lecture 8 - university of pittsburgh school of …ncell breathing n the boundary of a cdma cell is...
TRANSCRIPT
![Page 1: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/1.jpg)
Lecture 8
SpreadSpectrumandOFDM
![Page 2: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/2.jpg)
+Time Domain View (Sieve)
2
Channel
DirectSequenceSpreadSpectrum
![Page 3: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/3.jpg)
+Spread Spectrum
n Usuallythespectrumofasignalisrelatedtothedata(symbol)raten Thenull-to-nullbandwidth@ 1/Tn T isthesymbolduration
n Spread-spectrumn Thespectrumismuchwiderthan1/Tn Thespreadingisachievedusinga“spreadingsignal”alsocalleda
“codesignal”or“spreadingcode”n Thereceiverusescorrelationormatchedfilteringtorecoverthe
originaldata
3
![Page 4: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/4.jpg)
+Types of Spread Spectrum
n Direct-sequencespreadspectrum(DSSS)n Eachinformationsymbolis“chipped”intoapatternofsmallersymbols
n Thepatterniscalledthespread-spectrum“code”or“sequence”n ItisusedinIS-95,W-CDMA,cdma2000andIEEE802.11
n Frequencyhoppingspreadspectrum(FHSS)n Symbolsorpacketsaretransmittedondifferentfrequencycarrierseachtime
n Slowfrequencyhopping– thesamefrequencycarrierisusedoverseveralsymbolsorapacket(common)
n Fastfrequencyhopping– thefrequencycarrierischangedwithinasymbolperiod
n UsedinGSM,IEEE802.11(legacy)andBluetooth
4
![Page 5: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/5.jpg)
+Systems using Spread SpectrumnDSSSisemployedin2GCDMAsystemsn IS-95,cdma2000
nDSSSisemployedinall3GcellularsystemsnUMTSandHSPA
nDSSSwasusedinlegacyIEEE802.11(WiFi)
5
![Page 6: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/6.jpg)
+DSSS Modulation
n Theoriginaldatastreamis“chipped”upintoapatternofpulsesofsmallerduration
n Goodautocorrelationproperties
n Goodcross-correlationpropertieswithotherpatterns
n Eachpatterniscalledaspreadspectrumcodeorspreadspectrumsequence
6
Data Bit
“Spread” Bits
chip
1 2 3 4 5 6 7 8 9 10 11
PeriodicSpreadingCode
DataIn
SpreadingCodeIn
![Page 7: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/7.jpg)
+DSSS details
n Insteadoftransmittingarectangularpulseforazerooraone,wetransmitasequenceofnarrowerrectangularpulses
n Thenarrowpulsesarecalled“chips”n Youoftenseereferencesto“chips/sec”insteadofbits/sec
n TheeasiestwayofcreatingaDSSSsignalistomultiplyoneperiodofthespreadingsequencewitheachdatasymboln Example:IEEE802.11
n Barkersequence:[111-1-1-11-1-11-1]n Totransmita“0”,yousend[111-1-1-11-1-11-1]n Totransmita“1”yousend[-1-1-1111-111-11]
n Sometimespartsofthespreadingsequencearemultipliedwiththedatasymbol
7
![Page 8: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/8.jpg)
+Processing gain
n Definitionofprocessinggainn ThedurationofachipisusuallyrepresentedbyTcn ThedurationofthebitisTn TheratioT/Tc =N iscalledthe“processinggain”oftheDSSSsystem
n Theprocessinggainisalsotheratiobetweenthebandwidthofthespreadsignaltothebandwidthofthedatasignal
n Inmanycases,thisisalsotheratiooftheheightoftheautocorrelationpeaktothemaximumsideloben Thisratiodependsonthespreadingcodeproperties
8
