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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

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WirelessCommunications

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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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