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Introduc)ontoRadioResource
Management
1-Introduc)onToRRM
2-CDMASYSTEMS
Radio ResourceManagement
Mohammed Elmusrati University ofBenghazi
1
PART I: INTRODUCTION
CourseContents
Introduc-ontoRRM CDMAmodula-on/demodula-on WirelessChannelAnalysis OFDM PerformanceMeasureandDiversityTechniques PowerControl:Theore-calAnalysis PowerControlinUMTS AdmissionandLoadControl:Theore-calandPrac-cal JointRadioResources SmartAntennas. Packetscheduling:HSDPAandLTEsystems
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CourseReference
H.KoivoandM.Elmusra-,SystemEngineeringinWirelessCommunica4on,Wiley2009,ISBN:
9780470021781
Papersfromliterature.
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TheEvalua-on
Thefinalgradeisbasedon:Quizzes:25%Midterm:25%FinalExam:50%
Op-onalBonus:Maximumof10pointscanbeearnedbySimula-onproject.
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Introduc-on Beforeyoustartanyprojectyouneedtotakecarebyatleastthefollowingitems
Yourgoalobjec-vesfromthisprojectshouldbeclear.Yourcurrentandexpectedresourcestoachievethe
objec-ves(money,computers,people,bandwidth,strength,etc.)shouldbepreciselydetermined.
Planstousetheseresourcesandtoassesstheprojectstagesshouldbeclarified.
Thewirelesscommunica-onisnotadis-nc-onfromtheaboverules.
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Introduc-on(2)
Wehavecertainobjec-vesforeachwirelesscommunica-onproject.
ForexampleifourprojectistocreateFMradiochanneltocovercertaingeographicalareathenweneedtoconsidermanythingssuchas:
Therequiredmoneyforfirstcostsandrunningcosts. Takepermissionforcertainfrequencybandwidth. Es-matethetotalincomefromthisproject(thisincomeisnotneeded
tobemoneybutcanbealsodeploymentofcertainideasorideologies).
Therearealsotechnicalissuesshouldbetakenintoconsidera-onsuchasthetransmiedpowerwhichshouldbejusttherequiredpowertocoverthetargetedarea.Otherwisewemaymakeinterferencetootherco/crosschannelusers.
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Introduc-on(3) Fromtechnicalpointofviewtheradioresourcemanagement
ofradioorTVbroadcas-ngissimplerthanthemanagement
ofmul-transceiversystemssuchasmobilephonenetworks.
Somereasonsare: Insinglebroadcas-ngsystemwehaveonlyonetransmierandmany
receivers.Sothatthetransmierneedsonlytodeterminethe
reasonabletransmingpower,carrierfrequencyandbandwidth(with
permissionfromtheauthori-es).
Incellularphonesystemseveryterminalisatransceiver(transmierandreceiver)whichmeansthatwehavemanytransmiersandreceivers.Thismassivecommunica-onsystemshouldbeachievedina
specifiedlimitedbandwidth.Theradioresourcemanagementisvery
cri-calissueinthiscase.
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Introduc-on(4)
Theimportanceandchallengesofop-mumradioresourcemanagement(RRM)increaseswiththenumberofservicesintroducedbythewirelesscommunica-onsystem.
GenerallyspeakingtheRRMincludingthetransmingpoweralloca-on(orcontrol),thebandwidthalloca-on(ordataratecontrol),admissioncontrolandthehandoveropera-ons,andthe-meoftransmissionfornonreal-meapplica-ons(scheduling).
TherearemanyotherspecifictasksfortheRRMbutwewillnotgothroughthembecausetheyaremostlyrelatedtothesignallingplan.TheyhavebeencoveredinMobileCommunica-oncourse.
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Introduc-on(5) InthiscoursewewillconcentrateoncellularsystemsusingCDMAscheme(suchasUMTS)becauseofthefollowingthreereasonsTheperformanceandcapacityofGSMnetworksare
mainlyspecifiedbythenetworkplanning,whereastheperformanceandcapacityofUMTS(CDMA)systemaremainlyspecifiedbytheRRMalgorithms.
Themoderncellularsystems(e.g.,UMTS)aswellasmanyotherrecentcommunica-onsystems(802.11b,Zigbee,..)useCDMAschemeforitssuperiorityover
otherclassicalschemes.Itshouldbenotedthatitcanbealsousedashybridwithotherschemessuchas(CDMA/TDMA)
Similarconceptscouldbeappliedalsoforothermodernmodula-onandmul-pleaccessschemesuchastheOFDM.