![Page 9: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/9.jpg)
+Operation of a DSSS Transceiver
9
Demodulationinvolvesaprocesscalled“correlation”
![Page 10: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/10.jpg)
+Spectrum and Autocorrelation
10
Original signal
Spread Signal
⌧
⌧
Autocorrelationof Rectangle
Autocorrelationof Barker-11
PSD
PSD
E
E
fc +1T
fc �1T
fc �1Tc
fc +1Tc
f
f
![Page 11: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/11.jpg)
+Autocorrelation properties of the Barker sequence
nThewidthofthemainlobeis2T/11n Aboutone-tenththewidthoftheautocorrelationoftherectangularpulse
nTheheightofthemainlobeis11timestheheightofthesidelobes
nTheratioofmainlobepeaktosidelobeisanimportantmeasureofhow“good”aspreadingcodeis
11
![Page 12: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/12.jpg)
+7- Chip M-sequence
12
PeriodicAutocorrelation
Data Bit
Spreading Code
[1 -1 -1 1 -1 1 1 ]
Tschip
Ts
time time
-1
7
Ts
time
(a) (b)
(c)
chiptime
![Page 13: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/13.jpg)
+Autocorrelation
n Considerthespreadingsequencen [1-1-11-111]
13
1 -1 -1 1 -1 1 1
1 -1 -1 1 -1 1 1 Result:1x-1+-1x1+-1x1=-3
Aperiodicautocorrelation
1 -1 -1 1 -1 1 1
1 -1 -1 1 -1 1 1 1 -1 -1 1 -1 1 1
Result:1x-1+-1x1+-1x1+1x1+-1x-1+1x-1+1x1=-1
Periodicautocorrelation
![Page 14: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/14.jpg)
+Example in a two-path channel
n Randomdatasequenceoftendatabitsn Spreadingby11chipsusingaBarkerpulse
n Twopathchannelwithinter-pathdelayof17chips>bitduration
n Multipathamplitudesn Mainpath:1n Secondpath:1.1
n Justforillustration!
n Reality:n Manymultipathcomponentsn Rayleighfadingamplitudesn Noise!
14
![Page 15: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/15.jpg)
+Data and Channel
15
10 20 30 40 50 60 70 80 90 100 110-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 0 0 0 0 0011 1
![Page 16: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/16.jpg)
+
0 20 40 60 80 100 120-15
-10
-5
0
5
10
15
20
25
Output without spreading16
0 20 40 60 80 100 120-15
-10
-5
0
5
10
15
Without Multipath With Multipath
Output of a Matched Filter
Errorsintroducedbythechannel
0 0 0 0 1 1 0 0 1 00 0 0 0 1 1 0 0 1 0
Signalaftercorrelationissampledatgreenlines
![Page 17: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/17.jpg)
+
0 20 40 60 80 100 120-15
-10
-5
0
5
10
15
Output with spreading17
Without Multipath With Multipath
Output of a Matched Filter
0 20 40 60 80 100 120-15
-10
-5
0
5
10
15
Errorsintroducedbythechannelareremoved
0 0 0 0 0 0011 1 0 0 0 0 0 0011 1
![Page 18: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/18.jpg)
+Summary of DSSS and Combatting Multipath
18
Data Bit
Ts
chip
Ts
time
time
Intersymbol Interference
Symbol 1 Symbol 2
Reduced Intersymbol Interference& In-band Diversity
Traditional Transmission
DSSS Transmission
Channel
![Page 19: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/19.jpg)
+The RAKE receiver
n ObservethepeaksinthechanneloutputinthepreviousslidesthatareNOTsampled(Peaksthatarenotatthegreenverticalline)n Theycontainthe“same”informationasthesampledpeaks– butthese
peaksaredelayed!
