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DownloadaFREEtrialofHeraldProfessionalfromWWW.HERALDPRO.COM

Copyright 2001-2013, Luigi Moreno, Torino, Italy - All rights reserved

POINTTOPOINTRADIOLINK ENGINEERING

ASELFLEARNINGEBOOK BASEDCOURSE,BYRADIOENGINEERINGSERVICES

AUTHOR: LUIGIMORENO

Availablefromwww.heraldpro.com


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Copyright Notice

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2.COPYRIGHTS. ThisDocumentiscopyrightedbyLuigiMoreno,aProfessionalEngineerbasedin

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Italy,

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international

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Acknowledgments

Editorial setup and E-Book layout

TheAuthorishighlyindebtedtoStevenIrwin,aTelecommunicationsConsultantspecialisingin

MicrowaveRadiobasedinSouthEastQueenslandinAustralia,forsupportandappreciated

suggestionsintheeditorialsetupofthisdocumentandforthefinallayoutinEBookformat.

WithoutSteve'shelpthisEBookwouldhaveneverbeenpublished.

Reproduction of Figures

TheAuthorthankstheInternationalTelecommunicationUnion(ITU)forpermissionofreproducing

somefiguresfromITUtexts.ThecompleteandexactsourceofITUmaterialisindicatedattheplace

wheresuchreproductionismade.Pleasenotethat:

1. thismaterialisreproducedwiththepriorauthorizationofITU,ascopyrightholder;

2. thesoleresponsibilityforselectingextractsforreproductionlieswiththeAuthorandcanin

nowaybeattributedtoITU;

3. thecompletevolumesoftheITUextractscanbeobtainedfrom:

InternationalTelecommunicationUnion

SalesandMarketingDivisionPlacedesNations CH1211GENEVA20(Switzerland)

Phone:+41227306141(English)/+41227306142(French)/+41227306143(Spanish)

Telex:421000uitch/Fax:+41227305194

email:mailto:[email protected] / http://www.itu.int/publications


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About the Author

LuigiMORENOisaRadioCommunicationsConsultantwithmorethan25years'experience.A

graduateofthe"PolitecnicodiTorino"(Italy),hewasatCSELT(TelecomItaliaResearchLabs)(1973

82),thenatGTETelecomunicazioni(198285).HehasbeenadelegateatmanyITURmeetingsanda

teacherat

SSGRR

(Telecom

Italia

Training

School).

His

activity

as

aconsultant

includes

many

assignmentswithmanufacturingandoperatingcompanies(softwaredevelopmentsforradiosystem

simulation,analysis,andtesting;designofradiolinksandnetworks;trainingcourses). HeisanIEEE

SeniorMemberandservesasanIEEETransactionsTechnicalReviewer.Heistheauthorofsome30

journalandconferencepapersandoftwopatentsformobileradioreceivers. Hecanbecontacted

byemailatmailto:[email protected].


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POINT-TO-POINT RADIO LINK ENGINEERING.........................................................................1

.....................................................................................................................1

ASELF-LEARNING E-BOOK BASED COURSE,BY RADIO ENGINEERING SERVICES............1

Copyright Notice.....................................................................................................................................2

Acknowledgments...................................................................................................................................3

Editorial setup and E-Book layout..........................................................................................................3

Reproduction of Figures.........................................................................................................................3

About the Author....................................................................................................................................4

STUDENT GUIDE...............................................................................................................................11

Introduction.......................................................................................................................................11

Course Notes.....................................................................................................................................12

Herald Lab........................................................................................................................................13

SECTION 1 RADIO HOP CONFIGURATION...............................................................................14

Summary...........................................................................................................................................14

Point-to-Point radio-relay links.........................................................................................................

14

Site and Hop parameters...................................................................................................................16

Radio Equipment..............................................................................................................................17

Advanced - Digital Equipment Signature.................................................................................20

Antennas...........................................................................................................................................25

Antenna Parameters for hop design..............................................................................................27

Advanced - More on the Antenna radiation diagram................................................................29

Ancillary equipment..........................................................................................................................

31

Branching system..........................................................................................................................31

Tx / Rx Attenuators.......................................................................................................................31

Feeder Line...................................................................................................................................31

Advanced - Hops with a Passive Repeater................................................................................32

SECTION 2 BASICS IN LINK ENGINEERING.............................................................................35

Summary...........................................................................................................................................35

Free Space propagation.....................................................................................................................35

Comments on Free Space Loss.....................................................................................................37

Terrestrial radio links........................................................................................................................38


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Link Budget..................................................................................................................................40

Fade Margin and Outage prediction..............................................................................................41

ADVANCED - Link Equation with Passive Repeater..............................................................43

SECTION 3 PATH CLEARANCE...................................................................................................45

Summary...........................................................................................................................................45

Refractivity in the Atmosphere.........................................................................................................45

Propagation in Standard Atmosphere...............................................................................................46

The k-factor.......................................................................................................................................47

k-Factor variability............................................................................................................................49

Fresnel Ellipsoid...............................................................................................................................50

A note on radio propagation and visual analogies........................................................................52

Obstruction Loss...............................................................................................................................

53

Single obstacle loss.......................................................................................................................53

Advanced - More on obstruction loss computation..................................................................54

Knife-edge obstacle......................................................................................................................54

Single rounded obstacle................................................................................................................54

Spherical earth..............................................................................................................................55

Multiple obstacles.........................................................................................................................55

Clearance Criteria.............................................................................................................................

56

SECTION 4 GROUND REFLECTIONS..........................................................................................59

Summary...........................................................................................................................................59

Paths with ground reflection.............................................................................................................59

Reflection coefficient........................................................................................................................60

Summary of results...........................................................................................................................60

Advanced - Reflection coefficient computation.......................................................................61

Received signal level........................................................................................................................65

Vectorial addition of two signals..................................................................................................65

Reflected signal amplitude............................................................................................................66

Reflected signal phase...................................................................................................................67

Rate of change in the Rx signal amplitude........................................................................................67

Antenna height and k-factor effect....................................................................................................67

Diversity reception............................................................................................................................69

Advanced -Average degradation estimate................................................................................71

Advanced - Effect of time delay on digital signals...................................................................72

SECTION 5 MULTIPATH FADING................................................................................................72


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Summary...........................................................................................................................................73

Refractivity in the atmosphere (II)....................................................................................................73

Observed impairments in Rx signal..................................................................................................74

Signal attenuation..........................................................................................................................74

Signal distortion............................................................................................................................75

Advanced - Degradation of Cross-pol discrimination..............................................................76

Modeling multipath activity..............................................................................................................77

Radio and environmental parameters............................................................................................77

Statistical observation of multipath events...................................................................................78

Multipath Occurrence Factor........................................................................................................79

Advanced - ITU-R Multipath occurrence model......................................................................82

Performance prediction.....................................................................................................................

83

Outage prediction in Narrowband systems...................................................................................83

Advanced - Outage prediction in Wideband systems...............................................................84

Advanced - Outage contribution from X-pol interference........................................................85

Countermeasures...............................................................................................................................85

Space Diversity.............................................................................................................................86

Advanced - ITU-R model for Space Diversity improvement...................................................86

1+1 Frequency Diversity...............................................................................................................

87

Advanced - N + 1 Frequency Diversity....................................................................................87

Advanced - Outage in Wideband systems with Diversity........................................................88

Advanced - Adaptive equalizers...............................................................................................89

SECTION 6 RAIN ATTENUATION................................................................................................91

Summary...........................................................................................................................................91

EM wave interaction with atmosphere..............................................................................................91

Water vapour and Oxygen attenuation in clear air............................................................................91

Rain attenuation................................................................................................................................92

Worldwide rain intensity statistics....................................................................................................94

ITU-R rain attenuation model...........................................................................................................97

Rain intensity model.....................................................................................................................97

Advanced - Frequency / polarization scaling model.................................................................99

Rain unavailability prediction.........................................................................................................100

Advanced - Effect of cross-polarized interference..................................................................101

SECTION 7 FREQUENCY PLANNING AND INTERFERENCE...............................................103

Summary.........................................................................................................................................103


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Use of frequencies in P-P links.......................................................................................................103

Frequency Bands.............................................................................................................................104

Channel arrangements, ITU-R Recs...............................................................................................104

Go - Return Frequency plans......................................................................................................104

Interleaved and co-channel frequency arrangements..................................................................106

Comment.....................................................................................................................................107

Interference classification...............................................................................................................107

Source of Interference.................................................................................................................108

Propagation conditions....................................................................................................................109

Internal Interference sources...........................................................................................................110

Co-site Interference.....................................................................................................................110

Degradation due to Interference......................................................................................................

