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5/21/2018 Wireless Communications TLSingal Chapter5 PowerPointSlides R
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@ McGraw-Hill Education1
T L SINGAL : Wireless Communications McGraw-Hill Education 2010
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PROPRIETARY MATERIAL. 2010 The McGraw-Hill Companies, Inc. All rights reserved. No part of this PowerPoint slide may be displayed, reproduced ordistributed in any form or by any means, without the prior written permission of the publisher, or used beyond the limited distribution to teachers and educators
permitted by McGraw-Hill for their individual course preparation. If you are a student using this PowerPoint slide, you are using it without permission.
PowerPoint Slides
WirelessCommunications
T L Singal
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5
Cellular Antenna
System DesignConsiderations
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Cellular Antenna System Design
Considerations
Antenna Characteristics
Antennas at Cell SiteMobile Antennas
Design of Omni-directional AntennaCellular System
Design of Directional Antenna CellularSystems
Antenna Parameters and their Effects
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What is an Antenna?
An interfacebetween RF cable connected to
transmitter/receiver units and the space
Tx antenna -
converts the electrical energytraveling along a RF cable from a Tx unit into
electromagnetic waves in space
Rx antenna -the electric and magnetic fields
in space cause current to flow in the conductorsthat make up Rx antenna and some of this
energy is transferred to RF cable connected to it
and Rx unit
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Basic Antennas
An isotropic antennais defined as a hypothetical
loss less antenna having equal radiation in all
directions.
An isotropic radiatoris taken as a referencefor expressing the directional properties of actual
antennas.
A directional antennais one having the
property of radiating or receiving electromagnetic
waves more effectively in some directions than in
others.
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Antennas Characteristics
An antennaradiation patternor antenna patternis defined as a mathematical function or graphical
representation of the radiation properties of the
antenna as a function of space coordinates.A graph of the spatial variation of the electric or
magnetic field along a constant distance path, is
called a field pattern.
The linear dipole is an example of an omnidirectional antenna-- an antenna having a radiation
pattern which is non-directional in a plane.
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Omni-directional Antenna
Radiation Patterns
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EIRP and ERP
The power radiated within a given
geographic area is usually specified either
with reference to isotropic antenna or anomnidirectional dipole antenna.
The effective isotropic radiated power
(EIRP) is referenced to an isotropic antenna.
The effective radiated power (ERP) isreferenced to an omnidirectional antenna.
ERP = EIRP + 2 dB
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Directivity and Absolute gain
Directivityof a Tx antenna is the ratio of the
radiation intensity flowing in a given direction to the
radiation intensity averaged over all direction.
Directivity is sometimes referred to as
directive gain.
Absolute gainof a Tx antenna in a given direction
is the ratio of the radiation intensity flowing in that
direction to the radiation intensity that would be
obtained if the power acceptedby the antenna were
radiated isotropically.
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Antenna gainAntenna gainis directional gain, not power
gain, due to focusing of the radiated energy in
specified direction.
Absolute gain (Power gain) is closely related to
directivity, but considers efficiency of antenna as
well as its directional characteristics.
The efficiencyof a Tx antenna is the ratioof total radiated power radiated by antenna to
the input power to the antenna.
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Effective area of Rx antenna
It is the ratio of the available power at theterminals of the antenna to the radiation
intensity of a plane wave incident on theantenna in the given direction.
The gain of a Rx antenna is the ratio of
the antennas effective area to that of an
isotropic antenna
Antenna gain, Gr= 4 Aeff / c2
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Other Antenna Parameters
The radiation resistanceof a half-wave
dipole antenna situated in free-space and fed
at the center is approximately 70.
The polarizationof a radio wave is the
orientation of its electric field vector. It could
be horizontal or vertical or hybrid.
The ratio between the gains to the front andback lobes is the front-to-back ratio,
expressed in dB.
