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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
6
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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As the MS moves away from the Base Station the signal
strength reduces.
Path LossPath Loss
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As the MS moves away from the Base Station the signal
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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Antenna PropertiesAntenna Properties
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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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Antenna PropertiesAntenna Properties
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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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Comparison betweenComparison between MechMech && ElecElec TiltTilt
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Mechanically tilted Antenna Electrically tilted Antenna
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Comparison betweenComparison between MechMech && ElecElec TiltTilt
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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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