114

Interference power estimate............................................................................................................115

Effect of Interference......................................................................................................................116

SECTION 8 ITU OBJECTIVES......................................................................................................118

Summary.........................................................................................................................................118

Overview.........................................................................................................................................118

ITU-T and ITU-R Recommendations.............................................................................................118

Unavailability and Error Performance Objectives......................................................................

119

A brief history and overview of ITU Recs..................................................................................119

Definitions...................................................................................................................................120

Advanced - ITU-T Error Performance Recs...........................................................................121

Advanced - Error performance in a radio link........................................................................124

Error objectives for real links using equipment designed prior to approval of [revised] ITU-T

Recommendation G.826 in December 2002...............................................................................124

Practical rules in applying ITU-R Recs......................................................................................134

How to identify SDH and PDH sections.....................................................................................134

How to apportion Section objectives to each hop.......................................................................134

How to apportion Short-haul or Access section objectives on a distance basis ..........................134

Advanced - Unavailability Objectives....................................................................................135

ITU-T Recs. G.826 and G.827....................................................................................................135

Radio Link Availability Objectives............................................................................................136

Advanced - BER vs. Errored Blocks Performance Parameters..............................................137

Objectives vs. Propagation impairments.........................................................................................

139

HERALD LAB...................................................................................................................................141


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Working with HERALD Lab..........................................................................................................141

Using the HERALD program.........................................................................................................141

Instructions to HERALD Demo Users............................................................................................142

HERALD Lab #1 - Hop Configuration...............................................................................................143

HERALD Functions........................................................................................................................143

Exercise 1.1 : Radio Equipment data.............................................................................................143

Exercise 1.2 : Antenna data.............................................................................................................145

Exercise 1.3 : Feeder data...............................................................................................................146

Exercise 1.4 : Site & Hop definition...............................................................................................147

Exercise 1.5 : Hop configuration....................................................................................................149

Exercise 1.6 : Passive repeater........................................................................................................150

Exercise 1.7 : Export Network Topology to Google Earth.............................................................

151

Exercise 1.8 : Import Radio Sites....................................................................................................151

HERALD Lab #2 - Link Budget and Fade Margin............................................................................152


Exercise 2.1 : Compute Link Budget.............................................................................................152

Exercise 2.2 : Adjust Fade Margin................................................................................................153

Exercise 2.3 : Print Hop Report.....................................................................................................154

Exercise 2.4 : Include a Passive Repeater......................................................................................

155

HERALD Lab #3 Path Clearance....................................................................................................156


Exercise 3.1 : Define path profile..................................................................................................157

Exercise 3.2 : Check clearance.......................................................................................................159

Exercise 3.3 : Modify antenna height.............................................................................................160

Exercise 3.4 : Estimate obstruction loss..........................................................................................161

Exercise 3.5 : Display Profile Report..............................................................................................162

HERALD Lab #4 Ground Reflections.............................................................................................163


Exercise 4.1 : Estimate reflection parameters.................................................................................164

Exercise 4.2 : Analyze Rx power level...........................................................................................165

Exercise 4.3 : Design diversity Rx..................................................................................................166

Exercise 4.4 : Change reflection parameters...................................................................................167

Exercise 4.5 : Move radio site position...........................................................................................168

HERALD Lab #5 Multipath Fading................................................................................................169



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Exercise 5.1: Estimate multipath occurrence..................................................................................171

Exercise 5.2: Estimate multipath outage.........................................................................................172

Exercise 5.3: Frequency selective multipath...................................................................................173

Exercise 5.4: Design space diversity...............................................................................................174

Exercise 5.5: Use frequency diversity.............................................................................................175

HERALD Lab #6 Rain Attenuation.................................................................................................176


Exercise 6.1 : Predict rain unavailability........................................................................................177

Exercise 6.2 : Optimize hop design................................................................................................178

Exercise 6.3 : Include atmospheric absorption...............................................................................179

Exercise 6.4 : Revise design for tropical regions............................................................................180

Exercise 6.5 : Use Freq./Pol. scaling..............................................................................................

181

HERALD Lab #7 Interference Analysis..........................................................................................182


Exercise 7.1 : Search interference...................................................................................................183

Exercise 7.2 : Modify antennas.......................................................................................................184

Exercise 7.3 : Modify power levels................................................................................................185

Exercise 7.4 : Modify frequency/pol...............................................................................................186

Exercise 7.5 : Test rain correlation model......................................................................................

187

HERALD Lab #8 ITU Objectives...................................................................................................188


Exercise 8.1 : Set basic parameters................................................................................................189

Exercise 8.2 : International section.................................................................................................190

Exercise 8.3 : Long-haul section.....................................................................................................191

Exercise 8.4 : Access section..........................................................................................................192

Exercise 8.5 : North America Standard Objectives........................................................................193

Further Readings.............................................................................................................................194


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STUDENT GUIDE

Introduction

WelcometotheCourseon"PointtoPointRadioLinkEngineering".

TheaimofthisCourseistogiveyouapracticalguide,movingfrombasicnotionsinRadio

PropagationatmicrowavefrequenciesandcomingtoapplicationsinRadioLinkDesign.

TheCourseNotescoverthemaintopicsinRadioPropagationandPointtoPointradiolink

engineering.TheyapplytothedesignofMicrowaveLinksinthefrequencyrangefromabout450

MHzupto60GHz.

Withthe"HERALDLab",youcanusetheHERALDprogram(asoftwaretoolforradiolinkdesign)to

performanumberofguidedexercisesandtesttheHERALDapproachinimplementingthedesign

process.

Asawhole,theCoursehasbeendesignedwiththeobjectiveofactivelyinvolvingthereaderin

navigatingthroughthetextandinpracticingwithexercises. Themostrelevantaspectisthatthe

HERALDLabexercises,inadditiontotheCoursenotes,provideguidancetopracticalapplicationsin

thefieldofmicrowavelinkdesign.

WedonotexpecttoofferacompleteRadioEngineeringmanual,noratutorialoneveryaspectsof

RadioPropagation. Allthetopicsarepresentedinanintuitive,practicalstyle,notwithatheoretical

/academicapproach.

Itisassumedthatthereaderissomewhatfamiliarwithbasicnotionsinmodulationtechniques,

radioequipment

and

systems,

as

well

as

in

elementary

electromagnetic

physics.


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Course Notes

TheCourseNotesareorganizedineightsections. Werecommendyouproceedthroughthesections

inpublishedordertoensurecompleteunderstandingofthematerial. Thefirsttwosectionsare

introductory:

Section1introducestheradiosystemanditsstructure

Section2introducesthebasicsofmicrowavelinkengineering

Thenextfoursectionsarealmostselfcontainedandthefinaltwocoverpropagationprinciples.

Amoderatenumberofadditionalhypertextlinkshavebeenincludedthroughoutthetext. We

recommendyoureturntothelinkandcontinuethesectionafterfollowinghypertextlinkstoensure

youdontloseyourplaceinthiscourse.

TheCourse

Notes

are

subdivided

into

a"Basic

Techniques"

(white

background)

and

an

"Advanced

Techniques"(greenbackground)course. Initially,youmayskiptheadvancedtechniquesandreturn

tothematalaterreading.

