cellular_network_planning_and_optimization_part5.pdf
TRANSCRIPT
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Cellular Network PlanningandOptimizationPart V:GSMJyriHmlinen,
Communications andNetworking Department,TKK,18.1.2008
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2GSMBriefly
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3General
GSMwasthefirstdigitalcellularsystem. GSMwaslaunchedin1992 Over2billionsubscribersworldwide Over600millionsubscribersinEurope
ConventionalGSMvoiceisCircuitSwitched(CS) Calloccupiesthechannelduringthedurationofth ecall
Thelateststandardphaseincludes GeneralPacketRadioService(GPRS) EnhancedDataRatesforGSM(EDGE) GPRSandEDGEare packet switched systems
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4GSMSpectrum
OriginallyGSMoperatedonlyin900MHzband Laterextendedto1800MHzand1900MHzbands MostGSMnetworksoperateinthe900MHzor1800
MHzbands Also 450MHzband isused insome countries
Spectrumexample Inso-calledprimaryGSM900MHztheuplink
frequencybandis890915 MHz,andthedownlinkfrequencybandis935960 MHz.
This25MHzbandwidthissubdividedinto124carrie rfrequencychannels,eachspaced200 kHzapart.
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5GSMSystemarchitectureOurmainfocusarea
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6BaseStationSubsystem(BSS) BaseTransceiverStation(BTS)
RadiointerfacecontrolbetweenBTSandMS Transmissionexecution,channelencryption,diversi ty,
frequencyhopping BaseStationController(BSC)
Handovercontrol,channelassignments,collectiono fcellconfigurationdataetc
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7NetworkSwitchingSystem(NSS) MobileSwitchingCenter(MSC)
Callswitching;MSCcontrolcallsbetweenBSSando thernetworks(PSTN,PLMN)
GatewayMSC(GMSC) ExecutesgatewayfunctionsforMSC
HomeLocationRegister(HLR) Databasethatcontainsinformationofoperatorsown subscribers.
VisitorLocationRegister(VLR) Databasethatcontainsinformationofvisitingsubs cribers.Thereis
oneVLRperMSC ServingGPRSSupportNode(SGSN)
HandlespacketdatafromGPRS/EDGE GatewayGPRSSupportNode(GGSN)
GatewayfunctionsforSGSN
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8GSMFramestructure
TimeDivisionMultiplexing(TDMA)isused 8full-rateor16half-ratespeechchannelsper200 kHz
channel.Thereareeightradiotimeslotswithinar adioframe.
Halfratechannelsusealternateframesinthesametimeslot.Thechanneldatarateis270.833 kbit/s,a ndtheframedurationis4.615 msTDMA/FDMAscheme
Ontopofthis frame structure there ismultiframe,superframe andhyperframe structures.
0 1 2 3 4 5 6 7
4.615ms
200kHz
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9LogicalchannelstructureGSMchannels
Traffic channels (TCH) Signaling channels
Full rate Half rate
TCH/F TCH/H
BCCH(broadcast ch)
DCCH(dedicated ctrlch)
CCCH(commonctrlch)
DLDL UL slow fast
FCCH SCH PCH AGCH RACH SACCH
SDCCH FACCH
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Channelstructure
Traffic channels (our focus isinspeech service) Full rate:9.6kbps forspeech (1time slot/radioframe ) Half rate:4.8kbps forspeech (1time slot in
alternating radioframes) BCCH
Provides generalcell specific information MScan register tocell only if it can detect BCCH
From our perspective TCHandBCCHare most important.