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GeneralView
Theimportanceoftheefficientuseoftheavailableradioresourcessuchasthespectrum,thebasesta-ons,etc.isvery
cri-calformanyreasons.
Somereasonsare Therapidincreaseinthewirelessnetworksizes Thenewserviceswhichcanbeachievedbynewmobilenetworks
suchasMMS,GPRS,highspeedinternetexploring(e.g.,overHSPA
+,LTE),videoconferencing,mobilelocaliza-on,...etc.
Ratherlimitedavailablebandwidth,mostofthespectrumisalreadyoccupiedbymanyotherservicessuchasbroadcas-ng,GPS,military
applica-ons,naviga-on,satellitecommunica-on,..etc.
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GeneralView(2):ProblemFormula-on
AssumethatthereareQMobilephonesinaspecificareacontainsNbasesta-ons.
AssumethattherearethetotalofMavailablechannelsforconcurrentuseineachbasesta-onsuchasDifferentcarrierfrequencies(FDMA)Mul-plexed-meusage(TDMA)
Different(almost)orthogonalcodes(CDMA)Spa-alDivisionMul-pleAccess(SDMA)
Hybridbetweenanyoftheaboveschemes.
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GeneralView(3):ProblemFormula-on
Thetransmiedpowerfromeachmobile(uplink)isbetweenzero(mobileoff)toPu_max
Thetotaltransmiedpowerofthebasesta-on(downlink)isboundedbyPd_max
Themobilesta-oncantransmitatBdifferentdatarates{Rm1,Rm2,,RmB}
Thebasesta-oncantransmitatCdifferentdatarates{Rb1,Rb2,,RbC}
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GeneralView(3):ProblemFormula-on
NowtheRRMproblemcanbeformulatedasfollows:1. Howtodistributetheavailablechannelsbetweencells(This
distribu-oncanbesta-cordynamic)?
2. Whatistheproperbasesta-onforeachmobileataspecific-me,giventhattheassignedbasesta-oncanbechanged
duringlivecall(handover)?
3. Whatistheop-mumuplinktransmiedpowerfromeachmobileinordertoachievethetargetQoSwithoutlargeinterferencetoothers?
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GeneralView(4):ProblemFormula-on
4. Whatistheop-mumdownlinktransmiedpowerfromeachbasesta-oninordertosa-sfyallserveduserswithout
largeinterferencetoothercells?
5. Whatistheop-mumuplinktransmiedratefromeachmobileinordertoachievefastuploadingortransmissionbutwithouthighlybotheringotherscellsmates?
6. Whatistheop-mumdownlinktransmiedratefromeachbasesta-ontoeachserveduserinordertoachievehigh
totalcapacity,fairness,andgoodQoS?
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GeneralView(5):ProblemFormula-on
Therearemanyconstraintsshouldbetakenintoconsidera-onduringthealloca-onofthe
previous6itemssuchastheminimum
requiredQoSlevelforeachuser.
ForgoodRRMalgorithmsitisratherimportanttosa-sfyusersaswellasoperators.
Inthiscoursewewillshowhowtoaddressthepreviousitems.
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GeneralView(5):ProblemFormula-on
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?
P(t) ?R(t) ?
P(t) ?R(t) ?
P(t) ?R(t) ?
P(t) ?R(t) ?
r ?
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Mul-pleAccess(FDMA) InFDMA,thespectrumisdividedintomanysubbands.Each
subbandisassumedorthogonaltotheothersubbands,becausethereisnooverlappingbetweenthem.
Thisisnotcompletelytruebecauseasitiswellknownfromthebandwidthdilemmathatthereisnorealizablesignalwhichhasstrictlylimitedbandwidth.Sothatany-medura-onlimitedsignalhasunlimitedbandwidth.
Theoverlappingcanbeminimizedbylargesepara-onbetweenthesubbandswhichloosesthespectrumefficiency. Thisproblemhasbeenhandledbyusingtheimpressiveperformancefeaturesoforthogonalfrequencydivisionmul-plexing(OFDM)whichwillbediscussedindetailslater.
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Mul-pleAccess(TDMA)
InTDMA,eachterminalsendsinaveryshort-methenstopforawhiletogivetheopportunitytootherstosend.
Itusestheconceptofsamplingtheoremthatifyoudiscre-zeyourcon-nuoussignalsuchawaythatthe-medifferencebetweeneachsampleandthenextsampleislessthanor
equaltooneoverdoublethebandwidthofthesignal{i.e.t1/(2B)}thenthesignalcanberecoveredatthereceiverendcompletely(atleasttheore-cally).