Paragraphsgivingonlyadditionalcommentsaboutsometopicsareindicatedbyapinkbackground.


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Herald Lab

TheHERALDLabhasbeenincludedinthisCoursewithtwoobjectives:

asacomplementoftheCourseNotespresentation,showinghowthepropagationconcepts

andengineering

rules

are

applied

in

practical

cases;

asanintroductiontoHERALDfunctionsandcommands.

HERALDLabwillimprovebothyourunderstandingofradiolinkengineeringandyourskillsinusing

theHERALDprogram.

EachSessionintheHERALDLabstartswiththe"HERALDFunctions"chapter. Thischapterbriefly

explainshowdesignrules,presentedintheCourseNotes,areimplementedintheHERALD

program. ItdoesnotsubstitutetheHERALDHelp,whereyoufindamoredetailedguidetothe

programuse.

TheHERALDLabSessioncontinueswithexercises.Eachexerciseprovidesdetailedinstructionson

programstepstoexecuteagiventask. Someexercises(inparticularinthefirstsessions)may

appearrathereasyandeventedious. However,wesuggesttoskipthemonlyifyoualreadyhavea

goodpracticeinHERALDuse.

HeraldDemoversionmaybedownloadedbyregisteringatthefollowinglink:

http://www.activeonline.com.au/products/hp_register.php

HeraldDemoprovidesallthefeaturesrequiredtocompletetheHeraldLabexercise.


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SECTION 1 RADIO HOP CONFIGURATION

Summary

InthisSessionweintroducethebasicconfigurationofaPointtoPointRadioLink.Thefundamental

parametersusefultodescriberadiositeinstallationsarepresented,includingantenna,radio

equipmentandancillarysubsystems. Hopswithpassiverepeatersarefinallydiscussed.

Point-to-Point radio-relay links

APointtoPointradiorelaylinkenablescommunicationbetweentwofixedpoints,bymeansof

radiowavetransmissionandreception. Thelinkbetweentwoterminalradiositesmayincludea

numberofintermediateradiosites.

Thedirectconnectionbetweentwo(terminalorintermediate)radiositesisusuallyreferredasa

"RadioHop". Insomecases,aradiohopmayincludeapassiverepeater.

A multi-hop radio-relay link, connecting A to B, divided in two Radio Sections

Amultihopradiorelaylinkcanbedividedinanumberof"RadioSections",eachofthembeing

madeofoneormoreradiohops. Transmissionperformanceisusuallysummarisedonaradio

section

basis.


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Generalcriteriaforradionetworkplanninganddesignisnotdiscussedhere; justaverybrief

summaryisgivenbelow. Theoverallprocesscanbeusuallydividedintwosteps:

1) Preliminarynetworkorlinkplanning.Apartiallistofactivitiescarriedonatthisstageis:

a) Consideration

of

Regulatory

environment;

b) IdentificationofTerminalradiosites;

c) ServiceandCapacityrequirements;

d) Performanceobjectives;

e) Frequencybandselection;

f) IdentificationofsuitableRadioequipmentandAntennas;

g) Sampledesign

of

typical

hops,

estimate

of

maximum

hop

length.

2) RouteandIntermediateSiteSelection. Anumberoffactorshaveinfluenceonthischoice;

amongothers:

a) Maximumhoplength;

b) NatureofTerrainandEnvironment;

c) SitetoSiteterrainprofile;visibilityandreflections

d) Angularoffsetfromonehopandadjacenthops(toavoidcriticalinterference);

e) Needforpassiverepeaters.

f) Availabilityofexistingstructures(buildings,towers);

g) Newstructuresrequirements;

h) Accessroads(impactoninstallationandmaintenanceoperations);

i) Availabilityof

Electric

Power

sources;

j) Weatherconditions(wind,temperaturerange,snow,ice,etc.);

k) Localrestrictionsfromregulatorybodies(authorizationfornewbuildings,airtraffic,RF

emissioninpopulatedareas,etc.);

Whenatentativeselectionofintermediatesitesisavailable,thefinaldesigngoesthroughan

iterativeprocess:

Hopconfigurationanddetailedhopdesign;

PredictionofHopandSectionperformance;

Identificationofcriticalhops;


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RevisionofRouteandSiteselection;

RevisionofHopconfigurationandofdetailedhopdesign.

Inthe

following

Sections

we

focus

on

this

design

process,

going

through

site

and

hop

configuration

andleadingtoperformancepredictions. Asafirsttopic,wediscusstheparametersusefulto

describethesiteandhopconfiguration.

Site and Hop parameters

Aradiohopisdescribedintermsof:

1) Topographicaldataandterraindescription:

a) Radiositeposition:geographicalcoordinatesorothermappinginformation;elevationabove

sealevel(a.s.l.);

b) Pathlengthandorientation(azimuth: notethatinplanegeometrytheazimuthscomputed

atthetwoextremesofalinesegmentdifferby180deg,whilethisisnottrueinspherical

geometry;

so,

two

path

azimuths,

referred

to

each

radio

site,

are

usually

indicated);

c) Pathprofileasderivedfrompaperordigitalmaps:notethataccuracyrequirementsare

widelydifferentthroughoutaradiopath,sincetheelevationofpossibleobstructionsshould

beaccuratelyestimated,whilesignificantprofileportions(wherenoobstructionor

reflectionisexpected)couldbealmostignored.

2) Radioequipment,antennasandancillarysubsystemsinstalledateachradiosite;inthe

followingsections,themainparametersusefultodescribetheradiositeinstallationwillbe

discussed.

3) Specificaspectsonequipmentinstallationandoperation:

a) Antennapositioning:installationheightandpointing;spacediversityoption,antenna

spacing;

b) Frequencyused: (average)workingfrequency(usuallyreferredinhopcomputationsand

linkbudget);detailedfrequencyplan(goandreturnRFchannelsateachradiosite,required

forinterference

analysis);

c) RFprotectionsystems(useof1+1orn+1frequencydiversity,hotstandby,etc.)


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d) Useofpassiverepeaters:flatreflectororbacktobackantennasystem,repeatersite

parameters,reflectororantennapositioningandpointing.

4)Climaticandenvironmentalparameters:theyareusuallyrequiredbypropagationmodels

(atmosphericrefraction,rain,etc.),sotheywillbediscussedwhilepresentingsuchmodels.

Finally,letusconsiderseveralattenuationordegradingfactors,suchas:

Atmosphericabsorptionloss;

Obstructionloss;

Anyothersystematiclossthroughouttheradiopath(additionallosses);

Rxthresholddegradationduetogroundreflections;

Rxthresholddegradationduetointerference.

Theaboveimpairmentswillbediscussedinthefollowingsessions,wheresuitablemodelsto

estimatetheirimpactonhopperformanceareconsidered.

However,itmayhappenthattheinputsrequiredtoapplysuchmodelsarenotfullyavailableorthat

otherreasons

suggest

not

to

go

through

aspecific

analysis.

Inthatcase,wecanincludeamonghopparametersalsoaroughestimate(oraworstcase

assumption)oflossesordegradationscausedbytheimpairmentslistedabove.

Radio Equipment

Asimplifiedblockdiagramofasampleradiositeinstallationisshownbelow.


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Anexampleofradioequipmentblockdiagram,inthecaseofmultipleRFchanneloperation,usingasingle

antennaforbothtransmissionandreception.

Evenifthisexampleshowsaspecificconfiguration,itisusefulasareferenceinthefollowing

presentation. Otherconfigurationsofparticularinterestare:

SingleRFchannelinstallations,wherenobranchingsystemisneeded;

Outdoor

installations,

where

radio

equipment

is

directly

connected

to

the

antenna,

without

feederline.

Fromtheviewpointofasingleradiohopdesign,wecanlimitinformationaboutRadioEquipmentto

theverybasicparameters:

Rangeofoperatingfrequencies;

TransmittedpowerPT;

ReceiverthresholdPRTH(minimumreceivedpowerrequiredtoguaranteeagiven

performancelevel);

Notethat:

1) Boththetransmittedpowerandthereceiverthresholdareusuallyreferredattheequipment

input/outputflanges,notincludingbranchingfilterlosses.

