point to point radio link engineering.pdf
TRANSCRIPT
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POINTTOPOINTRADIOLINK ENGINEERING
ASELFLEARNINGEBOOK BASEDCOURSE,BYRADIOENGINEERINGSERVICES
AUTHOR: LUIGIMORENO
Availablefromwww.heraldpro.com
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Copyright Notice
Theuse
of
this
EBook
is
regulated
by
the
following
terms.
1.DOCUMENT. ThisDocumentismadeofthisEBookpageandofanyotherpage(inEBookformat
oranyotherformat)whichisdirectlyorindirectlylinkedtothisone,includinggraphicfilesandany
otherfileembeddedinanypage,unlessotherwisespecified,andexcludingwebpagesoutsidethe
domain"radioengineering.it".
2.COPYRIGHTS. ThisDocumentiscopyrightedbyLuigiMoreno,aProfessionalEngineerbasedin
Torino,
Italy,
and
is
protected
by
copyright
laws
and
international
copyright
treaties,
as
well
as
other
intellectualpropertylawsandtreaties.Youarenotallowedtocopy,modify,deleteormanipulate,in
anyway,partorthewholeofthisDocument,includinganychangeintheembeddedhyperlinks.
3.PUBLICPRESENTATION. ThisDocumentisforpersonaluseonly.TheuseofthisDocumentfor
ClassroomPresentationsorforanypublicpresentationispermittedonlyafterwrittenagreements
withtheAuthor.
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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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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
HERALD Functions........................................................................................................................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;
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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(
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=Echodelay,assumedasconstant(=6.3nsintheoriginalBellLabs/Rummlermodel);
fo=/2 =NotchFrequency(correspondingtotheminimumamplitudeofthetransfer
function);
B=Notch
depth
(in
dB)
=
20Log10(1
b).
Twopathchanneltransferfunction,withdefinitionofNotchFrequencyandNotchDepth.
TheabovedefinitionreferstoaMinimumPhaseTransferFunction. Otherwise,ifthesignal
amplitudeisb(
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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.
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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.
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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).
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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.
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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;
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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:
Rangeofoperatingfrequencies;
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.
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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.
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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.
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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:
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Rangeofoperatingfrequencies;
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:
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whereAREAListherealreflectorareaandistheanglebetweenthetworays.
Itisestimatedthatfor>120(correspondingto
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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
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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
:
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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".
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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.
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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:
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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.
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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
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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).
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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:
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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:
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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
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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:
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