RRM Lecture 1

8/22/2019 RRM Lecture 1

1/32

1

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



2


2/32

2

CourseReference

H.KoivoandM.Elmusra-,SystemEngineeringinWirelessCommunica4on,Wiley2009,ISBN:

9780470021781

Papersfromliterature.



3

TheEvalua-on

Thefinalgradeisbasedon:Quizzes:25%Midterm:25%FinalExam:50%

Op-onalBonus:Maximumof10pointscanbeearnedbySimula-onproject.



4


3/32

3

Introduc-on Beforeyoustartanyprojectyouneedtotakecarebyatleastthefollowingitems

Yourgoalobjec-vesfromthisprojectshouldbeclear.Yourcurrentandexpectedresourcestoachievethe

objec-ves(money,computers,people,bandwidth,strength,etc.)shouldbepreciselydetermined.

Planstousetheseresourcesandtoassesstheprojectstagesshouldbeclarified.

Thewirelesscommunica-onisnotadis-nc-onfromtheaboverules.



5

Introduc-on(2)

Wehavecertainobjec-vesforeachwirelesscommunica-onproject.

ForexampleifourprojectistocreateFMradiochanneltocovercertaingeographicalareathenweneedtoconsidermanythingssuchas:

Therequiredmoneyforfirstcostsandrunningcosts. Takepermissionforcertainfrequencybandwidth. Es-matethetotalincomefromthisproject(thisincomeisnotneeded

tobemoneybutcanbealsodeploymentofcertainideasorideologies).

Therearealsotechnicalissuesshouldbetakenintoconsidera-onsuchasthetransmiedpowerwhichshouldbejusttherequiredpowertocoverthetargetedarea.Otherwisewemaymakeinterferencetootherco/crosschannelusers.



6


4/32

4

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.



7

Introduc-on(4)

Theimportanceandchallengesofop-mumradioresourcemanagement(RRM)increaseswiththenumberofservicesintroducedbythewirelesscommunica-onsystem.

GenerallyspeakingtheRRMincludingthetransmingpoweralloca-on(orcontrol),thebandwidthalloca-on(ordataratecontrol),admissioncontrolandthehandoveropera-ons,andthe-meoftransmissionfornonreal-meapplica-ons(scheduling).

TherearemanyotherspecifictasksfortheRRMbutwewillnotgothroughthembecausetheyaremostlyrelatedtothesignallingplan.TheyhavebeencoveredinMobileCommunica-oncourse.



8


5/32

5

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.



9

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.



10


6/32

6

GeneralView(2):ProblemFormula-on

AssumethatthereareQMobilephonesinaspecificareacontainsNbasesta-ons.

AssumethattherearethetotalofMavailablechannelsforconcurrentuseineachbasesta-onsuchasDifferentcarrierfrequencies(FDMA)Mul-plexed-meusage(TDMA)

Different(almost)orthogonalcodes(CDMA)Spa-alDivisionMul-pleAccess(SDMA)

Hybridbetweenanyoftheaboveschemes.



11


Thetransmiedpowerfromeachmobile(uplink)isbetweenzero(mobileoff)toPu_max

Thetotaltransmiedpowerofthebasesta-on(downlink)isboundedbyPd_max

Themobilesta-oncantransmitatBdifferentdatarates{Rm1,Rm2,,RmB}

Thebasesta-oncantransmitatCdifferentdatarates{Rb1,Rb2,,RbC}



12


7/32

7


NowtheRRMproblemcanbeformulatedasfollows:1. Howtodistributetheavailablechannelsbetweencells(This

distribu-oncanbesta-cordynamic)?

2. Whatistheproperbasesta-onforeachmobileataspecific-me,giventhattheassignedbasesta-oncanbechanged

duringlivecall(handover)?

3. Whatistheop-mumuplinktransmiedpowerfromeachmobileinordertoachievethetargetQoSwithoutlargeinterferencetoothers?



13


4. Whatistheop-mumdownlinktransmiedpowerfromeachbasesta-oninordertosa-sfyallserveduserswithout

largeinterferencetoothercells?