2) WhenthetransmitterisequippedwithanAutomaticTransmittedPowerControl(ATPC)device,

theTxpowertobeconsideredinhopdesignisthemaximumpowerlevel(whichshouldbe

applied

every

time

the

received

signal

quality

is

deeply

affected

by

propagation

impairments);


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3) Thereceiverthresholdistheminimumreceivedpowerrequiredtoachieveagivenperformance

level;indigitalsystems,thereferenceperformanceisusuallyset

atBitErrorRate(BER)=103, whileotherreferencelevelsmaybeadoptedifneeded.

Performance

objectives

in

digital

radio

links

will

be

discussed

in

thefinal

Sessionof

this

course.

Additionalparameterscanbeusefulforamorecompleteunderstatingoftheequipmentoperation,

eveniftheyarenotdirectlyinvolvedinthehopdesign:

Equipmentusercapacity;fordigitalsystems,bitpersecondornumberofstandardized

signals,likeSTM1orDS1signals; foranalogsystems,numberoftelephoneortelevision

channels;

Bit

rate

(R)

of

the

modulated

(emitted)

signal

(this

may

differ

from

the

user

capacity,

mentionedabove,sincethetransmissionequipmentmayincludeadditionalbitsforservice

andmonitoringchannels,channelcoding,etc.);

Modulationtechnique;

Symbolrateofthemodulated(emitted)signal;inanalogsystems,anequivalentparameter

isthebaseband(modulating)signalbandwidth;

Emittedspectrumandmodulatedsignalbandwidth.

TheSymbolrateSRdependsontheemittedsignalbitrateRandonthemodulationtechnique:

whereListhenumberofbitscodedinasinglemodulatedwaveform(L=2inQPSK

modulation, L=6in64QAMmodulation).

ForadvancedtasksinRadioHopdesign,moredetaileddataonradioequipmentarerequired. This

includes:RxnoisebandwidthBNandRxnoisefigureNF;

SignaltoNoise(S/N)ratioattheRxthreshold;

CochannelCarriertoInterferenceratioatreceiverinput,producingthethresholdBER,in

theabsenceofthermalnoise(highRxlevel);

TypicalspacingbetweenadjacentRFchannel;

NetFilterDiscrimination(NFD)attheabovespacing;

Resultsof

signature

measurement;

PossibleuseofAutomaticTransmittedPowerControl(ATPC)andrelatedparameters;


20/196



PossibleuseofCrossPolarInterferenceCanceller(XPIC)andrelatedparameters.

TheSignaltoNoise(S/N)ratiocanbeexpressedintermsofreceivedpowerPR,receivernoise

bandwidth

BN,

and

receiver

noise

figure

NF

:

wherePRisexpressedindBmandBNinMHz;theexpressioninsquarebracketsgivesthe

receiverthermalnoisepower.

Advanced - Digital Equipment Signature

Theequipmentsignaturegivesameasureofthesensitivityofradiosystemstochannel(amplitude

andgroup

delay)

distortions

as

produced

during

multipath

propagationevents.

More

specifically,

it

isusedfordigitalradiosystemswithsignalbandwidthlargerthanabout1012MHz(onthistypeof

signals,significantfrequencyselectivedistortionisnotproducedifthebandwidthisnarrower;other

signalsmaybesensitivetofrequencyselectivemultipathevenwithanarrowerbandwidth).

MeasurementSetup - TheTxsignalismodulatedbyatestsequenceandistransmittedthrougha

simulatedmultipathchannel,modeledasatwopathchannel(directplusechobranches).

Signaturemeasurementsetup.

Asshownintheabovefigure,thepowerlevelandthephaseofthedelayedsignalcanbeadjustedby

meansofavariableattenuatorandavariablephaseshifter.

Assuminganormalizedsignalamplitudeequalto1inthedirectbranchandb(


21/196



=Echodelay,assumedasconstant(=6.3nsintheoriginalBellLabs/Rummlermodel);

fo=/2 =NotchFrequency(correspondingtotheminimumamplitudeofthetransfer

function);

B=Notch

depth

(in

dB)

=

20Log10(1

b).

Twopathchanneltransferfunction,withdefinitionofNotchFrequencyandNotchDepth.

TheabovedefinitionreferstoaMinimumPhaseTransferFunction. Otherwise,ifthesignal

amplitudeisb(


22/196



EquipmentSignatureintheNotchDepth/NotchFrequencyplane.

Inthatplaneeachpointcorrespondstoapairofnotchparameters,soitisrepresentativeofa

particularchannelstate. ThepointsbelowthesignatureshowthechannelstatesforwhichBER>

Threshold. Therefore,theareabelowthesignaturegivesameasureofthereceiversensitivityto

multipathdistortions.Foranunequalizedsignal,typicalsignaturewidthmaybeoftheorderof1.5

timesthesymbolrate,whileusingequalizationitishalvedatleast.

Topredictmultipathoutage,itisoftenrequiredthattheequipmentsignaturebedefinedbyonly

twoparameters(signaturewidthanddepth).Inmostcasestheshapeofactualequipment

signaturesallowfora"squarebrick"approximation.

EquipmentparametersusedinInterferenceanalysis

NetFilterDiscrimination(NFD) - Itisusedtocharacterizetheradiosystemabilitytolimitthe

interferencecomingfromanadjacentradiochannel.

NFDgivestheimprovementintheSignaltoInterferenceratiopassingthroughtheRxselectivity

chain(RF,Intermediate,basebandstages):

where(C/I)RFisdefinedattheRFinputstageand(S/I)DECatthedecisioncircuitstage.


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SignalspectraatthereceiverinputantoutputandRxselectivity.

Asshownbythefigureabove,theNFDdependson:

Interferingsignalspectrum(Txfiltering);

Channelspacing;

OverallRxselectivityintheUsefulChannel.

TheNFDcanbemeasuredorevaluatedforinterferencebetweenidenticalsignals(adjacentchannel

interferenceinahomogeneouschannelarrangement)andalsowhentheinterferingsignalis

different(incapacityand/ormodulationformat)fromtheusefulone(interferenceinamixedsignal

network).

So,foranypairofusefulandinterferingsignalsandforeachvalueofthechannelspacing,aNFD

valuecanbeevaluated.

ThresholdCarriertoInterferenceratio - Insomeapplicationsthereceivedsignalmaybe

interferedbyacochannelsignal,withidenticalcapacityandmodulationformat(forexampleinco

channelfrequencyarrangements,withuseofbothorthogonalpolarizations).

ThereceiversensitivitytocochannelinterferenceisestimatedbyaBitErrorRate(BER)vs.C/I

curve,asshowninthefigurebelow.


24/196



BitError

Rate

(BER)

vs.

Carrier

to

Interference

ratio

(C/I),with

indication

of

C/I

threshold

for

BER

=10

3

.

Themeasurementismadeinabsenceofanysignificantthermalnoisecontribution(highRxpower

level).

Fromthismeasurement,itispossibletoknowtheCarriertoInterferenceratiocorrespondingtothe

thresholderrorrate(forexampleBER=103).

CrossPolarInterferenceCanceller(XPIC)Gain - TheInterferenceCancellerisusedtoreducethe

interferencecomingfromasignaltransmittedonthesamefrequencywithorthogonalpolarizations

(usuallytheusefulandinterferingsignalshaveidenticalcapacityandmodulationformat).

WeassumethatthesignaltointerferenceratioatthereceiverRFinputis(C/I)RF.

Theinterferencecancellerworksinsuchawaythatthesignaltointerferenceratioappearstobe

improvedtoahighervalue(C/I)APP definedas

whereXPICGainisdefinedasthegainproducedbythecrosspolarcanceller. Theinterference

impairmentiscomputedbyassuming(C/I)APP tobetheactualsignaltointerferenceratio.