n ARAKEreceiverconsistsofatappeddelay-linethatsamplesthesepeaks
n Eachpeakusuallysuffersindependentfadingn ThisisaformofdiversityinherentlyavailableinDSSSsystems
n InIS-95systemstheRAKEreceiverhasthree“fingers”n Itcansamplethreesuchpeakssimultaneouslyn A4thfingerisusedtolistentoadjacentcellsforRSSmeasurementsand
tosupportsofthand-offn Themobilestationistemporarilyconnectedtomorethanonebasestation
19
![Page 20: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/20.jpg)
+Principle of RAKE Receiver
n Stepsn Multipleversionsofasignalarrivemorethanonechipinterval apartn Receiverattemptstorecoversignalsfrommultiplepathsandcombinethem
n Thismethodachievesbetterperformancethansimplyrecoveringdominantsignalandtreatingremainingsignalsasnoise
20
![Page 21: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/21.jpg)
+CDMA/DSSS Summary
![Page 22: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/22.jpg)
+CDMA Properties: Near-Far Problemn ACDMAreceivercannotsuccessfully
de-spreadthedesiredsignalinahighmultiple-access-interferenceenvironment
n Unlessatransmitterclosetothereceivertransmitsatpowerlowerthanatransmitterfartheraway,thefartransmittercannotbeheard
n Powercontrolmustbeusedtomitigatethenear-farproblem
n Mobilestransmitatsuchpowerlevelstoensurethatreceivedpowerlevelsareequalatbasestation
n Powercontrolandchannelproblems!
22
Base station
![Page 23: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/23.jpg)
+CDMA Deployment Issues
nRadioplanninginCDMAsystemsisdifferentfromstandardTDMA/FDMAsystemsn Reuseisdefineddifferentlyn Capacitycalculationsaredifferent
23
![Page 24: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/24.jpg)
+Network planning for CDMA
nThereisnoconceptofco-channeloradjacentchannelinterferencen Interferencearisesfromusersinthesamecellandfromneighboringcells
n Codingandspreadspectrumplayaveryimportantroleinthemitigationofinterference
n InsteadofdefininganSr basedonsignalstrength,itismorecommontouseavalueofEb/It thatprovidesagiven“qualityofsignal”n Usuallythisisthevaluethatprovidesaframeerrorrateof1%– thisprovidesagoodMOSforvoice
n ThequantityIt isthetotalinterference
24
![Page 25: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/25.jpg)
+More on Eb/It
nThevalueofEb/Itdependsgreatlyonn Propagationconditionsn Transmitpowersoftheinterferingusersn SpeedoftheMSn Numberofmultipathsignalsthatcanbeusedfordiversity
nCellbreathingn TheboundaryofaCDMAcellisnotfixedanddependsonwheretheEb/It isreached
n Capacitymustbeoffloadedtoothercarrierstoovercomethiseffect
25
![Page 26: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/26.jpg)
+Coverage holes in CDMA
nPowercontrol,softhandoffandRSSthresholdsplayaveryimportantroleinthedesignn IftoomanyBSs(orsectors)coveranarea,thismaycreatea“coveragehole”
n Usually,notmorethanthreeBSsorsectorsshouldcoveranarea
26
Single CDMA Cell
Multiple CDMA Cells
High interference
holeSoft
handoffregions
![Page 27: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/27.jpg)
+Approach
n Somewhatsimplified,butworksingeneralforM usersinacell
n Letusconsiderthereverselink(uplink)n Therearetwocomponentsoftheinterference
n Owncellinterference- Ion Othercellinterference– Ioc
n Assumingperfectpowercontrol,theowncellinterferenceisgivenby:
Io =(M-1)Svfn S istheaveragepowerreceivedfromeachoftheM mobilestationsn Thereverselink “activityfactor”isvf
n Theactivityfactorisameasureof whatfractionoftimeatransmissionoccurs
27
![Page 28: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/28.jpg)
+Other Cell Interference
n Interferencefromothercellsfluctuatesasafunctionoftheload
n TheaveragevalueIoc canbeexpressedasfollowsIoc =f MSvf
n Assumptionisthatallothercellsaresimilartothecurrentcell
n Thefactorf indicatesfractionofothercellreceivedpowercomparedtotheown-cellreceivedpower
n Insomeways,f isameasureofthereusefactor
n Thefactorf dependsonthesizeofthegivencell,thepathlossexponent,shadowfadingdistribution,softhandoffparameters,etc.