Itexploitsalsothefactthatthehumaneyeandearcannotobservethediscre-za-onoflightorsoundiftheyarefastenough.
Fordatatransmissionthecon-nuityisnotnecessary.
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Mul-pleAccess(TDMA) Frombandwidthdilemma,strictlylimitedsignalin-medomainmeansunlimitedbandwidth,butsincethebandwidthislimitedthenthewidthaswellastheformofsignalpulseshouldbecarefullyselected.Sincethetransmissionisdoneoverlimitedbandwidth,thenthe-medura-onofthereceivedsignalwillbesmearedandthenwewillhavetheintersymbolinterference(ISI)problem.
Theaboveproblemlimitsthenumberoftransmierswhichcanconcurrentlyusethespectrum.
Toincreasetheefficiency,theTDMAcellularsystems(suchasGSM)useshybridmul-pleaccessmethodasFDMA/TDMAasdescribedinMobilecommunica-oncourse.
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Mul-pleAccess(CDMA)
Inthecodedivisionmul-pleaccessalltransmierstreattheirsignalsbeforetransmissiontogivethem
uniquecharacteris-cssothatitiseasytoextract
themattheintendedreceivers. InCDMAallsignalsoccupythebandwidthatthesame-me.
TheCDMAisthemostefficientmul-pleaccessmethodfromseveralperspec-ves.
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Mul-pleAccess(SDMA/PDMA) Thereareotherdifferentmul-pleaccessmethods.InSpace(orSpa-al)DivisionMul-pleAccessthetransmierscanusethesamefrequency,-me,andcodeifwecanseparatebetweenthemspa-ally.
Thesignalsarriveatthereceiverantennasatdifferentdirec-onsofarrival(DoA),canbeseparatedusingsmartantennas(atsomeperfectcondi-ons).
Itisalsopossibletoseparatebetweensignalsusedifferenttransmissionpolariza-ons.
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CSMA/CA
Thecarriersensemul-pleaccesswithcollusionavoidanceisveryusefulmul-ple
accesstechniqueswhenthereisno
communica-oninfrastructurewhichcanorganizethepreviousmul-pleaccess
methods.
Itiscommonlyusedinelas-corbesteffortnetworksaswellasadhocnetworks.
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Introduc-on
Ashasbeenmen-onedinfirstpartofthislecturethemobilecommunica-onsystemistwowaycommunica-on,i.e.,everymobilephoneisatransceiver(transmierreceiver)device.
Thebandwidthwhichisassignedformobilecommunica-onssystemsisratherrestricted.Forthisreasonverysophis-catedmethodsforthemul-pleuse(mul-pleaccess)oftheavailablebandwidthisnecessaryforefficientsystems.
Themul-pleaccessisthemethodofsharingtheavailableradioresources(suchasbandwidth)byallmobiles.
Whenthemobilesareseparatedgeographicallyenough,thereshouldbenoproblembecauseinthiscasetheinterferencebetweenthemcanbeneglected,sothatthemobilesmayreusethesameresources.
Theproblemariseswhenthemobilessharethesamegeographicalarea,inthiscasesomemul-pleaccessmethodmustbeusedsuchas(FDMA,TDMA,CDMA,SDMA)asdescribedinthefirstpart.
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Introduc-on TheCDMAu-lizesSpreadSpectrum(SS)techniques. ThemainideaoftheSpreadSpectrumtechniquesistospreadthe
originalsignalinfrequencydomainusinganothersignalwhichisindependenttotheinforma-onsignal.
Formanyreasons,manymoderncommunica-onsystemspreferCodeDivisionMul-pleAccess(CDMA)whichisoneoftheSSmethods.
SomeofthebenefitsoftheCDMAsystemsare:itisnotneededtodividethe-meresource(asin
TDMA)orthebandwidth(likeFDMA)betweenusers.Allusersuseallavailablebandallthe-mebutu-lizing(ideally)orthogonalspreadingcodes.
Itdoesnotneedveryaccurate-minglikeinTDMAnorfrequencyplanningasinFDMA.
Morebenefitsaregiveninthenextslide,buts-lltherearemanyothers!
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CDMA Ithassmoothcapacity,whichmeansthatthecapacitycanbeincreasedand
decreasedbasedontherequiredQualityofService(QoS)aswellasthenetworksitua-on.
Ithaswidebandwidthwhichofferfrequencydiversity,i.e.,ifpor-onofthebandwidthlost,thereceiverwills-llbeabletorecoverthesignalwithcertainprobabilityoferror.