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Antennas at Cell-site
Cell-site antennasare mounted on a
single microwave tower located in the center
of the cell
An adequate nos. of antennas are
placed to cover all of 360 degrees of a cell
The effect of an omni directional antenna
can be achieved by employing severaldirectional antennas to cover the whole of 360
degrees
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Space Diversity AntennasTwo-branch space-diversity antennas are
used to receive the same signal with two
different fading envelopes.
Spacing between two Rx antennas (s)
depends upon the degree of correlation
between two fading envelopes and is given by
s=h/11, where h is the antenna height.On combining the two received signals, the
degree of fading is reduced.
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Diversity Antenna Mounting
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Cell-site Antenna Tower
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Cell-site Antenna Mounting
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Mobile AntennasThe requirement of a mobile (motor-vehiclemounted) antenna is an omnidirectionalantenna.
Can be located as high as possible fromthe point of reception.
The physical limitation of antenna height onthe vehicle restricts this requirement
Generally the antenna should at leastclear the top of the vehicle.
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Mobile Antenna TypesRoof-mounted antennas 3 dB high-gainantenna
Glass-mounted antennas 1 to 3 dB gain,
lower height than roof-mounted antennasMobile high-gain (2 to 3 dB) antennas
Horizontally oriented space-diversity
antennasVertically oriented space-diversityantennas
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Design of Omni-directional
Antenna Cellular System
Cochannel Interface with
Omnidirectional Cell Site
.
..
.
.. Interfering
Cell
First Tier
1
IN
i
C R
ID
4.61 q
IC
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Worst-case Omni Antenna Design
D1 = D
D2 = (D+R)
D3 = (D+R)D4 = D
D5 = (D-R)
D6= (D-R)
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C/I in Worst-case Omni Directional
Cellular Antenna Design
)(22)(2
)(
RDDRD
Romni
I
C
)1(2)(2)1(2
1)(
qqq
omni
I
C
For q = 4.6 (K=7), and =4,C/I 17 dB,
which is less than the desired C/I =18 dB.
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Cell Sectoring using Directional
Antennas
The process of reducing the co-channel
interference and thereby improving C/I by
using directional antennas is known as Cell
Sectoring.
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Cell Sectoring
An omni cell is partitioned into three 120
degree sectors or six 60 degree sectors
The channels used in a particular cell arebroken down into sectored groups and are
used only in a particular sector
Each sector is assigned a set of channels
The interference between two cochannel
cells decreases with cell sectoring
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3-sector Cellular System Design
D1 = D+0.7R
For q = 4.6, and =4,C/I 24.5 dB,
which is more than
desired C/I =18 dB.
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6-sector Cellular System Design
D1 = D+0.7R
For q = 4.6, and =4,C/I 29 dB,which
is more than desired
C/I =18 dB.
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C/I for different Antenna Systems
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Cell SectoringPros & Cons
Depends upon proper installation of cell-site directional antennas
Enhances signal qualityImproves system capacity
Increases handoff occurrences
Increases trunking inefficiencies
Reduces available number of channelsin each sector
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Microcell Zone Concept
Related to sharing the same radioequipment by different microcells
Three directional antennas installed at ajunction point, referred to as zone-site
Results in reduction of cluster size
Used to expand the capacity of cellularsystems
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Lees Microcell Zone Concept
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Effects of Antenna Parameters
Antenna parameters that affect coverage andreduce cochannel interference include
Antenna radiation pattern
Antenna beamwidthAntenna gain
Antenna height
Separation between transmitting andreceiving antennas
Antenna tilting
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Effects of Lowering Antenna Heights
In fairly flat ground or in a valleysituation, lowering the antenna height isvery effective for reducing the cochanneland adjacent channel interference.
However, lowering the cell-site antenna
height may or may not reduce the
interference on a high hill or a high spot,In a valley, or in a forest area
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Summary
Antenna characteristics includeradiation pattern, directivity, gain, etc.
Cell site and mobile antennas must beinstalled properly
Omnidirectional antenna systemdesign does not meet C/I requirements
Cell sectoring improves C/I andsystem capacity but increases handoffs
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