

dB is a relative unit of measurement used to describe power gain

or loss.Decibel is logarithmic value that represent ratio of two given

power.

dB can be expressed as follows:

X dB = 10 log (Po/ Pin)

The powers Poad Pinmust be in the same units. If the units are

not compatible, then they should be transformed.Equal power corresponds to 0dB.

Decibel(dB)Decibel(dB)

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Decibel with respect to milliDecibel with respect to milli--watt (watt (dBm)dBm)

dBm is a standard unit for measuring levels of power in relation

to a 1 milli-watt reference signal.

It is different from the dB because A dB has no particular

defined reference while a dBm is referenced to a specific

quantity: the milli-watt (1/1000 of a watt).

dBm can be expressed as follows:

dBm =10 log10 (Signal Power / .001 watt )

Decibel with respect to mW (dBm)Decibel with respect to mW (dBm)

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Free Space LossFree Space Loss

FreeFree--space lossspace loss ((FSLFSL)) is the loss in signal strength of an

electromagnetic wave that would result from a line-of-sight path

through free space (usually air), with no obstacles nearby to cause

reflection or diffraction.

It does not include factors such as the gain of the antennas used at

the transmitter and receiver, nor any loss associated with hardware

imperfections.

Formula for Free Space Loss is given by

Free Space Loss (dB) = 32.44 + 20log F + 20log D

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F is frequency in MHz

D is Dist in KM


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Propagation MechanismsPropagation Mechanisms

ReflectionReflection

Occurs when a wave impinges upon a smooth surface.Dimensions of the surface are large relative to .

Reflections occur from the surface of the earth, from buildings,

walls, mountains, etc.

DiffractionDiffraction

Occurs when the path is blocked by an object with large

dimensions relative to and sharp irregularities (edges).

Diffraction gives rise to bending of waves around the obstacle.

Propagation MechanismsPropagation Mechanisms


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ScatteringScattering

Occurs when a wave impinges upon an object with dimensions

on the order of or less, causing the reflected energy to spreadout or scatter in many directions.

Small objects such as street lights, signs, & leaves cause

scattering

Propagation MechanismPropagation Mechanism


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Transmission Problems in RF PropagationTransmission Problems in RF Propagation

In radio communication the transmission quality varies from

one place to another.

Two major factors affecting quality of transmission are:

1. Distance between transmitter and receiver

2. Reflections or obstructions due to man made or

natural structures

General Transmission problems affecting Transmission

Quality:

Path Loss

Shadowing

Multi path Fading


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Path LossPath Loss

As the MS moves away from the Base Station the signal

strength reduces


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strength reduces.

Path LossPath Loss


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strength reduces

Path LossPath Loss


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This decrease in strength is called PATH LOSS.

Power Loss d2f2, where d is the distance from the BTS and f is the

frequency

Thus power decreases with distance and frequency

But in the cellular system, due to the presence of Base Station this

problem does not occur as a new transmission path is established

before the previous is broken

As the MS moves away from the Base Station the signal strength

reduces

Path LossPath Loss


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More often we observe hills ,trees and buildings between MS and the BTS

This environmental attenuation of radio signal is called SHADOWING.

Thus signal strength increases or decreases depending on the presenceof obstacles between the Tx and Rx antenna

Thus a mobile subscriber experiences both PATH LOSS and

SHADOWING

ShadowingShadowing


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Multipath, is the result of reflections and scattering of radiowaves due to buildings, mountains and other objects, etc.

Multipath, in effect, creates multiple transmission path of the

signal which arrive at the receiver at different times When the components are in phase, the signal gets added.

When they arrive out of phase, they cancel each other.

Give rise to

Rayleigh Fading

Ricean fading

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Multipath PropagationMultipath Propagation


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Rayleigh FadingRayleigh Fading

The Rayleigh fading is applicable to obstructed propagationpaths.

All the signals are NLOS signals and there is no dominant directpath.

Signals from all paths have comparable signal strengths.

The instantaneous received power seen by a moving antenna

becomes a random variable depending on the location of theantenna.

Rayleigh FadingRayleigh Fading


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Ricean FadingRicean Fading

This phenomenon is due to multipath propagation of thesignal.

In this case there is a partially scattered field.

One dominant signal.

Others are weaker.

Ricean FadingRicean Fading


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AntennasAntennas

Antenna is an essential part of a transmitting or receiving

system which is designed to radiate or to receive

electromagnetic waves.

An antenna can also be viewed as a transitional structure

between free-space and a transmission line (such as a coaxial

line).

An important property of an antenna is the ability to focus and

shape the radiated power in space e.g.: it enhances the power

in some wanted directions and suppresses the power in other

directions.

Many different types and mechanical forms of antennas exist

and each type is specifically designed for special purposes.

AntennasAntennas


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DirectionalDirectional antennaantenna

These antennas are mostly used to get higher gain comparedto Omni-directional antenna and to minimise interference

effects in the network.

In these type of antennas, the radiation is directed at aspecific angle instead of uniformly across all azimuth angles

in case of Omni antennas.

Antenna TypesAntenna Types


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RadiationRadiation PatternPattern

The main characteristics of antenna is the radiation pattern.