Ithasan-jamcapability,becauseusuallythejammingsignalhasstrongpowerbutinfinitebandwidth.Sothatthejammingsignalwillaffectonlysmallpo-onofthetransmiedsignal.
TheSSsignalhassecuritynaturebecauseitisdifficulttodetectit.ThelevelandthenatureoftheSSsignalissimilartotheaddi-vewhitenoise.
SincetheSShaswidebandwidththenitwillbemuchlessaffectedbytheFastFadingnatureofthechannelaswillbedescribedinthenextLecture.
Thesignalcanbespreadanddespreadusingrela-velysimplecircuits.Ifweuse(1)binaryformatthenwecanspreadanddespreadthesignalsusingsimplemul-plierasshowninthenextslide.Ifweuse(0,1)binaryformatthenwecanspread/despreadthesignalsusingasimpleXORgate.
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CDMA
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CDMA
Initssimplestformat,theDirectSequenceCDMA(DSCDMA)canbeachievedusingsimpleXORlogicgateasshowninthenextslideincaseof(0,1).
TheideaistoapplyourdatabitstooneinputandapplythespreadingcodetotheotherinputoftheXORgate.
Thespreadingcodehasmuchhigherratethanthedataratetoachievethespreadingprinciple.Thechiprateofthespreadingcodedividedbythebitrateofthedataiscalledtheprocessinggainaswillbeexplainedlater.
FromtheXORtableshowninthenextslideweseethatwhenthedatainputis0thentheXORoutputwillbeiden-caltothespreadingcode.Andifitis1,theXORoutputwillbethecomplementofthespreadingcode.
Atthereceiver,ifweapplythereceivedsignaltooneinputandthesamespreadcodetotheotherinput,thenifthetwoinputsareiden-caltheoutputisZeroandiftheyaredifferentthentheoutputisOne,whichresultthesamedatabitsagain.
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CDMASystems
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A B C
0 0 00 1 1
1 0 1
1 1 0
A
B
CXOR
Same spreading code
CHANNEL
Tb
Tc
Usually fc>> fb
The original
signal recovered
If the binary signal
is NRZ (-1,+1) thenwe use sameconcept but with
normal multiplierinstead of the XOR
gates
XOR
PG=Processing Gain
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CDMASystems
30
f fInformation spectrum
The spectrum after spreading
Wireless Channel
The received spectrum
StrongjammingInterference
Additivewhite noise
The received spectrum after de-spreadingff
( )b f
( )c f
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CDMASystems TogainmaximumbenefitsfromSS,thespreadingcodesignalshouldhavesimilar
characteris-csasthewhitenoise,i.e.,uncorrelatedin-medomainwhichmeansithasflatpowerspectraldensityinfrequencydomain.
OnekindofthisspreadingcodefamilyiscalledPseudorandom(PN)Sequence. Formul-pleaccessapplica-ons,thedifferentusersshould(ideally)u-lize
orthogonalPNsequences(i.e.,thecrosscorrela-onbetweenthemiszero),toseparatethemeasilyatthereceiveraswillbeshownlater.
Prac-callyweuselowcorrelatedcodesratherthanorthogonalonesbecauseofatleastthreereasons:
Thenumberoforthogonalsequencesisasmallfrac-onnumberfromthesequencelengthwhichmeansthatfewnumberofuserscanbesupported.
Evenifweuseorthogonalsequencesatthetransmiers,theywillnotbenecessarilyorthogonalatthereceiverbecauseofthemul-pathcharacteris-csofthechannels.Soitisbeertorelaxtheorthogonalityconstrainttoincreasethenumberofsupportedusers.
Thereareothernonorthogonalcodeswhichhavebeerperformanceinmul-pathchannels(e.g.,Goldencodes).
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PNSequenceGenerator
Therearedifferentmethodstogenerateasequencewhichhassimilarproper-eslikenoise.
OnesimpleandefficientmethodusesshiregisterswithfeedbackstogeneratethePNsequence.OneexampleisshownintheFigure.
Theshiregistergeneratorsproducesequencesthatdependonthenumberofshiregisters,feedbacktapconnec-ons,andini-alcondi-ons.
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+1x 2x 3x 4x 5x y
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PNSequenceGenerator Theoutputsequencey(t)canbeclassifiedaseithermaximallengthornonmaximallength.
Maximallengthsequenceshavethepropertythatforannstagelinearfeedbackshiregisterthesequencerepe--onperiodinclockpulsespis(allzerocodeisnotincluded):
Onecharacteris-cofthePNsequenceisthatifwecomparethePNsequencewithanycyclicshiofitselfthenumberofagreementchipsanddisagreementchipsaredifferbyatmostonecount.
