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Application note

Link Budget

3.LINKBUDGET

The basic elements to calculate a link budget can be illustrated by considering the example showninFigure4.ItinvolvesaGSM900radiocoverageinadual-boretunnelthatis900minlength.Itshallbeassumedthat:

➔ thepowerperchannelavailableforthedownlinkis1W(+30dBm);➔ theRC ineachbore is fedviaapower splitter (“T-feed”configuration) the insertion lossofwhich

isequalto3.5dB;➔ jumper cables are used to connect the repeater, power splitter and the RCs. Their total insertion

lossisequalto1.5dB;➔ thespecification imposesa receivedsignal (measuredwithahalf-wavedipoleantenna)ofat least

-88dBmat95%ofthepointsinthevicinityofthecableendandat6mdistance.

It shall also be assumed that the RC has the following characteristics at 960 MHz (upper limit of theGSM900frequencyband):

➔ longitudinalattenuation:3.1dB/100m;➔ CL50%and equalto58and62dBrespectively.

Table1summarisesthelinkbudget.Thelastlineindicatesthatthespecificationissatisfied,i.e.aminimumreceivedsignal(measuredwithahalf-wavedipoleantenna)higherthan-88dBmat95%ofthepointsinthevicinityofthecableendandat6mdistance.Thevariouslinesofthislinkbudgetarecommentedhereafter.

Uplinkperformances(i.e.frommobilestationtobasestation)canbecomputedinthesameway.

Table1:Linkbudgetexample

Availablepowerperchannel +30dBm

Jumpercableloss -1.5dB

Powersplitterinsertionloss -3.5dB

RCinsertionloss:900mwith3.1dB/100m -28dB

CL95%at2m=62dB -62dB

Correctionforlongerdistance=20log(d/2)=20log(6/2)= -9.5dB

Penetrationloss* 0dB

Mobileantennalossrelativetodipole* 0dB

Safetymargin -10dB

Minimum received signal at 6 m from the RC (95% percentile) - 84.5 dBm

Figure4:Dual-boretunnel

withonebasestationandapowersplitter

*Note:Itresultsfromthespecificationofthisparticularexamplethatthepenetrationlossandmobileantennalossrelativetodipoleareequalto0dB.

RCinbore1

RCinbore2

RadiatingCablesApplicationnote03/2010

1.

3.1. RC insertion loss

The RC insertion loss is equal to the cable length multiplied by the longitudinal attenuation. Thislongitudinalattenuationissomewhatinfluencedbythestandoffdistance(betweentheRCandthewallorceilingtowhichitishung).Forexample,iftheRCisdirectlyagainstaconcretesurface,theimpactonthelongitudinalattenuationisfrequencydependentandisobviouslynotidenticalforallRCs.IftheIECstandardconditionsareconsideredasreferencevalues,measurementscarriedwithvariousEupenRMCsindicatethatinstallingtheRCdirectlyagainstaconcretesurfaceinvolvethefollowinglongitudinalattenuationincreases:

➔ below300MHz,theimpactisnegligibleandevensometimesnegative;➔ typicallyrangesfrom5to10%around450MHz;➔ typicallyrangesfrom10to20%around900MHz;➔ typicallyrangesfrom25to60%around2000MHz.

ThelongitudinalattenuationisalsoinfluencedbyhumidityanddustdepositontheRCjacket.Eveninrathersevereconditions,thelongitudinalattenuationincreaseneverexceeds10%.

3.2. RC Coupling loss

Some RC manufacturers use the free space method or specify the CLs for the antenna orientationcorrespondingtothebestresult.ThedifferencesofinterpretationinthemeaningoftheCLparametermayleadtosignificanterrorsinlinkbudgetorwhentheperformancesofdifferentproductshavetobecompared.

TheCL50%andCL95%specifiedintheEupendatasheetsaremeasuredwiththegroundlevelmethodaccordingthe IEC61196-4standard1.Thegroundlevelmethodhasbeenpreferredbecause itdefinesconditions which are closer to those actually met in practice. Indeed, in almost all the applications,theRCishungatshortdistancefromasurface(ceilingorwall).Adetailedanalysisofthisissueispresentedinsection4.

However,CLsmeasuredwiththefreespacemethodarealsoavailableforsomeEupenRCs.

TheCL50%andCL95%specifiedintheEupendatasheetsareaveragedoverthreeantennaorientations(radial,orthogonalandparallel).Asexplainedinsection4.2.,theCL50%orCL95%thatshouldbeusedforlinkbudgetcorrespondtothesymbolsCL50%-meanorCL95%-mean.

1IEC61196-4standard-Coaxialcommunicationcables-Part4:Sectionalspecificationforradiatingcables.

RadiatingCablesApplicationnote

03/2010

1.1.