The antenna pattern is a graphical representation in threedimensions of the radiation of the antenna as a function of

angular direction.

Antenna radiation performance is usually measured and

recorded in two orthogonal principal planes (E-Plane and H-plane or vertical and horizontal planes).

The pattern of most base station antennas contains a main

lobe and several minor lobes, termed side lobes.

A side lobe occurring in space in the direction opposite to themain lobe is called back lobe.

Radiation PatternRadiation Pattern


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RadiationRadiation PatternPattern

Radiation PatternRadiation Pattern


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Directivity of Antenna:Directivity of Antenna:

Directivity is the ability of an antenna to focus energy in a

particular direction when transmitting or to receive energy

better from a particular direction when receiving.

Gain of Antenna:Gain of Antenna:

The gain is a measure of how much of the input power is

concentrated in a particular direction.

It is expressed with respect to a hypothetical isotropic

antenna, which radiates equally in all directions.

The relation between gain and directivity includes a new

parameter which describes the efficiency of the antenna.

G=.D

Antenna PropertiesAntenna Properties

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Beamwidth of an Antenna:Beamwidth of an Antenna:

Beamwidth is usually understood to mean the half-power

beamwidth.

Half power Beamwidth (HPBW) is the angle between the half-

power points of the main lobe, referenced to the peak power of

main lobe.

Beam width is categorized as:

Horizontal Beamwidth

Vertical Beamwidth

Beamwidth is measured in degrees.

Beamwidth

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Horizontal Beamwidth:Horizontal Beamwidth:

Horizontal beamwidth is the beamwidth of the radiation pattern

in Horizontal Plane .

Generally, in City, antenna with 65Horizontal Beamwidth is

used and in highway antenna with 33Horizontal Beam width

is used.

Vertical Beamwidth:Vertical Beamwidth:

Vertical Beamwidth of an antenna is the beamwidth of the

radiation pattern in vertical plane.

Vertical beamwidth Varies from 4 - 8 depending upon themodel of the antenna

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Azimuth:Azimuth:

An azimuth is an angular measurement in a spherical

coordinate system. It gives apparent direction of an object

It is measured in degrees.

Measured with help of compass

AzimuthAzimuth

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Obstacle requirementObstacle requirement

Obstacle requirement for AzimuthObstacle requirement for Azimuth


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Antenna Tilt:Antenna Tilt:

The tilting of the vertical main lobe towards the ground by

specific angle with respect to horizon is known Antenna tilting.

The Maximum power of the lobe is at zero and can be place

within the cell by down tilting the antenna.

Down tilt can be use to:

Improve the coverage

Reduce interference

Adjust the cell radius

Basically there are two types of Down tilting

Mechanical TiltElectrical Tilt

Antenna TiltAntenna Tilt

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Mechanical Down tiltMechanical Down tilt

In Mechanical down tilting,

antenna is physically down

tilted towards the ground.

In a mechanical down tilt as

the front lobe moves downward

the back lobe moves upwards.

Side lobe increase withmechanical down tilting

Mechanical tilt have no impact

on the gain.

Electrical Down TiltElectrical Down Tilt

In Electrical down tilt, the

beam is tilted by altering the

signal phasing.

Electrical tilting is done by

using the tilt knob integrated in

the antenna

In Electrical down tilting, allthe lobes (main, back as well as

side) has distinct advantage of

tilting

In electrical down tilting, gainalso reduces in all direction

Comparison betweenComparison between MechMech && ElecElec TiltTilt

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Mechanically tilted Antenna Electrically tilted Antenna

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Diversity:Diversity:

Diversity techniques is an effective means which enhances the

immunity of the communication system to the multipath fading.

Diversity reception is a way to improve the quality and strength

of the signal arriving at the base station, by receiving it in several

independent ways

Two forms of diversity reception often employed are:

Space diversity

Polarization diversity

DiversityDiversity

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Spatial (Space) DiversitySpatial (Space) Diversity

The spatial diversity antenna system is constructed by physically

separating two receiving base station antennas.

If they are far apart, the received signals will be independent of

each other.

If one has suffered fading, the other may not.

A suitable distance is generally about 10 wavelengths (10)

Plan views of two possible tri-sectored site antenna systems

Spatial DiversitySpatial Diversity

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PolarisationPolarisation

Polarisation is the propagation of the electric field vector .

Antennas used in cellular communications are usually verticallypolarised or cross polarised.

Dual Polarised AntennasDual Polarised Antennas

A dual-polarisation antenna consists of two sets of radiating

elements which radiate or, in reciprocal, receive two orthogonal

polarised fields.

Dual Polarised antenna are of 2 types

Vertical Polarised

Crossed Polarised

PolarizationPolarization

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Vertical Polarized antenna:Vertical Polarized antenna:

Good diversity performance only in cities

Horizontal termination cannot be used for Tx

Cross Polarized antennaCross Polarized antenna

Equivalent diversity results compared to space diversity either in

cities or in rural areas

Both terminations are suitable for Rx and Tx

PolarizationPolarization

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Alcatel-Lucent GSM Basic Part-1 PPT