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2 1n
p =
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PNSequenceGenerator
Fromthelastcharacteris-cinthepreviousslideofthePNsequence,itiseasytoseethat
theautocorrela-onfunc-onofthePN
sequencehasthefollowingform
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1
p
2 1n
p =
Period in number of chips
( )R
cT
Chip duration
From the figure we
see that asp
increases and Tc
decreases we
approach to the
correlation
characteristic of the
white noise (delta
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PNSequenceGenerator Example:FindtheoutputsequencefromthefollowingPNsequencegenerator.
Assume1000istheini-alcontentsoftheshiregisters.Rememberthat(+)inbinaryrepresentsmodulo2adder(i.e.,XOR).
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1x
2x
3x
4x
y
+
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Solu-on
Aerfirstclock,ytakesthevalueofx4y=0.Andx1takesthevaluefromthefeedback,i.e.,x1=x3+x4=0+0=0.x2takesthepreviousvalueof
x1,i.e.,x2=x1.Finallyx3takesthepreviousvalueofx2andx4takesthe
previousvalueofx3.Werepeatthesameprocedureforthenextclock
andsoon.Thecontentsoftheshiregistersaereveryclockwillbe:
1000010000101001110001101011010110101101111011110111001100011000(sameasini-alstate,itmeansthatitwillrepeat
itself).
Sinceevery-mey=x4thenfromabovey=000100110101111
Itisclearthatyrepeatsitselfaer15digits,i.e.,maximallengthbecause
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42 1 16 1 15p = = =
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PNSequenceGenerator Example:PNsequencegeneratorwith23stageshiregisterhas
beenusedtogeneratethespreadingcodeforcertainCDMA
communica-onsystem.Thechiprateis10Mchip/s.Aerhowlong
-methecodewillrepeatitself.Sketchtheautocorrela-on
characteris-csofthegeneratedcode.Repeattheabove
calcula-onswith43stages
Solu-on
37
23
6
1243 12 5
6
8388607
2 1 8388607 chips 0.8410 10
8.7961 102 1 8.7961 10 chips 8.8 10
10 10
10
p
p
p
p T s
p T s
T days
= = =
= = =
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DSCDMATransmierandReceiver
ThefollowingFigureshowsimplifiedmodelforthetransmierandreceiverofDSCDMAsystems
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Digital
modulator
(e.g. BPSK)
( )i
x t
( )ic tDigital
Demodulator
( )ic t
differentmultipaths
with different
delays
( ),j ib t
other interferenceusers
NoiseReceiver fori
Transmitter #iInformation
( )1 b
b
j T
jT
+
jth bit
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t( )
s
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DSCDMATransmierandReceiver
Thetransmiedsignalfromusericanbeexpressed(withBPSK)as
( ) ( ){ }2
2i i
E x t c t =
39
( ) ( ) ( ) ( ) ( )cosi i i i cs t P t x t c t t=
Transmitted power
NRZ waveform of users data
Spreading code
Carrier frequency
( ) ( ),i i k b b
k
x t b h t kT= ( ) ( ),
i i k c c
k
c t c h t kT =
User data bits
Code sequence
chips
Assume
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bit shape
(e.g. rectangular)
DSCDMATransmierandReceiver
Thereceivedsignalattheinputofthereceiveri,inthisexpressionweassumedNuserssendtogetherandforsimplicityeachuserhasonlyasinglepath
Whereisthechannelgainbetweentransmierkandreceiveri.Itshouldbenotedthatthechannelgainisaffectedmainlybythedistance,lognormalshadowingandfadingaswillbedescribedinthenextlecture.
40
( ) ( ) ( ) ( ) ( ) ( ) ( )01, cos 2
N
i ki k k k k k k k
kr t g t P t x t c t f t n t
=
= + +
( )kig t
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DSCDMATransmierandReceiver
Aerthedemodula-onstage(basebandsignal)weobtainthefollowingsignal(weassumedthatthecoherentdetectorisperfect)
Thefirsttermintheaboveequa-onrepresentsthesignalofrequireduser,thesecondtermistheinterferencefromotherusers,andfinallythethirdtermistheaddi-vewhitenoise.
41
( ) ( ) ( ) ( ) ( )
( ) ( ) ( ) ( ) ( )1
,
i ii i i i i i i
N
ki k k k k k k
kk
r t g t P t x t c t
g t P t x t c t n t
=
=
+ +
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Inidealsitua-onthecorrela-onbetweendifferentPNsequencesshouldbezero,sothatthereceivercaneasilycancelouttheotherusers,butprac-cally
thisisnotpossible.