25/196



Antennas

Gaindefinitionandrelatedparameters

LetusconsideraradiotransmitterwithpowerpTcoupledtoanIsotropicAntenna(anidealsource

ofEMRadiation,thatradiatesuniformlyinalldirections). AtthedistanceLfromtheantenna,the

emittedpower

will

be

uniformly

distributedon.

the

surface

area

of

asphere

of

radius

L,

so

that

thePowerDensityIis:

EMpoweremissionfromanIsotropicAntenna(left)andfromaDirectiveAntenna(right)

ThenwesubstitutetheIsotropicAntennawithaDirectiveAntenna,whilethetransmittedpoweris

againPT. Weimagine

to

measure

the

Power

Density

where

the

antenna

axis

intercepts

the

sphere

surface,withresultD

Theantennagaingivesameasureofhowmuchtheemittedpowerisfocusedinthemeasurement

direction,comparedwiththeisotropiccase. Asaresultofthe"experiment"describedabove,the

antennagainisdefinedas:

Thisdefinitionleadstog=1fortheisotropic antenna.

Generallyspeaking,theantennagainisrelatedtotheratiobetweenantennadimensionandthe

wavelength Morespecifically,inthecaseofreflectorantennas,theantennagaingisgivenby:

whereDisthereflectordiameter,iscalled"antennaefficiency"(typicallyintherange

0.55 0.65),AisthereflectorareaandAE=A istheAntennaEffectiveArea.(or

Aperture).


26/196



Inlogarithmic(decibel)units:

wherethe0.5dBtermdependsagainonantennaefficiency;itisassumedtoexpressthe

diameterD

in

meters

[m]

and

the

frequency

Fin

GigaHertz

[GHz].

Notethat,forgivendimension,theantennagainincreaseswithfrequency(6dBhigherifthe

frequencyisdoubled). Similarly,atagivenfrequency,thegainincreases6dBiftheantenna

diameterisdoubled.

Below,someexamplesofantennagainvs.diameterandfrequencyaregiven.

Antennagainvs.diameterandfrequency;thedouble(red,black)linegivesarangeofpossiblegains,

dependingonantennaefficiency.

The3dBbeamwidthBW(seegraphicaldefinitionbelow)isrelatedtoantennagain;asthegain

increases,theEMenergyisfocusedinanarrowerbeam.


27/196



Definitionoftheantenna3dBBeamwidthBW.

Forreflectorantennas,somesimple"rulesofthumb"areusefulinrelatingantennadiameterD[m],

workingfrequencyF[GHz],gainG[dB],andthe3dBbeamwidthBW[deg]:

Notethattheseareapproximaterelations,fittingwith"real"valueswithinsomemargin;in

particularcases

this

margin

may

be

even

large.

AnadditionalconceptinantennaoperationistheFarFieldRegion. Itistheregionsufficiently

distantfromtheantenna,wheretheelectromagnetic(EM)fieldcanbewellapproximatedasaplane

waveandtheantennadiagramisstabilized.Closertotheantenna,theNearFieldRegionandthe

Fresnel(transition)Regionaredefined,wheretheantennaradiationdiagramisnoteasilypredicted.

TheboundarybetweentheFresnelandtheFarFieldRegionisapproximatelyatthedistance:

Antenna Parameters for hop design

Pointtopointradiohopsusuallymakeuseofhighgaindirectiveantennas,whichofferseveral

advantages:

bothTransmissionandReception: theantennagainismaximizedinthedesireddirection.

Transmission: theemittedradioenergyisfocusedtowardthereceiver,thusreducingthe

emissionof

interfering

radio

energy

in

other

directions;


28/196



Reception: thereceiversensitivitytointerferingsignalscomingfromotherdirectionsis

reduced.

However,inspecialcases,alsoantennaswithsectorialorevenomnidirectionalcoveragemaybe

used(thisistruemainlyforpointtomultipointapplications).

Inmostcases,directiveantennasareparabolicantennasorotherreflectorantennas(likeHornor

Cassegrainantennas). Thedirectivitypatternscanbemeasuredbothinthevertical(elevation)and

inthehorizontal(azimuth)planes; however,wecanoftenadoptthesimplifyingassumptionthat

onediagramisapplicablebothtotheverticalandtothehorizontalplanes.Inthatcase,alsothe3dB

antennabeamwidthisassumedtobethesameinthetwoplanes.

Asfarasinterferenceproblemsarenotconsideredinasingleradiohopdesign,wecanlimit

informationabouttheantennastotheverybasicparameters:


SingleorDoublePolarizationoperation;

Antennagain;

3dBbeamwidthintheverticalplane(thismaybeusefultoanalyzereflectionpaths).

AnexampleoftheantennaconnectiontoradioequipmentisgivenintheBlock diagramshown

above. Notethattheantennagain(aswellasotherantennaparameters)isreferredtotheantenna

I/Oflange.

Additionalparameters

can

be

useful

for

amore

complete

description

of

antenna

operation:

Antennatype(Parabolic,Horn,Cassegrain,etc.);

Coveragetype(omnidirectional,sectorial,directive);

3dBbeamwidthinthehorizontalplane(forsectorialantennas);

Diameter(ormoregenerally,physicaldimensions);

Voltagestandingwaveratio(VSWR);

Weight.

Moreover,theantennadiagram,asmentionedabove,illustratestheantennaoperationin

directionsotherthanthepointing(maxgain)direction.


29/196



Antenna radiation diagram (mask) for Co-polar e X-polar operation (a different horizontal scale is

used in the 0 - 20 deg range and in the 20 - 180 deg range).

Advanced - More on the Antenna radiation diagram

Someadditionalcommentsonantennadiagrams:

Theresultoftheantennadirectivitymeasurementusuallyexhibitsmultiplelobesand

nulls.

Asidelobe

envelope

is

estimated,

giving

a"mask

diagram",

useful

to

characterize

the

antennadirectivity. Ininterferenceanalysistheneedarisestoestimatetheantennagainin

anydirectionandtheantennamaskgivesaconservativeresult.

Thepatternofcopolandcrosspolantennadiagrams,closetothepointingdirection,are

significantlydifferent,asshowninthefigurebelow. Whilethecopolpatternisratherflat,

intherangeofsometensofdegreearoundpointingdirection(maximumgain),thecrosspol

patternhasaverynarrowminimuminthesamedirection. Insomecasesitisconvenientto

pointtheantennabysearchingfortheminimumcrosspolsignallevel,insteadofsearching

forthemaximumcopolsignal.Bythisway,itisassuredthat,notonlythemaximumgain,

butalsothemaximumcrosspoldiscriminationareobtained.


30/196



Exampleof

Co

pol.

and

Cross

Pol.

antenna

diagrams,close

to

the

antenna

pointing

direction

Theantennadirectivitydiagramisusuallymeasuredinacontrolledenvironment,inorderto

characterizethe"true"antennaresponse,withoutinfluenceorerrorsproducedbyanyexternal

element.

Inactualoperation,theantennaresponsemaybysignificantlyalteredbythesurrounding

environment.Forexample,anobstacleclosetothemainantennalobemayproduceasignal

reflection,about180fromtheantennapointingdirection.Thisapparentlyreducestheantenna

fronttobackdecoupling,bothinthecopolandcrosspoldiagrams.

Thecorrectantennapositioningisakeyfactorinordertogetantennaperformanceinreal

operatingconditionsascloseaspossibletomeasuredparameters.


31/196



Ancillary equipment

Anumberofadditionalequipmentandsubsystemsareworkinginaradiosite.Inthepresent

context,weconsideronlywhatisstrictlyrelatedtothedesignofaradiohop(so,wedonotdiscuss

powerlinesandbackups,airconditioning,grounding,andothersubsystems,eveniftheyareof

significantimportanceintheoverallsiteoperation).

Branching system

Asshownintheblock diagramabove,abranchingfilterisrequiredinradiotransceiversfor

multipleRFchanneloperation.