5. Whatistheop-mumuplinktransmiedratefromeachmobileinordertoachievefastuploadingortransmissionbutwithouthighlybotheringotherscellsmates?

6. Whatistheop-mumdownlinktransmiedratefromeachbasesta-ontoeachserveduserinordertoachievehigh

totalcapacity,fairness,andgoodQoS?



14


8/32

8


Therearemanyconstraintsshouldbetakenintoconsidera-onduringthealloca-onofthe

previous6itemssuchastheminimum

requiredQoSlevelforeachuser.

ForgoodRRMalgorithmsitisratherimportanttosa-sfyusersaswellasoperators.

Inthiscoursewewillshowhowtoaddressthepreviousitems.



15




16

?

P(t) ?R(t) ?

P(t) ?R(t) ?

P(t) ?R(t) ?

P(t) ?R(t) ?

r ?


9/32

9

Mul-pleAccess(FDMA) InFDMA,thespectrumisdividedintomanysubbands.Each

subbandisassumedorthogonaltotheothersubbands,becausethereisnooverlappingbetweenthem.

Thisisnotcompletelytruebecauseasitiswellknownfromthebandwidthdilemmathatthereisnorealizablesignalwhichhasstrictlylimitedbandwidth.Sothatany-medura-onlimitedsignalhasunlimitedbandwidth.

Theoverlappingcanbeminimizedbylargesepara-onbetweenthesubbandswhichloosesthespectrumefficiency. Thisproblemhasbeenhandledbyusingtheimpressiveperformancefeaturesoforthogonalfrequencydivisionmul-plexing(OFDM)whichwillbediscussedindetailslater.



17

Mul-pleAccess(TDMA)

InTDMA,eachterminalsendsinaveryshort-methenstopforawhiletogivetheopportunitytootherstosend.

Itusestheconceptofsamplingtheoremthatifyoudiscre-zeyourcon-nuoussignalsuchawaythatthe-medifferencebetweeneachsampleandthenextsampleislessthanor

equaltooneoverdoublethebandwidthofthesignal{i.e.t1/(2B)}thenthesignalcanberecoveredatthereceiverendcompletely(atleasttheore-cally).

Itexploitsalsothefactthatthehumaneyeandearcannotobservethediscre-za-onoflightorsoundiftheyarefastenough.

Fordatatransmissionthecon-nuityisnotnecessary.



18


10/32

10

Mul-pleAccess(TDMA) Frombandwidthdilemma,strictlylimitedsignalin-medomainmeansunlimitedbandwidth,butsincethebandwidthislimitedthenthewidthaswellastheformofsignalpulseshouldbecarefullyselected.Sincethetransmissionisdoneoverlimitedbandwidth,thenthe-medura-onofthereceivedsignalwillbesmearedandthenwewillhavetheintersymbolinterference(ISI)problem.

Theaboveproblemlimitsthenumberoftransmierswhichcanconcurrentlyusethespectrum.

Toincreasetheefficiency,theTDMAcellularsystems(suchasGSM)useshybridmul-pleaccessmethodasFDMA/TDMAasdescribedinMobilecommunica-oncourse.



19

Mul-pleAccess(CDMA)

Inthecodedivisionmul-pleaccessalltransmierstreattheirsignalsbeforetransmissiontogivethem

uniquecharacteris-cssothatitiseasytoextract

themattheintendedreceivers. InCDMAallsignalsoccupythebandwidthatthesame-me.

TheCDMAisthemostefficientmul-pleaccessmethodfromseveralperspec-ves.



20


11/32

11

Mul-pleAccess(SDMA/PDMA) Thereareotherdifferentmul-pleaccessmethods.InSpace(orSpa-al)DivisionMul-pleAccessthetransmierscanusethesamefrequency,-me,andcodeifwecanseparatebetweenthemspa-ally.

Thesignalsarriveatthereceiverantennasatdifferentdirec-onsofarrival(DoA),canbeseparatedusingsmartantennas(atsomeperfectcondi-ons).

Itisalsopossibletoseparatebetweensignalsusedifferenttransmissionpolariza-ons.