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Channelstructure
FCCH=Frequency correction channel SCH=Synchronization channel PCH=Paging channel AGCH=Accessgrant channel RACH=Random access channel SACCH=Slow assosiated control channel SDCCH=Stand-alone dedicate control channel FACCH=Fast assosiated control channel
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Network planning issues inGSM
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Frequency reuse
Cluster sizes up toK=12or even more are used insome extreme cases. Due toincreasing traffic cluster sizes tend todecrease = >
replanning andoptimization ofpresent networks isongo ingactivity
Cluster size K=1isnot used due tohigh co-channelinterference
BCCHandTCHmay have different cluster sizes BCCHiscrucial forconnection =>larger cluster sizes
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Frequency reuse Frequencyreuserate
Measureforeffectivenessoffrequencyplan Trade-off:Effectivenessvs interference
Multiplere-useratesincreaseeffectivenessoffre quencyplan Compromisebetweensafe,interferencefreeplanning and
effectiveresourceusage1 3 6 9 12 15 18 21
safeplanning(BCCHlayer)
normalplanning(TCHmacrolayer)
tightre-useplanningsamefrequencyineverycell
Cluster size
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TRX
Atransmission/receptionfacilityiscalledasatr ansceiver(TRX). Typically,acellhasseveralTRXs,andonefrequen cyisallocatedto
eachTRX. Thecapacityofacellcanthereforebemeasuredin thenumberofTRXs.
Atimeslotthatcarriesusertrafficisalsocalle dasTrafficChannel(TCH).
Foruser speech service 1time slot/radioframe isreser ved inTCH/Fand1time slot inalternating radioframes inTCH/H
Foradministrativepurposes,e.g.providingmobile terminalswithcontrolinformation,thereisalsooneBCCHineach cell.
Thischannelconsumesonetimeslot(slot#0),and isoperatedbyoneTRXineachcell.
InphysicalBCCHthereareatleastFCCHandSCH+ basicsystemrelatedinformation
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SINRandTXpowers
According toGSMspecifications C/I>9dBfornominal performance. C/I=carrier tointerference power ratio Here interfere nce term
contains also AWGN. MStransmissionpower
InGSM850/900TXpowerisbetween0.8Wand8W.Usua lvalueis2W=33dBm
InGSM1800/1900TXpowerisbetween0.25Wand4W BStransmissionpower
InGSM-900and1800TXnormal BTS(=macro BTS)power isusually between 5Wand40W=46dBm.
Outputpowers inmicro BTSs are between 14dBmand32dBm FormacroBTSpowerismeasuredincombinerinput FormicroBTSpowerismeasuredinantennaconnecto r
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Example
Assume that Operator has 5MHzavailable forGSM Operator makes theTCHfrequency plan according to
specifications (SINR>9dB)but add 6dBmarginal forBCCHSINR.
Propagation exponent is4,system isinterferencelimited
Problems What are thecluster sizes forBCCHandTCH? How many TRXs are needed What isthenumber ofTCH/FandTCH/Hspeech
channels percell?
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Solution
5MHz/200kHz=25subcarriers,8time slots ineach =>200time slots intotal
SINRforTCH=9dB=> =7.943 SINRforBCCH=15dB=> =7.943 Propagationexponent =4=>C( )~7 Interference limited system =>we can useequation
After solving KforTCHwe find that K=2.4855 Similarly we find forBCCHthat K=4.959
=7.943 (TCH)
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Solution
Closest cluster sizes from ForTCHcluster size =3 ForBCCHcluster size =7
One TRXcan handle 1subcarrier (200kHz) We need 1BCCH-TRX/cell K=7forBCCH=>7subcarriers needed forBCCH There are 18subcarriers left forTCH.Since K=3forTCH we need 6
TRX/cell inaddition totheone that iscarrying BCCH Intotal we need 7TRX/cell
Number ofspeech channels TCHTRXs:7x8speech channels forfull rate and7x16f orhalf rate BCCH-TRX:7speech channels forfull rate and14speech channels
forhalf rate Intotal there will be 63speech channels forfull rate and 126forhalf
rate.
This isabout themaximum capacity configuration forthe operator
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Sectorizing andrange
Three-sector sites are most commoninGSM Sectorization gives antenna gain inBS
Maximum cell size isround 35km Cell size depends onthetiming advance which is232.47 s
inbasic GSMsystem Usually cell sizes are between few hundreds meters tofew
kilometers.Very large cells may occur e.g.insea coastswhere bunch ofislands are covered by asingleBS.