Intransmission,thefunctionofthebranchingsystemistomultiplexRFchannelsonasinglewide

bandRFsignal,suitabletobetransmittedonasingleantenna. Similarly,inreception,thebranching

systemsplitsthemultichannelsignalcomingfromtheantennaintomultipleRFchannels,each

addressedtothecorrespondingreceiver.

ThebranchinglossisdifferentforthevariousRFchannels(inTxandRx),dependingonthenumber

offilterportsandcirculatorstobepassedthroughbythesignal. However,inhopdesign,itis

advisabletotakeaccountofhighestloss,resultingfromTxandRxbranching.

InabranchingconfigurationwithacommonTx/Rxantenna(seeblock diagram),thebranchingloss

includethelossofthecirculatorusedtoseparatetheTxandtheRxbranches.

Tx / Rx Attenuators

Powerattenuatorsmaybeaddedinthetransmitterorinthereceiverchain,mainlytoavoidan

excessivepowerlevelatthereceiverinput(whichmaysaturatetheRxfrontendstage)and/orto

avoidunnecessarypoweremissioninshorthops(interferencereduction).

NotethatmanyradioequipmentsnowincludepowersettingoptionsorATPC(Automatic

TransmittedPowerControl)devices,sothatinmostcasestheuseofexternalattenuatorsisno

longerrequired.

Inthecontextofradiohopdesign,theonlyparametertobeassociatedwithTxandRxattenuators

isthe

attenuation

level

itself.

Feeder Line

AfeederlineisrequiredtoconnecttheantennaI/OflangetotheradioequipmentI/Oport(ortothe

branchingsystemI/Oport). Theexceptionistheoutdoorconfiguration,withdirectequipmentto

antennaconnection.

Thebasicfeederparametersforradiolinkdesignare:


32/196




Specificloss(expressedindBperunitlength).

Additionalparameters,givingmoredetailsonfeederdescription:

Feedertype(cable,rectangularwaveguide,etc.);

Weight(expressedinkgperunitlength).

Advanced - Hops with a Passive Repeater

PassiveRepeatersareusedmainlyinhopsoverirregularterrain,tobypassanobstructionalongthe

pathprofile.

ThreePassiveRepeaterconfigurationsaredescribedbelow,whilethecorrespondingLinkBudget

equationsarepresentedinthenextSession.

Singleplanereflector - itisimplementedasametalsurface,whichisclosetoa100%reflection

efficiency.Thesurfaceflatnessmustbemoreandmoreaccurateforincreasingfrequency(smaller

signalwavelength).

Thereflectorworkstodeviatetheincomingsignaldirectionbyanangle. Thegeometryisshown

inthefigurebelow

Passiverepeaterimplementedasasingleplanereflector

Eachpathfromaradiositetotherepeateriscalleda"leg". Soaradiohopwithasinglereflectoris

madeoftwolegs.

Notethattheusefulor"effective"areaAEoftheplanereflectorisgivenby:


33/196



whereAREAListherealreflectorareaandistheanglebetweenthetworays.

Itisestimatedthatfor>120(correspondingto


34/196



Passiverepeaterimplementedasatwoantennabacktobackarrangement

Fromageometricalpointofview,thebacktobackantennasystemhasawiderandmoreflexible

applicationfield,

compared

with

asingle

reflector

system.

From

agiven

repeater

position,

any

changeinsignaldirection()canbeobtained.

However,singleordoublereflectorsmaybeimplemented,ifneeded,withsurfacesmuchwider

thantheusualantennasize. Moreover,thereflectorefficiencyiscloseto100%,comparedtosome

55%antennaefficiency.

So,whenthepowerbudgetislimited,thebacktobackantennasystemmaybeapoorsolution.

This

concludes

Section

1

of

the

PPRLE.

Please

proceed

to

Herald

Lab

Exercise

1.

End of Section #1


35/196



SECTION 2 BASICS IN LINK ENGINEERING

Summary

InthisSessiontheFreeSpaceradiolinkequationispresented,togetherwiththeconceptofFree

SpaceLoss.Then,terrestrialradiohopsareconsideredandabriefsummaryisgivenofthemost

significantpropagationimpairments. WediscusstheLinkBudget,inordertoestimatetheFade

Margin,andhowtousetheFadeMargininpredictingtheoutageprobability. Finally,theradiolink

equationisrevisedtoincludetheuseofpassiverepeaters.

Free Space propagation

Weapproachradiolinkengineeringbyfirstconsideringanidealpropagationenvironment,where

transmissionofradiowavesfromTxantennatoRxantennaisfreeofallobjectsthatmightinteract

inanywaywithelectromagnetic(EM)energy. Thisassumptionisusuallyreferredas"FreeSpace"

propagation.

LetusconsideraradiotransmitterwithpowerpTcoupledtoadirective antennawithmaximum

gainontheaxisgT.

AtdistanceDfromthetransmittingantenna(sufficientlylarge,inorderthatFarFieldconditionsare

satisfied),thePowerDensityontheantennaaxisis:

Computation of Received Power in Free Space propagation

Nowweimaginethat,atthedistanceD,areceivingantennaisinstalled.Theantenna "effective

aperture"or"effectivearea"AEgivesameasureoftheantennaabilitytocaptureafractionofthe

radioenergydistributedatthereceiverlocation.Assumingnoreceivermismatch,thepowerpR,at

thereceiver

antenna

output

flange,

is

:


36/196



TakingaccountthattherelationbetweentheRxantenna gainandtheantenna"effective

aperture"is:

thereceivedpowerequationbecomes:

whereFisthefrequencyofthetransmittedsignal,isthewavelength,andc=Fisthe

propagationspeed,

which

can

be

assumed

to

be

about

3108m/s,

with

good

approximation,

bothinthevacuumandintheatmosphere.

Thisisusuallyknownasthe"FreeSpaceRadioLinkEquation." Usinglogarithmicunits,itcanbe

writtenas:

whereuppercaselettersareusedtoexpresspowerindBmandgainsindB,whilethesame

lettersinlowercasehadbeenpreviouslyusedfornonlogarithmicunits.

NotethatfrequencymustbeexpressedinGHzanddistanceinkm,otherwisethe92.44constantis

tobemodifiedaccordingly(e.g.:withdistanceinmiles,theconstantis96.57;withfrequencyin

MHz,theconstantis32.44).

Theaboveequationcanbealsowrittenas:

where

FSL

is

called

Free

Space

Loss,

given

by:

IfweassumetouseIsotropic Antennas(G=0dB)bothatthetransmittingandatthereceiving

site,then:

soFSLisalsodefinedas"lossbetweenisotropicantennas".


37/196



Free Space Loss vs. distance and frequency

Comments on Free Space Loss

TheconceptofFreeSpaceLoss,andtherelatedformulas,needsomecomments. First,theterm

"loss"couldsuggestsomesimilaritywithlossesincoaxialcablesorotherguidedtransmissionof

electromagnetic(EM)energy,whereweobserveaninteractionandpowertransferfromtheEM

wavetothepropagationmedium. Here,wearetalkingabout"FreeSpacePropagation":the

propagationmediumisthevacuumandnointeractionexists.TheFreeSpaceLossisjusttobe

referredtothedensityofEMenergy,whichfollowstheinversesquarelawdependenceversus

distancefromthesource.

AsecondproblemistheroleoffrequencyintheFreeSpaceLossformula.IstheFreeSpacea

transmissionmediummorelossyasfrequencyincreases? Letusconsiderthetwoequivalentforms

oftheradiolinkequationgivenabove:

Thefirstexpressionisprobablymoreintuitiveandshouldbepreferredwhenwetrytounderstand

thephysicalconceptunderlyingfreespacepropagation. TheTxantennaisdescribedbyitsgain(the

abilitytofocustheEMpowertowardagivendirection),whiletheRxantennaisdescribedbyits

equivalentaperture(theabilitytocapturetheEMpowerdistributedatthereceiverlocation).