1.2.

a)E.M.wavedepolarisationduetoreflectionsonobstacles

In thecaseofcommunicationswithhand-heldmobileequipmentonboard train,awavepenetratingintoacarriageexperiencesreflectionsonthecarriagewalls,ceiling,floor,seats,etc.Ateachpoint,thefieldstrengthisthevectoradditionofseveralwavesandthepolarisationofthesumcanbeconsideredaselliptical ratherthan linear.Figure5showsthesimplecasewhereadirectwaveradiatedbytheRCinterfereswithanotherwavewhichhasbeenreflectedbythecarriageceilingandwindow.Thedashedarrows(atrightanglewiththedirectionofpropagation)indicatethewavepolarisation.Ifweconsiderthedifferenceofpropagationdelay,itisclearthatthepolarisationoftheresultingE.M.fieldatthereceptionpoint R is very complex. Figure5 is a very simple casewithonlyone reflectedwave. Inpractice, thesituationsaremuchmorecomplexassuggestedinFigure6wherethedirectwavemaybeblockedbytravellersorbyanothertrain.

b)Mobileantennaorientation

Withhand-heldequipments,themobileantennaorientationisneitherperfectlyverticalnorhorizontalbutratheracombinationoftheseasshowninFigure7.Indeed,innormaluse,anhand-heldequipmentisslightlydown-tiltedandnotnecessarilyorientatedformaximumresponse.

Figure5:Wavedepolarisation

duetoreflections

Figure6:Propagationintocarriage

Figure7:Mobileantenna

orientation

Figure6 Figure7

RadiatingCablesApplicationnote03/2010

c)Mobilestationareratherinsensitivetoantennaorientation

Asexplainedinsection4.2,thedifferencebetweentheCLsinthedifferentantennaorientationisduetothedirectivityofthehalf-wavedipolewhichisusedforCLmeasurements.

Converselymobilestationantennas(suchasGSM,PCN,UMTS,etc.)aremoresophisticatedthatthesingledipoleormonopole(whichisillustratedinFigures6and7).Theirspatialresponseisdifferentandmuchmore“isotropic”thanquarter-wavemonopoleandhalf-wavedipoleantennas.Measurementsperformedwithvariousmobilestationsdemonstratethatthereceivedpowerisnearlyindependentoftheantennaorientation.So,mobilestationantennasbehaveasnearlyisotropicantennaswhichpickupthestrongestfieldcomponentwitharatherlowgain(about–10dB).

If the symbols CLr, CLp and CLo designate the coupling losses in the radial, parallel and orthogonalorientationrespectively(whichevertheprobabilitylevel)andifthesymbolCLmeancorrespondstothemeancoupling lossasdefined in the IEC61194-4standard, it is shown insection4.2thatCLmean isgenerally about 4 dB higher than the lowest coupling loss. It results that, for practice, the followingapproximationcanbemade:

CLmean = min (CLr, CLo, CLp) + 4

wherethe“min”symboldesignatestheminimumofthevaluesinbrackets.

This results means that calculating the link budget with the lowest coupling loss (CLr, CLp or CLo) instead of the average value (CLmean) is equivalent to a 4 dB decrease of the safety margin.

3.3. Correction for longer distance

With“classical”transmittingantennas,thereceivedpowerdecreasesasafunctionofthesquareofthedistanced,i.e.:

This is a consequence of the “spherical symmetry” (the radiated energy is contained in a sphere ofradiusequaltod.WithRCs,theradiatedenergyiscontainedinacylinderofradiusequaltod,hencea“cylindricalsymmetry”.Consequently,thereceivedpowerdecreasesasafunctionofthedistanced,i.e.:

1➔Prec÷— d2

1➔Prec÷— d

RadiatingCablesApplicationnote

03/2010

1.3.

TheCL50%isspecifiedat2moftheRCaccordingtotheIEC61196-4standard.Ifitisrequiredatanotherdistance,thefollowingcorrectionshouldbeapplied:

FortheCL95%,alongerdistanceinvolvesastrongerinfluenceofscatteredradiationsandreflectionsonwallsandceiling,henceafadingincrease.Thefollowingcorrectioncanbeapplied:

3.4. Penetration loss

For communications into vehicles, the link budget must take a penetration loss into account. Thispenetrationlossisstronglyinfluencedbythefrequency,thewidowsizes,theglasstype(singleordoublelayer) and the possible presence of metal coating (for thermal insulation). For example, at 900 MHz,penetrationlossmayrangefrom2or3dBforasinglelayerglass.Itreaches30dBinthecaseofmetalcoatedglasses.

3.5. Mobile antenna loss relative to dipole

Theantennasusedinmobilephones(suchasGSM,PCN,UMTS,etc.)haveanegativegainwithrespecttothehalfwavedipolenormallyusedtomeasuretheCLs.Theirspatialresponseishowevermore“isotropic”.A10dBmobileantennalossrelativetohalfwavedipoleseemsarealisticvalue.

3.6. Safety margin

A10dBsafetymarginisrecommendedtoaccountfor:

➔ thedifferencesbetweenthestandardconditionsinwhichtheCLsaremeasuredandthoseactuallymetinarealtunnelenvironment;

➔ thevariousfactorswhichmayimpairtheRCperformances.

As explained in section 3.2., a link budget based on a CL averaged over three antenna orientationsprovidesasafetymarginwhichis4dBsuperiortoabudgetcalculatedwiththevaluemeasuredinthebestorientation.

d➔CL50%(d)=CL50%+10log(——)

2

d➔CL95%(d)=CL50%+20log(——)

2

RadiatingCablesApplicationnote03/2010

1.4.

1.5.

1.6.

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