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Cell categories
Commonlyusedcellcategories Macrocell=cellwherethebasestationantennais
installedonamastorabuildingaboveaverageroo ftoplevel.Macrocellsarecommoninalloutdoorenvironments
Microcell=cell whereantennaisplacedunderrooftoplevel.Microcellsareusedinurbanareastoc overfewblocksofbuildings
Picocell=cell thatadmitcoverageofsometensofmeters(coveragecanvarydependingontheenvironment).Usedindoors.
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Indoor coverage
Usually cheapest andmost commonway toprovide indoor co verageistouse outdoor macro base stations forthis purpose
=outdoor toindoor coverage. Indoor users onthecell edge usually define thecell size since
penetration loss due tobuiding walls can be tens ofdecibe ls (usualvalue forpenetration loss is20dB)
Indoorcoveragemaybeprovidedalsobyindoorpico basestations. Suitable inoffice buildings;each floor can be covered by few pico
base stations Thelarge number ofbase stations may increase thenetwor k costs
Radiorepeaters andRFheads provide another solution Inrepeater distributed antennas (e.g.antenna/floor) are fed through
power splitters Repeater system applies anoutdoor antenna that receives t he
outdoor BSsignal after which thesystem repeates thesig nal fromindoor antennas.
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Frequencyhopping(FH)inGSM FHmeansthataTRXshiftsfrequencyateverynew
radioframe =>hoppingisperformedapproximately1/(4.615*0.00 1)=
217timespersecond. Foraspecificuser,anewfrequencyisusedfromo netime
slottothenext. TheadvantagesofFHaretwofold
Frequencydiversity Interferencediversity.
ThedrawbackwithFHisthatthenetworkcomponentsbecomemorecomplexandexpensive.
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FHdiversity gain
Transmittedsignalissubjecttofastfadingduetomultipathnatureofthepropagation.
Especially,ifthereceiverismoving,theuserwil lexperiencefadingdipsfrequently,andthedistancebetweenthefadingdipsdependsonthefrequencyus ed.
Changingfrequencycontinuouslyisonewaytoreduc etheinfluenceoffadingdipsandtheprobabilityof goodlinkqualityisincreased.TheadvantageofusingF Histhatthechannelwillusuallynotsufferseverefad ingdipsunderlongertimeperiods.
Thenumberofhoppingfrequenciesisanimportantfactorthataffectsthefrequencydiversitygain.H ighergainisachievedwithmorehoppingfrequencies,butmorethan8hoppingfrequenciesgivelessimproveme nt
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FHdiversity gain
0 1 2 3 4 5 6 7
0 1 2
Channelpower response isgood Channelpower response isbad
Channelbits from consecutive slots are mixed (interleavi ng inTXunitanddeinterleaving inRXunit)=>fading isaveraged a nddetection hasbetter probability tobe successfull
Frequency 1
Frequency 2after ahop
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FHinterference diversity gain WithoutFH,neighboringcellswithco-channelfrequ enciestothe
carrierfrequencywillinterferecontinuously. Theinterferencecanbesevereforcertainfrequenc iesandmaylead
toinformationloss. FHmeansthatdifferentfrequencieswillinterfere withthecarrierat
differenttimeslots. Insteadofhavingaconstantlevelofinterference betweentwocells,
theinterferenceisspreadamongseveralfrequencie s. Foraspecificcarrier,onlysometimeslotswills ufferfromhigh
interference.Fromasystempointofview,strongi nterferersaresharedbetweenusersandsocalledinterferenceave ragingisachieved.
AGSMnetworkcanbeplannedforaverageinterferen ceinsteadofworstcaseinterference .
Theinterferencediversitygainisdependentonthe numberofhoppingfrequenciesandinterferencelevelsfromtheinterf eringcells.Asignificantinterferencediversitygainisobtained withaslittleas3hoppingfrequencies.