28
![Page 29: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/29.jpg)
+Approach (II)
n Totalinterferenceisgivenby:Itotal =Io +Ioc =[(1+f)M-1]vf S=[M/h - 1]vf S
n Herethetermh referstothe“reuseefficiency”n Supposethereisimperfectpowercontrol,wecanrepresentthisbyafactorhc
Itotal =[M/h - 1]vf (S/hc)
n Ingeneral,therequiredSIRmustbesmallerthantheobservedSIR
(Eb/It)req <(SIR)systemn Ignorethermalnoise
n ThedesiredsignalhasapowerS multipliedbythe“processinggain”Gp
29
![Page 30: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/30.jpg)
+Approach (III)
n Proceedingfurther,weget:
n SolvingforMweget:
nMmax iscalledthe“polepoint”orasymptoticcellcapacity
30
Eb
It=
[M/h - 1]vf (S/hc)
SGp=
[(1+f)M-1]vfS/hc
SGp
M =1
1+fGphc
(Eb/It)vf (1+f)+
Mmax = 1Gphc
(Eb/It)vf (1+f)+
![Page 31: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/31.jpg)
+Cell Loading and Pole Point in IS-95
n Cellloadingn Ameasureofthetotalinterferenceinthesystemcomparedtothermalnoise
n Representedbythequantityr =M/Mmax
n Youcanshowthatitisalsoapproximatelyequaltotheratioofthetotalinterferencetothethermalnoise
n Samplecalculationn Let(Eb/It)reqd =6dB=4,R =9.6kbps,Rc = 1.2288Mcps,hc =0.8,vf =0.5,f =0.67
n Then,thepolepointorMmax willbe:n Mmax =1+ (1.2288´ 106/9.6´ 103)(0.8/(4´ 0.5´ [1+0.67])=1+30.65= 32
n Ifa3sectorantennaisused,typically,thegainincapacityisbyafactorof2.55sothatthepolepointis: 31.65´ 2.55=81
31
![Page 32: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/32.jpg)
+Comparison with AMPS/TDMA
n InAMPS,eachserviceproviderhas12.5MHzBWn Witha3sectorantenna,wecanhaveafrequencyreuseof7n Thereare30kHzchannelspervoicecalln Numberofchannels/cell=
(12.5´ 106 /30´ 103)´ (1/7)=57
n InthecaseofIS-136,witha3sectorantenna,wecanhaveafrequencyreuseof4n Each30kHzchannelcancarry3voicecallsn Numberofchannels/cell=
(12.5´ 106 /30´ 103)´ (1/4)´ 3=312.5
n WhatwasthepolepointofIS-95?n 81percarrierpercellsectorn With8cdma carriersina12.5MHzbandwidth,wecanhaveupto648
channelspercellsectorn With10cdma carriersina12.5MHzbandwidth,wecanhaveupto 810
channelspercellsector
32
![Page 33: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/33.jpg)
+Remarks
n Rangesofvaluesn Powercontrolinefficiencyhc variesbetween0.7and0.85n Voiceactivityfactorvf variesbetween0.4and0.6n Theothercellinterferencef variesbetween0.56and1.28forapathlossexponentof4andastandarddeviationofshadowfadingof6to10dB
33
![Page 34: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/34.jpg)
+Other issues
n ForwardLinkn Wehavetobeworriedaboutthepilot,sync,pagingandtrafficchannelsinIS-95and
manymoreincdma2000andUMTSn Thestrengthofthepilotchanneleffectivelydeterminesthesizeofthecelln Interferenceisfromclustersofhighpowertransmittersratherthanmanydistributedlow
powertransmittersn Designshouldtrytomaketheforwardandreverselinkcapacitiesasclosetooneanother
aspossiblen Thiswillreducetheamountofunnecessaryinterferenceandenablesmoothhandoffs
betweencells
n PNSequenceReusen Howcloselyshouldthesamepilotoffsetsbeused?(laterwhenwedoIS-95)
n Howdoesthelinkbudgetaffectthecapacity?
n Howdoessofthandoffaffectthecapacity?