Ideallythespreadingcodesachievethefollowingcondi-on(Assumingergodicopera-on):
Tobemorerealis-cweuseinouranalysisthefollowingcondi-on
Whereiscalledtheorthogonalityfactorbetweenthereceivedcodeofuserianduserj.Alsoweassumedthatwecanes-matethedelay
perfectlyinthereceiver,sothatwewillignorethedelaydifferences.
CDMASYSTEMS
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ij0 1
E ci
t( ) cj t( )!" #$= ci t( )0
Tb
cj t( ) dt=1 i = j
ij i j
'
()
*)
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E ci
t( )cj t+( ) = ci t( )0
Tb
cj t+ ( )dt= 1 i= j
0 i j
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CDMAReceiver ThenoisepartisusuallyzeromeanGaussiandistribu-onrandomvariable.
Forsimplicityweassumethedatabitsandthecodechipshaverectangularformwithan-podalvalues(i.e.,1).
Thenoiseandthedatabitsandthecodesequenceareindependenttoeachother.
Ifthespreadingcodesareorthogonalatthereceiverthentheinterferenceterm(secondtermintheequa-onofslide42)becomeszeroaerthecorrela-onprocess.
Wewillanalyzetheoutputofthereceiverforonebitdura-on(Tb).Theresultcanbegeneralizedforalltheotherdatabits.
Withoutanylossingeneralitywewillassumethatwearereceivingthefirstdatabit(i.e.,from-me0to-meTb).ForjthbitwecanintegratefromjTbto-me(j+1)Tb.
InthenextslideweshowagainthebockdiagramofthesimplifiedCDMAreceiver.
43Radio ResourceManagement
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CDMAReceiver
44
( )i
c t
0z
0
bT
( )dy t
( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )1
N
d ii i i i i ki k k k i i
kk
y t g t P t x t c t c t g t P t x t c t c t n t c t=
= + +
Threshold
( )0, 1ib
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CDMAReceiver
45
z0= y
dt( )
0
Tb
dt= giiPib0,i ci t( )ci t( )0
Tb
dt+ gkiPkb0,k ck t( )ci t( )0
Tb
k=1
k
N
dt+ n t( )
Because the integration is taken over very small time (bit duration time), it is
more convenient to assume fixed channel gain and transmitted power.
From slide# 43
z0= g
iiP
ib0,i+ g
kiP
k
ikb0,k
k=1
k
N
+ n t( )
Desiredsignal
Interference noise
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CDMAReceiver
Thesecondandthirdtermsinpreviousequa-oncontributerandomvaluestothedesiredsignal.
Assumingfixedtransmiedpoweratoneandalsochannelgainatone,ifthetransmiedsignalisb0,i=+1,andatthat-methetotalinterferenceandnoisewasforexample1.5,thenz0=11.5=0.5.Thethresholdstagegives+1iftheinputislargerthan0,and1ifit
islessthan0.Inthiscasetheoutputwillbe1,whichmeanswrongdecision(error).Sothattheinterferenceandthenoisemakescertainbiterrorrate(BER).
TheBERdependsontheaveragepoweroftheinforma-onsignalaswellastheaveragepoweroftheinterferenceandnoisesignals.ItmeansthatitdependsontheSignaltoInterferenceandNoisera-o(SINR).
ToderivetheSINR,wecomputefirstthetotalreceivedpowerasexplainedinthenextslide.
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CDMAReceiver
47
E z0
2!"
#$= E giiPi b0,i + gkiPkikb0,k
k=1
k
N
+ n t( )'
(
)))
*
+
,,,
2!
"
----
#
$
.
.
.
.
= giiPE b
0,i
2!"
#$+E gkiPkikb0,k
k=1
k
N
'
(
)))
*
+
,,,
2!
"
----
#
$
.
.
.
.
+ E n2
t( )!" #$= giiPi + gkiPkikk=1
k
N
+n
2
Please note that the previous result is obtained considering that data bitsfrom different users are independent with zero mean each.
( )0, 0, 0,1
, 00
i k k
i kE b b E b E n t
i k
= = = =
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SignaltoNoiseRa-o
48
Previous slide shows the average total received power after dispreading,
where the first part is the interested signal power and the second and
third parts are the interference and additive noise average power
respectively.