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FHStrategies CyclicFH
Incyclichopping,allinterferingcellsusethesa mehoppingsequence,andachannelwillbeaffectedbythesameinterfer ersallthetime.Nointerferencediversityisobtainedwithcyclichopp ing.
RandomFH Randomhoppingusessequencesthatarepseudo-rando mly
generated=>interferencediversityisobtained. ThephysicalwaystoperformFHinacellarebaseb andhoppingand
synthesizedhopping. Inbasebandhopping,everyTRXisassignedtoafix edfrequency,
whilethechannelisshiftedbetweentheTRXs.The numberoffrequencycarriersmustbeequaltothenumberofT RXs.
Insynthesizedhopping,channelstaysatacertain TRX,whiletheTRXshiftsamongtheavailablefrequencies.Inthelatt ermethod,thenumberofcarriersisgreaterthanorequaltothe numberofTRXs.
Sincethenumberoffrequenciescanbemorethanth enumberoftransceiversinacellwhensynthesizedhoppingis used,thereusefactorcanbechangedwhilethenumberofTRXs isfi xed.
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FHStrategies
Therearetwostrategiesforusingthefrequencyba ndfortheTCHandBCCHcarriers Incommonbandstrategy bothTCHandBCCHcarriersuse
theentireband,andaTCHcanreceiveinterference frombothTCHandBCCHcarriers.
Indedicatedbandstrategy thefrequencybandissplitintotwoparts,onefortheTCHfrequenciesandonefor theBCCHfrequencies.InthisstrategyaTCHcarrierca nonlybeinterferedbyanotherTCH.FHisnotperformeda ttheTRXs thatoperatetheBCCH.
Simulationshaveshownthatdedicatedspectrumband sgivelessinterferencethancommonbandstrategywh ichsufferedfromseveredisturbanceinthedownlink.I tiseasiertoputextraTCH-TRXinanexistingcellwhendedic atedbandsareusedsincetheBCCHfrequencyplandoesn othavetobechanged.
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Frequency assignment problem
Classicalfrequencyassignmentproblem(FAP)belong stotheclassof NP-completeproblems,whichmeansthattheproblemprobablycannotbesolvedinpolynomia ltime.
ThereexisttwomajorapproachestodealwiththeF APinwirelessnetworks FixedChannelAssignment(FCA). InFCA,achannelis
assignedtoaconnectionbeforehandandcannotbechangedon-line.
DynamicChannelAssignment(DCA). InDCAthechannelsarechangedon-linewhenevertheradioconnectionsuffersfrominterferenceandthequalit yrequirementsarenotfulfilled.
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FCAvs DCA
FixedChannelAllocation(FCA) Thisisconventionalapproachwhereeachcellisal locatedtoa
predeterminedsetofchannels.Channelsareallocat edaccordingtofrequencyplan
Radioresources(channels)cantbetransferredbet weencellsandnetworkplanningisusuallydoneonworstcase basis=>trunking losssincetrafficloadchanges.
DynamicChannelAllocation(DCA) Channelsarenotallocatedpermanentlytodifferent cellsbut
BSallocateschannelsdynamicallytocomingcalls. DCAappliesalgorithmthattakeintoaccounte.g.
Likelihoodofblocking,co-channelinterference,ot hercostfunctions
Inextremecaseallchannelsareavailableineach cellandDCAeliminatestheneedforfrequencyplanning
Radioresourcereuseischangingdynamically=>hig hertrunking efficiency
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Frequency assignment problem inFCA
ThereexistseveralversionsofFCAapproach MinimumspanFAP .IntheminimumspanFAP,certainsoft
and/orhardrestrictionsaregivenforthequality ofthenetwork.Theserestrictionscanbeinterferenceandseparati onrequirements.Thesoftrestrictionscanbeviolated atsomepenaltycost,butthehardrestrictionsmustbemet .Theobjectiveistominimizethedifferencebetweenthe minimumandmaximumfrequenciesassigned,thespan.
MinimumorderFAP .Insteadofminimizingthespanoffrequencies,thenumberoffrequenciesisminimized inminimumorderFAP.