Ontheotherhand,wepassedtothesecondexpression,whereboththeTxandRxantennagains

appear,sinceitlooksattractiveforitssymmetricform. Thefrequencydependenceinthiscaseis

duetothedecreasingeffectiveapertureofthereceivingantenna(foragivengain),asthefrequency

increases.It

is

just

aformal

artifice

to

include

frequency

dependence

in

the

so

called

Free

Space

Loss.


38/196



Asaconclusion,theFreeSpaceLossisaconvenientstepinevaluatingthereceivedpowerinaradio

linkanditisusefulinordertoputformulasinamanageableform. However,careshouldbepaid

aboutthephysicalconceptrelatedtoit,inordertoavoidmisleadinginterpretations.

Terrestrial radio links

WenowdepartfromtheFreeSpaceassumptionandweputagainourfeettotheearth. We

considerradiowavepropagationbetweentwoterrestrialradiosites,inthecontextofradiohop

design.

Transmittingandreceivingantennasareassumedtobeinstalledontowers/buildings,atmoderate

heightabovetheearthsurface(metersortensofmeters),sothatpropagationinthelower

atmosphere,closetoground, hastobeconsidered.

Moreover,weassumethattheradiowavefrequencyisintherangefromUHFband(lowerlimit300

MHz)uptosometensofGHz(60GHzcanroughlybetheupperlimit,accordingtopresent

applications).

ComparedwithFreeSpacePropagation,thepresenceoftheatmosphereandthevicinityofthe

groundproduceanumberofphenomenawhichmayseverelyimpactonradiowavepropagation.

Themajorphenomenaaredueto:

AtmosphericRefraction:

RayCurvature;

MultipathPropagation;

Interactionwithparticles/moleculesintheAtmosphere:

AtmosphericAbsorptionintheabsenceofrain;

RaindropAbsorptionandScattering;

EffectsoftheGround:

DiffractionthroughObstacles;

Reflectionsonflatterrain/watersurfaces.

Whenoneormoreoftheabovephenomenaaffectradiopropagation,theresultingimpairmentis:

usually,anadditionalloss(withrespecttofreespace)inthereceivedsignalpower;

inparticularcases,alsoadistortionofthereceivedsignal.

Propagationimpairments

will

be

considered

in

the

following

sessions.

In

most

cases

they

can

be

predictedonlyonastatisticalbasis. Theyaremainlyaffectedby:


39/196



Frequencyofoperation;

HopLength;

Climaticenvironmentandcurrentmeteorologicalconditions;

Groundcharacteristics(terrainprofile,obstaclesaboveground,electricalparameters).

Fromtheviewpointofthephenomenaduration,letusconsider:

temporaryimpairments,whichaffectthereceivedsignalonlyforsmallpercentagesoftime

(examplesarerain,multipathpropagation,...);

longterm(orpermanent)propagationconditions,whichaffectthereceivedsignalformost

ofthetime(examplesareatmosphericoxygenabsorption,terraindiffraction,...),evenif

theirimpactmaybevariableinsomemeasure.

Inmostcases,longtermpropagationimpairmentsdonotproduceasignificantpowerlossinthe

receivedsignal,comparedwithFreeSpaceconditions. So,thereceivedpowerobservedforlong

periodsoftimewillberatherclosetothatpredictedbytheFree Space Radio Link Equation.

Themostsignificantexceptiontotheaboveconditionisexperiencedinradiopathswithnotperfect

visibility. Inthatcase,attenuationcausedbyterraindiffractionresultsinasystematicloss,in

comparisonwithFreeSpaceconditions.


40/196



Link Budget

EvenindesigningTerrestrialRadioLinks,theFree Space Radio Link Equationisthebasisfor

receivedpowerprediction.

Theequationin

logarithmic

units

offers

avery

simple

and

convenient

tool,

sinceGains

and

Losses,

throughoutthetransmissionchain,areaddedwithpositiveornegativesign,asinafinancialbudget.

Theresultiswhatiscalledthe"LinkBudget".

TheFreeSpaceequationcanberewrittenwithmoredetail,takingaccountofactualequipment

structureandofsystematicimpairmentsthroughoutthepropagationpath.Anexampleisgivenin

theTablebelow.

PowerLevel

[dBm]

Gains

[dB]

Losses

[dB]

TxPoweratradioeqp.

outputflange

Txbranchingfilter

Txfeeder

OtherTxlosses

Poweratant.input

Txantenna

gain

Propagationlosses:

FreeSpace

Obstruction

Atm.Absorption

Other

RxAntenna

gain

Poweratant.output

Rxfeeder

Rxbranchingfilter

OtherRxlosses

NominalRxPowerat

radioeqp.inputflange


41/196



Asshownintheaboveexample,thelinkbudgetincludesanestimateofthepowerlossdueto

permanent(orlongterm)impairments(likeatmosphericabsorptionandobstructions). So,the

NominalRxPower(ascomputedatthelastline)isexpectedtobeobservedforlongperiodsoftime.

OncetheLinkBudgetiscomputed,otherimpairmentsatthereceiveraretakenintoaccountas:

adegradingeffectinreceiveroperation(Rx thresholddegradation):thisusuallyappliesto

theeffectofgroundreflectionsandinterference;

ashorttermattenuation(orevendistortion)inthereceivedsignal,whoseeffectmaybeto

fadethereceivedsignalbelowtheRxthreshold

Power

Threshold Margin

Nominal

Rx

Power

EquipmentThreshold

ThresholdDegrad.

Reflections

Interference

HopThreshold

Fademargin

WesummarizethefinalstepsinLinkBudgetanalysiswiththetwoequations:

NotethatThresholdDegradationcausestheactualHopThresholdtobehigherthantheEquipment

Threshold(onedBthresholdincreasemeansonedBreductionintheavailableFadeMargin).

Fade Margin and Outage prediction

Typically,pointtopointradiohopsaredesignedinawaythattheNominalRxPower(ascomputed

intheLink Budget)isfargreaterthanthereceiverthreshold. So,ratherlargeFadeMargins(ofthe

orderof3040dB,orevengreater)areusuallyavailable.

TheFade

Marginis

required

to

cope

with

short

term

attenuation

and

distortion

in

the

received

signal(mainlycausedbyrainandmultipath).


42/196



Asummaryofvariousdefinitionsisgiveninthediagrambelow.

AsummaryofdefinitionsinReceivedPowerlevels,thresholds,andmargins,withapplicationtoOutage

estimation.

Theabovefiguresuggeststhefollowingcomments:

TheRxpowermayexceedtheFreeSpacelevel: thesocalled"upfading"isaratherunusual

event(itmaybecausedbyparticularrefractionconditions,whichcreateasortofguided

propagationthroughtheatmosphere).Caremustbetakenthatthereceivedpowerlevelbe

in

any

case

below

the

maximum

level

accepted

by

the

Rx

equipment

(otherwise,

receiver

saturationandnonlineardistortionmaybeobserved).

TheRxPowerwillbeattheNominallevel(Normalpropagation)formostofthetime.

ModerateattenuationbelowtheNominalRxpowerdoesnotusuallyproduceanysignificant

lossinsignalquality.

TheEquipmentthresholdmaybedegradedinsomemeasurebyreflectionsand/or

interference,sothatahigherHopthresholdmustbeconsidered.

Startingfrom

the

very

low

Rx

power,

the

Outage

conditions

are:

o belowtheEquipmentthreshold,outageisproducedbythereceiverthermalnoise,

evenintheabsenceofanyadditionalimpairmentinthereceivedsignal;

o belowtheHopthreshold,outageiscausedbythecombinedeffectofreceivernoise

andotherimpairments(likereflectionorinterference);

o inthedeepfadingregion,abovetheHopthreshold,outagemaybeobservedwhen

thereceivedsignalisnotonlyattenuated,butalsodistortedbypropagationevents

(mainly,frequencyselectivemultipath).

Fromtheabovediscussion,theOutagetime,duringtheobservationperiodTo(typically,one

month)canbepredictedas:


43/196



ifnocontributiontooutageisexpectedfromsignaldistortion.