34
![Page 35: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/35.jpg)
+Frequency Domain View (Gate)
35
OR
OrthogonalFrequencyDivisionMultiplexing
Channel
![Page 36: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/36.jpg)
+Diversity (continued) – Frequency Hoppingn Traditional
n Transmitter/receiverpaircommunicateonafixedfrequencychannel.
n FrequencyHoppingIdean Noise,fadingandinterferencechangewithfrequencybandintimen Movefrombandtoband
n Timespentonasinglefrequencyistermedthe“dwelltime”
nOriginallydevelopedformilitarycommunicationsn Spendashortamountoftimeinonefrequencybandn Preventinterceptionorjamming
36
![Page 37: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/37.jpg)
+
DevelopedduringWWIIbyactressHedyLammar andclassicalcomposerGeorgeAntheil
Patentgiventogovernment
Frequency Hopping Spread Spectrum
37
![Page 38: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/38.jpg)
+Frequency Hopping Spread Spectrumn Twotypesofsystems
n SlowHoppingn Dwelltimelongenoughtotransmitseveralbitsinarow(timeslot)
n FastHoppingn Dwelltimeontheorderofabitorfractionofabit(primarilyformilitarysystems)
n Transmitterandreceivermustknowhoppingpatternoralgorithmthatdeterminesthepatternbeforecommunications.n Cyclicpattern – bestforlownumberoffrequenciesandcombatingsmall-scale
fading:n Examplewithfourfrequencies:f4,f2,f1,f3,f4,f2,f1,f3,….
n Randompattern – bestforlargenumberoffrequencies,combatingco-channelinterference,andinterferenceaveragingn Examplewithsixfrequencies:f1,f3,f2,f1,f6,f5,f4,f2,f6,…n Userandomnumbergeneratorwithsameseedatbothends
38
![Page 39: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/39.jpg)
+Frequency Hopping concept
39
ABC
One Period of Sequence = 1 0 0 1 0 1 1
0 01
10 0
110
111
1 01
11 0
1 00
1 0 0
ABC
t7
t6
t5
t4
t3
t2
t1
t0
CLK
CLK
f4
f1
f3
f7
f6
f5
f2
f4
fc
f1
f2
f3
f4
f5
f6
f7
frequ
ency
cha
nnel
s
time
![Page 40: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/40.jpg)
+Combatting Time Dispersion
Rec
eive
d SN
R
frequencyTransmission
Lost Here
Hop Frequencies
Retransmission Here Successful
![Page 41: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/41.jpg)
+Example Systems
nGSM(2GCellular)nVeryslowhopping
nOriginalIEEE802.11nSlowhopping
nBluetoothnAlsoslowhoppingover79frequencieseach1MHzwide
nPerpackethopping
41R
ecei
ved
SNR
frequency
……
2.402 GHz
2.480 GHz
1 MHz
Collision Different Users
![Page 42: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/42.jpg)
+How do you utilize the entire bandwidth?
IdeainIEEE802.11g/a
=OFDM!
![Page 43: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/43.jpg)
+Orthogonal Frequency Division Multiplexingn Ideainfrequencydomain:
n Coherencebandwidthlimitsthemaximumdatarateofthechanneln Senddatainseveralparallelsub-channelseachatalowerdatarateand
differentcarrierfrequency
n Ideaintimedomain:n Byusingseveralsub-channelsandreducingthedatarateoneach
channel,thesymboldurationineachchannelisincreasedn Ifthesymboldurationineachchannelislargerthanthemultipathdelay
spread,wehavefewerrors
n OFDMenablesn Spacingcarriers(sub-channels)ascloselyaspossiblen ImplementingthesystemcompletelyindigitaleliminatinganalogVCOs
43
![Page 44: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/44.jpg)
+What is OFDM?