MinimumblockingFAP .TheminimumblockingandminimuminterferenceFAP(seebelow)useafixedspanoffr equencies.IntheminimumblockingFAP,thegoalistoassign frequenciesinsuchawaythattheoverallblockingprobability ofthenetworkisminimized.
MinimuminterferenceFAP .Theobjectiveistominimizethetotalsumofinterferenceinthenetwork.
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Frequency assignment problem inDCA
All or almost all frequencies are available ineach cell Very generaloptimization problem faced Like inFCArelated FPAdifferent approaches can be
adopted (starting from blocking,interference or othermeasures)
Theinformation change between base stations isslow=>fast interference mitigation isdifficult
Slow interference avoidance methods can be designedbased oninterference matrices.
Different companies have their own solutions
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Simplified link budgets
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Backgroundinformation
RXsensitivity( 3GPPTS05.05V8.20.0(2005-11)) GSM900MS:
forGSM 900smallMS -102 dBm GSM900BTS
fornormalBTS-104 dBm formicroBTSfrom-87downto-97 dBm dependingont heBTS
class Forpico BTS-88dBm
DCS1 800MS forDCS 1 800MS-102 dBm (forsomeclassesalso-100d Bm)
DCS1 800BTS fornormalBTS-104 dBm formicroBTSfrom-92dBmdownto-102 dBm depending onthe
class forpico BTS-95dBm
Note:RXsensitivityisthethe minimumreceivedpow erforwhichthepredefinedSNRrequirementisachieved.
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Recall:SINRandTXpowers
According toGSMspecifications C/I>9dBfornominal performance. C/I=carrier tointerference power ratio Here interfere nce term
contains also AWGN. MStransmissionpower
InGSM850/900TXpowerisbetween0.8Wand8W.Usua lvalueis2W=33dBm
InGSM1800/1900TXpowerisbetween0.25Wand4W BStransmissionpower
InGSM-900and1800TXnormal BTS(=macro BTS)power isusually between 5Wand40W=46dBm.
Outputpowers inmicro BTSs are between 14dBmand32dBm FormacroBTSpowerismeasuredincombinerinput FormicroBTSpowerismeasuredinantennaconnecto r
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Simple ULlink budget:Illustrative example
Pathloss=152dBFeeder/cableloss=4dB
TXPower33dBm(2W)
Antennagain=16dBi
Diversitygain(2antennas)=3dB
33dBm
- 116dBm- 100dBm
- 104dBm
RXSensitivity-104dB Antenna gain isadesignissue,usually
16-18dBiwhen 3sectors (cells)insite.RXsensitivity isalso designissue butphysics andcosts put limits toit.Neither interference nor shadow fadingistaken intoaccount here
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Simple DLlink budget:Illustrative example
Pathloss=152dB
Combinerloss=5dB
Feeder/cableloss=4dB
RXSensitivity- 102dBm
TXPower43dBm(20W)
Antennagain=16dBi(nodiversity)
- 102dBm
50dBm34dBm
38dBm
Recall:Combiner isadevice thatcombines feedsfromseveralTRXs sothattheycouldbesentoutthroughasingleantenna
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Cell range
Afterthemaximumallowablepathlosshasbeendetermined,thecellsizecanbeevaluated
Determinationisdonebyusingbasicpropagationpredictionformulas Okumura-Hata Walfish-Ikegami
Wetypicallywanttohave90%locationprobabilityoverthecellarea=>shadowfadingmarginneedstobeaddedaccordingly.