21

CSMA/CA

Thecarriersensemul-pleaccesswithcollusionavoidanceisveryusefulmul-ple

accesstechniqueswhenthereisno

communica-oninfrastructurewhichcanorganizethepreviousmul-pleaccess

methods.

Itiscommonlyusedinelas-corbesteffortnetworksaswellasadhocnetworks.



22


12/32

12



23

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.

24Radio ResourceManagement



13/32

13

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!



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.




14/32

14

CDMA



CDMA

Initssimplestformat,theDirectSequenceCDMA(DSCDMA)canbeachievedusingsimpleXORlogicgateasshowninthenextslideincaseof(0,1).

TheideaistoapplyourdatabitstooneinputandapplythespreadingcodetotheotherinputoftheXORgate.

Thespreadingcodehasmuchhigherratethanthedataratetoachievethespreadingprinciple.Thechiprateofthespreadingcodedividedbythebitrateofthedataiscalledtheprocessinggainaswillbeexplainedlater.

FromtheXORtableshowninthenextslideweseethatwhenthedatainputis0thentheXORoutputwillbeiden-caltothespreadingcode.Andifitis1,theXORoutputwillbethecomplementofthespreadingcode.

Atthereceiver,ifweapplythereceivedsignaltooneinputandthesamespreadcodetotheotherinput,thenifthetwoinputsareiden-caltheoutputisZeroandiftheyaredifferentthentheoutputisOne,whichresultthesamedatabitsagain.




15/32

15

CDMASystems

29

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



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




16/32

16

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).



PNSequenceGenerator

Therearedifferentmethodstogenerateasequencewhichhassimilarproper-eslikenoise.

OnesimpleandefficientmethodusesshiregisterswithfeedbackstogeneratethePNsequence.OneexampleisshownintheFigure.

Theshiregistergeneratorsproducesequencesthatdependonthenumberofshiregisters,feedbacktapconnec-ons,andini-alcondi-ons.

32

+1x 2x 3x 4x 5x y




17/32

17

PNSequenceGenerator Theoutputsequencey(t)canbeclassifiedaseithermaximallengthornonmaximallength.

Maximallengthsequenceshavethepropertythatforannstagelinearfeedbackshiregisterthesequencerepe--onperiodinclockpulsespis(allzerocodeisnotincluded):

Onecharacteris-cofthePNsequenceisthatifwecomparethePNsequencewithanycyclicshiofitselfthenumberofagreementchipsanddisagreementchipsaredifferbyatmostonecount.

33

2 1n

p =



PNSequenceGenerator

Fromthelastcharacteris-cinthepreviousslideofthePNsequence,itiseasytoseethat

theautocorrela-onfunc-onofthePN

sequencehasthefollowingform

34

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

function)Radio ResourceManagement



18/32

18

PNSequenceGenerator Example:FindtheoutputsequencefromthefollowingPNsequencegenerator.

Assume1000istheini-alcontentsoftheshiregisters.Rememberthat(+)inbinaryrepresentsmodulo2adder(i.e.,XOR).

35

1x

2x

3x

4x

y

+



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

36

42 1 16 1 15p = = =




19/32

19

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

= = =

= = =



DSCDMATransmierandReceiver

ThefollowingFigureshowsimplifiedmodelforthetransmierandreceiverofDSCDMAsystems

38

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



ri

t( )

s

it( )


20/32

20


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



bit shape

(e.g. rectangular)


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




21/32

21


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

=

=

+ +



Inidealsitua-onthecorrela-onbetweendifferentPNsequencesshouldbezero,sothatthereceivercaneasilycancelouttheotherusers,butprac-cally

thisisnotpossible.

Ideallythespreadingcodesachievethefollowingcondi-on(Assumingergodicopera-on):

Tobemorerealis-cweuseinouranalysisthefollowingcondi-on

Whereiscalledtheorthogonalityfactorbetweenthereceivedcodeofuserianduserj.Alsoweassumedthatwecanes-matethedelay

perfectlyinthereceiver,sothatwewillignorethedelaydifferences.