The SINR can now be calculated at any time t as follows:
SINRi
t( ) = g
iit( )Pi t( )
ikg
kit( )Pk
k=1
k
N
t( )+n2
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CDMASystems
Thepreviousequa-oninthelastslideshowsonefigureforthequalityofthereceivedsignal.Thisfigureisverygoodforanalog
communica-onsystemsbutitisnotgoodmeasureforthequalityof
thedigitalcommunica-on.
Thereasonisthat,indigitalcommunica-on,onesymbolmayrepresent(2,4,8,)bits.ForexampleinBPSKeachsymbolrepresents
binaryinforma-on(1bit),whereas8PSK,eachsymbolrepresent3
bits.WithpreviousmeasuretheSINRwillbesamewhetherforBPSK
or8PSK,althoughtheBERforthesecondcaseisofcourseworst(for
sametransmissionandrecep-oncondi-ons).
Toovercomethisproblemitisbeertomeasurethereceivedpowerperbit,i.e.,thereceivedpowermul-pliedbythebitdura-onwhich
meansthebitenergy.
49Radio ResourceManagement
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CDMASystems
Tohavegoodandconsistentmeasureofthereceivedsignalqualitywedividetheenergyofthereceivedsignalbythespectraldensityofthe
interferenceandthenoise(itcanbeseenastheenergyofthe
interferenceandnoise).
Fromabove
50
( ) ( )( )0
bit energy average received signal power bit time duration
total noise and interfernce average powerNoise and interfernce spectral density
total bandwidth
b bE T
N
=
=
( )( ) ( )
( ) ( )
( ) ( )
( ) ( )2 201 1
1
ii i ii ib b c
N N
biik ki k n ik ki k n
k kk k
g t P t g t P tE T ft
N W fg t P t g t P t
= =
=
+ +
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CDMASystems
Asweexplainedbeforethetermiscalledtheprocessinggainwherefcisthechip
rateandfbisthebitrate.
ThetermEb/Noisdirectlyaffec-ngthebiterrorrateofthereceivedsignal,forexample
forBPSK,theBERisgivenby
WhereQfunc-onisgivenby51
c bf f
0
2 bE
BER Q N
=
( )221
2
u
x
Q x e du
= Radio ResourceManagement
Mohammed Elmusrati University ofBenghazi
Example
EachchanneloftheCDMAsystemIS-95occupies1.25MHzofthespectrumoneachone-waylink.Bandsof25MHzare
availableineachdirec4on.ThemaximumuserrateisR=9.6
kb/s.IfaminimumacceptableEb/N0is6dB,determinethe
capacityofaCDMAsystemintheuplinkifusing: a)Omni-direc4onalbasesta4onantennasand b)Three-sectoredantennasatthebasesta4onwithgain=2.4. ThereceivedsignalpowerfromallusersPis1011W,theone-
sidedAWGNpowerspectraldensityN0=1017W/Hz,andthe
other-cellrela4veinterferencefactorf=0.6.
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Solu-on
53
( ) ( )
( ) ( ) 201
ii ib c
N
biik ki k n
kk
g t P tE f
N fg t P t
=
=
+
For worst case calculation we assumed that ik=1, the above equation
can be rewritten as
( ) 2 20 1b c
b other ni
E f P
N f N P
= + +
Here we added new term which represents the interference comes
from other cells. It is given in the example that it is 60% from own cellinterference, i.e.,
2
other
( )2 0.6 1other P N =
From slide#49
( )
6 11
0.6
3 11 6 17
1.25 10 1010 20
9.6 10 1.6 1 10 1.25 10 10N
N
=
+
Received power
from user i
Received power
from other users
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Solu-on
Becausethesystemhas
channelsineachlink,thetotalcapacityisequalto
Forpartb,werepeatthesameprocedurewithgain2.4,i.e.,
Thenwecomputethecapacitypersector
54
6 625 10 1.25 10 20 =
Total Capacity 20 20 400= =
( ) 2 20
2.4
1
b c
b other ni
E f P
N f N P
= + +
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Exercise
EachchanneloftheWCDMAsystemoccupies5MHzofthespectrumoneachlink.Assume
thattheuserrate12.2kb/s.Theother
parametersarethesameasintheprevious
Example
FindthecapacityoftheWCDMAsystemunderthegivencondi4ons.
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CDMASystems
Itisclearthatasweincreasetheprocessinggain,asbeersignalequalityweget.Thiscanbeaccomplishedeitherbyincreasingthechiprate(increasingthespreadingbandwidthwhichislimited)ordecreasingthedatasymbolrate
(whichreducethedatarate!). Oneofthemaintasksoftheradioresourcemanagementistofindtheop-mumsolu-onfortheabovecontradic-ngproblem.