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Linkbudgettestexample
WehavemadeasimpleexceltoolforGSMlinkbudgetcomputation
Inthefollowingwegothroughanexample. Itissuggestedthatallparticipantsplaywiththetool(itcanbeloadedfromcoursepages) Learnhowdifferentparametersarecomputed Examinetheimpactofparameteres likeantennahight
etc
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Linkbudgetexample/TXcharacteristics
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Linkbudgetexample/RXcharacteristics
Downlink unit Uplink unit
Systemgaindefinestheattenuationthatsystemcan tolerate
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Linkbudgetexample/margins
Downlink unit Uplink unit
0 0.5 1 1.5 2 2.5 3 3.5 410-5
10-4
10-3
10-2
10-1
100
x
Q
(
x
)
Note:thisissimplifiedapproach.Usuallyweneedtocomputeshadowfadingmarginbasedonpredefinedcellcoverageprobability
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AllowedpropagationlossAllowedpropagationloss=Systemgain- margins
Allowedpropagationlossdefineshowmuchsystemca nstandpropagationloss.Hence,wecannextcalculatethe cellrange
ThereisslightimbalancebetweenULandDL
Okumura-Hata modelhasbeenused
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Exampleexercise
AssumeGSMsystemsuchthat TXpowerinBSis10Wand1WinMS ForBSantenna
Horizontal3dBbeamwidthis60degrees Fourdipolesareusedtoformverticalantennabeam
MSantennagainis0dBi Cablelossis3dB,combinerlossis3.5dB Bodylossis2.5dB
ComputetransmitterEIRPinbothBSandMS BSantennagainis16dBiandTXpoweris40dBm(fin dout
details).ThenBSEIRP=40dBm+16dBi-3dB-3.5dB=49 .5dBm MSTXpoweris30dBmandEIRP=30dBm+0dBi-2.5dB=
27.5dBm Itisusefultospendsometimewithlinkbudget!
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Howtofinddetailedtechnicalinformation
GSM(andGPRS,EDGE,WCDMA,LTE)specificationsareavailableon3GPPpages 3GPP=3rdGenerationPartnershipProject Theoriginalscopeof3GPPwastoproduceglobally
applicableTechnicalSpecificationsandTechnicalReportsfora3rdGenerationMobileSystembasedonevolvedGSMcorenetworksandtheradioaccesstechnologiesthattheysupport(e.g.WCDMA,HSPA,etc).
ThescopewassubsequentlyamendedtoincludethemaintenanceanddevelopmentoftheGSMTechnicalSpecificationsandTechnicalReportsincludingevol vedradioaccesstechnologies(e.g.GPRSandEDGE).
Specificationsdefinepreciselythefunctionalities andrequirementsofMS,BSothernetworkelementsof3GPPsystemslikeGSM, WCDMA,
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Standardization Inadditiontoreadyspecifications3GPPdatabases contain
technicaldiscussionconcerningtoongoingstandard izationofvarioustopics
Relateddocumentsarenoteasyreading buttheyco ntainlatesttechnicalideas.
CompanieslikeNokia,NSN,Ericsson,Motorola,Sams ung,Huaweiandmanyothersproposecompetingsolutions(about whichtheyhavepatentapplications)instandardizationdocuments( calledcontributes).
Oncestandardreleaseisreadycompaniesnegotiate ontheirshareofso-calledessentialpatents (patentsrelatedtost andardizedsolutions).
Thesharesofpatentsgiveguidelinesforcross-lic ensingbetweencompanies.Licensingcontractsdefinehowmuchmanu facturerpayorreceivefeeswhenusingthestandardizedtechnology initsproducts.
Somecompanies(thosewhoarenotdoingstandardiza tionresearch/arenotsuccessfulinit)mayneedtopay tensorevenhundredsofmillionsofeuros peryearinordertoh avearighttousecertaintechnologiesinproductstheymake.
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Standardizationdocuments
3GPPdatabasesaremostlypublic Easieswaytofindtechnicaldocuments
Google3GPP andgoto3GPPmainpage ClickTechnicalbodies
YoufindTSG(TechnicalSpecificationGroup)organization
UnderTSGGERAN youfindGSManditsevolution(GPRS,EDGE)radiospecifications
UnderTSGRAN youfindWCDMA,(+HSDPA,HSUPA,LTE)radiospecifications
Also,if youknownumberofthespecification/standardizationdocumentyoucangoogle itdirectly