Ontheotherhand,ifsignificantdistortionintheRxsignalisexpectedtocontributetothetotal

outage,thepredictionformulahastobecompletedas:

wherethesecondtermgivesthecontributiontooutageprobabilitywhenthereceived

signalisabovetheHopthreshold,butitisseverelydistorted(notethatProb{A/B}means

probabilityofeventA,giventhateventBistrue).

Theseformulas

only

help

to

clarify

how

the

outage

time

is

related

to

the

Rx

power

level

and

to

additionalimpairmentsinthereceivedsignal. Theydonotprovideapracticalmeanstopredict

outagetime;thisrequiresthatsuitablestatisticalmodelsofpropagationimpairmentsbeavailable:

SuchmodelswillbeconsideredinthefollowingSessions.

ADVANCED - Link Equation with Passive Repeater

WhenaPassiveRepeaterisusedinaradiohop,wehavetorevisethe"BasicRadioLinkEquation".

TobeconsistentwiththesimpleFreeSpaceformula,wewritethenewequationas:

where:

FSL(DTOT)istheFreeSpaceLossofaradiolinkwithpathlength DTOT=Di;

Di isthelengthofeachpath leg;

LPRisthepowerlosscausedbythepassiverepeater,incomparisonwiththeFreeSpacecase.

SingleReflector - Werefertothepathgeometry,asshowninaprevious figureandtothe

definitionofthereflectoreffective areaAE. Then,LPRisgivenby:

whereFistheworkingfrequencyinGHzandD1,D2aretheleglengthsinkm.

DoubleReflector - Again,werefertothepathgeometry,asshowninaprevious figureandto

thedefinitionofthereflectoreffective areaAE. Then,LPRisgivenby:


44/196



whereFistheworkingfrequencyinGHzandD1,D2,D3aretheleglengthsinkm.

BacktoBackantennasystem - Thepathgeometryisshowninaprevious figure. Then,LPRis

givenby:

whereFistheworkingfrequencyinGHz,D1,D2aretheleglengthsinkm,G1,G2arethe

antennagainsattherepeatersite(usually G1=G2)andLFisthelossduetothefeeder

connectingthetwoantennas.

NearFieldcorrection - Theaboveformulasarecorrectlyusedwhenthereflectorsarepositioned

outsidethe"nearfield"region. Ifthisconditionisnotsatisfied,thenacorrectionfactor(additional

loss)must

be

applied.

Thenearfieldregionisestimatedasafunctionoftheantennaandreflectordimensionsandofthe

signalfrequency(wavelength). Twonormalizedparameters(,)arecomputed:

whereDMinistheshortestlegfromoneantennatotheclosestreflector,distheantenna

diameterandAEisthereflectoreffectivearea.

Aruleofthumbisthefollowing:forintherange0.2 1.5(thiscoversmostpracticalconditions),

thenearfieldcorrectionfactorisnotnegligibleif 10dB

=0.40 1.7dB 3.9dB 7.1dB 9.8dB

=0.60 0.7dB 1.8dB 3.8dB 6.7dB

=1.00


45/196



End of Section #2

SECTION 3 PATH CLEARANCE

Summary

InthisSessiontheeffectoftheatmosphereonradioraytrajectoriesisfirstconsidered,by

introducingthekfactorconcept;possibledeviationsfromstandardconditionsareidentified,aswell

astheminimumkfactorvalue.ThentheFresnelellipsoidisdefined;thepartialobstructionofthe

ellipsoidleadstotheestimateoftheresultantloss.Finally,thepreviousconceptsareusedtoset

clearancecriteriaandtodiscusstheirapplicationtopathprofileanalysis.

Refractivity in the Atmosphere

TheRefractive

Index

nin

agiven

medium

is

defined

as

the

ratio

of

the

speed

of

radio

waves

in

vacuumtothespeedinthatmedium.Sincethespeedofradiowavesintheatmosphereisjust

slightlylowerthaninvacuum,thentheRefractiveIndexintheatmosphereisgreaterthan,butvery

closeto,1.

However,alsosmallvariationsintheatmosphereRefractiveIndexhavesignificanteffectsonradio

wavepropagation.Forthisreason,insteadofusingtheRefractiveIndexn(closeto1),itis

convenienttodefinetheRefractivityNas:

So,NisthenumberofpartspermillionthattheRefractiveIndexexceedsunity;itisadimensionless

parameter,measuredinNunits.

TheatmosphereRefractivityisafunctionofTemperature,Pressure,andHumidity.TheITURRec.

453givestheformula:

where:

T=absolutetemperature(Kelvindeg);

P=atmosphericpressure(hPa,numericallyequaltomillibar);

e=watervapourpressure(hPa).

Atsealevel,theaveragevalueofNisaboutNo=315Nunits.TheITURgivesworldmapswiththe

meanvaluesofNointhemonthsofFebruaryandAugust.

Temperature,atmosphericpressure,andwatervapourpressurearenotconstantwithheight.This

producesaVertical

Refractivity

Gradient

G

(measured

in

N

units

per

km,

N/km),

defined

as:


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whereN1andN2aretherefractivityvaluesatelevationsH1andH2,respectively.

Undernormal

(standard)

atmospheric

conditions,

Refractivity

decreases

at

aconstant

rate,

moving

fromgroundleveluptoabout1kmheight. ThismeansthattheRefractivityGradientGisconstant,

thetypicalvaluebeingabout 40N/km.

DeviationfromtheStandardAtmosphereconditionisusuallyassociatedwithparticularweather

events,liketemperatureinversion,veryhighevaporationandhumidity,passageofcoldairover

warmsurfacesorviceversa.Intheseconditions,theVerticalRefractivityGradientisnolonger

constant.Anumberofdifferentprofileshavebeenobservedandmeasured.Itisworthnotingthat,

atgreateraltitude,theRefractiveIndexis,inanycase,closerandcloserto1;sotheRefractivityN

decreasestozero.

Propagation in Standard Atmosphere

ARadioWavepropagatesinthedirectionnormaltotheisophaseplane(theplanewhereallthe

pointsarephasesynchronous,withrespecttothesinusoidalpatternofelectricandmagneticfields).

Inahomogeneousmedium,theisophaseplanesareparalleltoeachotherandthepropagation

directionisastraightlinenormaltothem.

As seen above,theAtmosphereisnotahomogeneousmediumandtheVerticalRefractivity

Gradient

gives

a

measure

of

that.

Different

Refractivity

at

different

heights

means

different

propagationspeeds.Thewavefrontmovesfasterorslower,dependingontheheight:thiscausesa

rotationofthewavefrontitself.

Wavefront

and

ray

rotation

caused

by

avertical

refractivity

gradient

in

the

atmosphere


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So,thepropagationtrajectory(normaltothewavefront)isnotastraightline,butitisrotated,as

shownintheabovefigure.Takingintoaccountthatthepropagationspeedisinverselyproportional

totherefractiveindex,itispossibletoderivethattheradiotrajectorycurvature1/risrelatedtothe

VerticalRefractivityGradientG,as:

InStandardAtmosphere,withatypicalvalueoftheRefractivityGradientG= 40N/km,the

curvatureoftheradioraytrajectoryis:

Thismeansthattheradiorayisbentdownward,withacurvature1/r,somewhatlower(lesscurved)

thantheEarthcurvature1/R:

Raybendinginstandardatmosphere(CL=clearance,verticaldistancefromgroundtoraytrajectory)

The k-factor

Aconvenientartificeisusedtoaccount,atthesametime,forboththerayandtheearthcurvatures.

An"equivalent"representationofthe above figurecanbeplottedbyalteringbothcurvaturesby

anamountequaltotheraycurvature1/r.

Inthenewfigure(seebelow)theradioraytrajectorybecomesastraightline,whilethemodified

("equivalent")earthcurvature1/REis:

8/14/2019 Point To P

point to point radio link engineering.pdf

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