CDMASYSTEMS

42

ij0 1

E ci

t( ) cj t( )!" #$= ci t( )0

Tb

cj t( ) dt=1 i = j

ij i j

'

()

*)



E ci

t( )cj t+( ) = ci t( )0

Tb

cj t+ ( )dt= 1 i= j

0 i j


22/32

22

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.



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




23/32

23

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



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.




24/32

24

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

= = = =



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




25/32

25

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.



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

= =

=

+ +




26/32

26

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


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.




27/32

27

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



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

= + +




28/32

28

Exercise

EachchanneloftheWCDMAsystemoccupies5MHzofthespectrumoneachlink.Assume

thattheuserrate12.2kb/s.Theother

parametersarethesameasintheprevious

Example

FindthecapacityoftheWCDMAsystemunderthegivencondi4ons.



CDMASystems

Itisclearthatasweincreasetheprocessinggain,asbeersignalequalityweget.Thiscanbeaccomplishedeitherbyincreasingthechiprate(increasingthespreadingbandwidthwhichislimited)ordecreasingthedatasymbolrate

(whichreducethedatarate!). Oneofthemaintasksoftheradioresourcemanagementistofindtheop-mumsolu-onfortheabovecontradic-ngproblem.

Un-lnowwetalkedaboutDSCDMA,therearetwomoretypesofSStechniques:FrequencyHoppingSpreadSpectrum(FHSS)TimeHoppingSpreadSpectrum(THSS)




29/32

29

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.



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.




30/32

30

FH-CDMA Ifanotherusertransmitsinthesamebandatthesame-meinaFHCDMA

system,acollisioncanoccur.

Frequencyhoppingcanbeclassifiedassloworfast.Slowfrequencyhoppingoccursifoneormoreqarysymbolsaretransmiedinthe

intervalbetweenfrequencyhops.Thusslowfrequencyhoppingimplies

thatthesymbolrate1/Tbexceedsthehoppingrate1/Th.Fastfrequency

hoppingoccursifthereismorethanonefrequencyhopduringone

symboltransmission-me.

NextslideshowsimpleblockdiagramforFHCDMAsystem



FH-CDMA

60

modulator

Frequencysynthesizer

PN sequencegenerator

Datainput

Mixer

Frequencysynthesizer

PN sequencegenerator

Dataoutput

LPF

Transmitter Receiver




31/32

31

TimeHoppingCDMA Digitalradiotransmissionhastradi-onallybeenbasedontheconceptthat

thecarrierfrequencyismuchlargerthanthebandwidthofthetransmiedsignal.

Whentherequiredbandwidthisoftheorderof100MHzthisapproachencountersmanyobstacles.Typically,thetransmierwouldoperateatacarrierfrequencyabove10GHz,andthuswouldsufferfromabsorp-onbyrainandfog.

Adifferenttechniqueistheimpulseradio,mul-pleaccessmodulatedbyapulseposi-onhopping(PPH).Theimpulseradiotechniqueisalsodenotedultra-wide-bandtransmission.

Impulseradiocommunicateswithpulsesofveryshortdura-on,typicallyontheorderofananosecond,therebyspreadingtheenergyoftheradiosignalverythinlyuptoafewgigahertz.Itisapromisingtechniqueforshortrangeandindoorcommunica-on.



TimeHoppingCDMA

Themainadvantagesofimpulseradioareasfollows.Inanimpulseradiosystem,thetransmiedsignalisaditheredpulsetrainwithouta

sinusoidalcarrierand,hence,carrierrecoveryatthereceiverisnot

required.

Aswemen-onedinthepreviousslide,inanultrawidebandsystem,suchasimpulseradio,fadingisnotnearlyasseriousaproblemasitisfornarrowbandsystems.

Impulseradiosystemscanoperateatvariablebitratesbychangingthenumberofpulsesusedtotransmitonebitofinforma-on.

WenotethatDSCDMAandFHCDMAusemorecomplexbitratevaria-ontechniques.

NextslideshowblockdiagramofsimpleTHCDMAsystem.




32/32

TimeHoppingCDMA



Documents

RRM Lecture 1