Un-lnowwetalkedaboutDSCDMA,therearetwomoretypesofSStechniques:FrequencyHoppingSpreadSpectrum(FHSS)TimeHoppingSpreadSpectrum(THSS)
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FH-CDMA Conven-onalfrequencyhopped(FH)CDMAisadigitalmul-pleaccess
systeminwhichindividualusersselectoneofQfrequencieswithina
widebandchannelascarrierfrequency.
Thepseudorandomchangesofthecarrierfrequenciesrandomizetheoccupancyofaspecificbandatanygiven-me,therebyallowingfor
mul-pleaccessoverawiderangeoffrequencies.
Inaconven-onalFHCDMAsystem,thetotalhoppingbandwidthWisdividedintoQnarrowbandseachofbandwidthB,whereB=W/Q.
EachoftheQbandsisdefinedasaspectralregionwithacentralfrequencycalledthecarrierfrequency.Thesetofpossiblecarrier
frequenciesiscalledthehopset.ThebandwidthBofabandusedina
hopsetiscalledtheinstantaneousbandwidth.
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FH-CDMA
ThebandwidthofthespectrumWoverwhichthehoppingoccursiscalledthetotalhoppingbandwidth.Informa-onissentbyhoppingthecarrierfrequencyaccordingtothepseudorandomlaw,whichisknowntothedesiredreceiver.
Ineachhop,asmallsetofcodesymbolsissentwithconven-onalnarrowbandmodula-onbeforethecarrierfrequencyhopsagain.
The-medura-onbetweenhopsiscalledhopdura4onorhoppingperiodandisdenotedbyTh.The-medura-onbetweentransmissionoftwoconsecu-vesymbolsisTb.
UsuallyinFHCDMAfrequencyshikeying(FSK)isused. Atthereceiverside,aerthefrequencyhoppinghasbeenremovedfrom
thereceivedsignal,theresul-ngsignalissaidtobedehopped.Beforedemodula-on,thedehoppedsignalisappliedtoaconven-onalreceiver.
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FH-CDMA Ifanotherusertransmitsinthesamebandatthesame-meinaFHCDMA
system,acollisioncanoccur.
Frequencyhoppingcanbeclassifiedassloworfast.Slowfrequencyhoppingoccursifoneormoreqarysymbolsaretransmiedinthe
intervalbetweenfrequencyhops.Thusslowfrequencyhoppingimplies
thatthesymbolrate1/Tbexceedsthehoppingrate1/Th.Fastfrequency
hoppingoccursifthereismorethanonefrequencyhopduringone
symboltransmission-me.
NextslideshowsimpleblockdiagramforFHCDMAsystem
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FH-CDMA
60
modulator
Frequencysynthesizer
PN sequencegenerator
Datainput
Mixer
Frequencysynthesizer
PN sequencegenerator
Dataoutput
LPF
Transmitter Receiver
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TimeHoppingCDMA Digitalradiotransmissionhastradi-onallybeenbasedontheconceptthat
thecarrierfrequencyismuchlargerthanthebandwidthofthetransmiedsignal.
Whentherequiredbandwidthisoftheorderof100MHzthisapproachencountersmanyobstacles.Typically,thetransmierwouldoperateatacarrierfrequencyabove10GHz,andthuswouldsufferfromabsorp-onbyrainandfog.
Adifferenttechniqueistheimpulseradio,mul-pleaccessmodulatedbyapulseposi-onhopping(PPH).Theimpulseradiotechniqueisalsodenotedultra-wide-bandtransmission.
Impulseradiocommunicateswithpulsesofveryshortdura-on,typicallyontheorderofananosecond,therebyspreadingtheenergyoftheradiosignalverythinlyuptoafewgigahertz.Itisapromisingtechniqueforshortrangeandindoorcommunica-on.
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TimeHoppingCDMA
Themainadvantagesofimpulseradioareasfollows.Inanimpulseradiosystem,thetransmiedsignalisaditheredpulsetrainwithouta
sinusoidalcarrierand,hence,carrierrecoveryatthereceiverisnot
required.
Aswemen-onedinthepreviousslide,inanultrawidebandsystem,suchasimpulseradio,fadingisnotnearlyasseriousaproblemasitisfornarrowbandsystems.
Impulseradiosystemscanoperateatvariablebitratesbychangingthenumberofpulsesusedtotransmitonebitofinforma-on.
WenotethatDSCDMAandFHCDMAusemorecomplexbitratevaria-ontechniques.
NextslideshowblockdiagramofsimpleTHCDMAsystem.
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TimeHoppingCDMA
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