nModulation/Multiplexingtechnique
nUsualtransmissionn Transmitssinglehigh-ratedatastreamoverasinglecarrier
nWithOFDMnMultipleparallellow-ratedatastreamsn Low-ratedatastreamstransmittedonorthogonalsubcarriers
n Allowsspectraloverlapofsub-channels
44
![Page 45: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/45.jpg)
+OFDM Remarks
n ItisNOTanewtechnologybuthasfoundnewimportancebecauseofapplicationsn DSLmodemswherethechannelisnotuniformn Digitalaudioandvideobroadcastn WirelessLANapplications
n IEEE802.11aandHIPERLAN-2
n FastimplementationusingFFT’sisnowpossible
n Canbeadaptive(usedin802.11a)
n Problemsn Synchronizationbetweencarriersn Peak-to-averagepower(PAP)ratios
n Requireslinearamplifiers
45
![Page 46: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/46.jpg)
+OFDM Advantages
nBandwidthefficiency
nReductionofISIn Needssimplerequalizers
nRobusttonarrowbandinterferenceandfrequencyselectivefading
nPossibilityofimprovingchannelcapacityusingadaptivebitloadingovermultiplechannels
46
![Page 47: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/47.jpg)
+OFDM in frequency and time domains
47
n Noteorthogonality inbothdomainsn Whatisone“OFDMsymbol”?
Frequency
Power Spectrum
sub-carrier
single carrier
4 Carriers Spanning the Bandwidth of One Carrier
Channel
Bc
frequency
Fourier transform of symbol
time
AmplitudeSub-Carriers
![Page 48: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/48.jpg)
+OFDM Signal/Symbol
48
Df
![Page 49: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/49.jpg)
+OFDM Symbol
n OneOFDM“symbol”lastsforsayTs secondsn Thesymbolconsistsofthesumoftheindividualsymbolsfromthemanysub-carriersn Example:ConsiderQPSKoneachcarrier
n Ingeneraln ForN subchannels,theN samplesofthei-th transmittedOFDMsymbolcanbewritten
as
49
IFFTComplexNumber
![Page 50: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/50.jpg)
+Guard Time and Cyclic Prefix
n GuardtimeeliminatesISIiflargerthanexpecteddelayspreadoccurs
n Iftheguardtimehasnosignal,intercarrierinterference(ICI)mayoccurn ICIislikeacrosstalkbetweensubcarriers
n AcyclicprefixeliminatesICIn EnsuresthatdelayedreplicasofOFDMsymbolsalwayshaveintegernumberofcycleswithintheFFTinterval
n Maintainsorthogonalitybetweensubcarriersn Cyclicprefixisremovedatthereceiver
50
![Page 51: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/51.jpg)
+OFDM Transmission – basic system
n N consecutivecomplexsymbolsareconvertedintoagroupofNparalleldatastreams,whichthenaremodulatedoverorthogonalsubcarriers
51
ChannelEncoding
SymbolMapping
SerialtoParallel
N-PointIFFT
ParallelToSerial
Guard/CPInsertion
ChannelDecoding
ParalleltoSerial Detector N-Point
FFTSerialtoParallel
Guard/CPRemoval
RadioChannel+AWGN
![Page 52: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/52.jpg)
+Adaptive OFDM
52
frequency
|H(f)|
![Page 53: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/53.jpg)
+Channel Partitioning for Multicarrier Modulation
n Asthechannelisfrequencyselective,itmakessensetosplitthechannelintoseveralsmallerpartsn EachsmallerchunkisnowanAWGNchannel
n EachAWGNchannelprovidesadifferent SNR
n Question:Howdoweallocatetransmitpowers/modulationschemestoeachchunk?Whatisthemostoptimal?
53
NoisePSD/|H(f)|
Allocationofpower
Water-fillingalgorithmAllocatemoreenergywheretheSNRisbetter!
![Page 54: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/54.jpg)
+Adaptive OFDM
n Improvechannelcapacityfurthern Changemodulationschemen Allocatingbits/powerpersubcarrieraccordingtothequalityofeachsubchannel
54
AOFDM Components
Channel Quality Estimator*
Set of Modulation Schemes
Adaptive Loading/Allocation Algorithm
+
+
![Page 55: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/55.jpg)
+Adaptive Modulation
55
No transmission (0 bits)
BPSK (1 bit/symbol)QPSK (2 bits/symbol)
16-QAM (4 bits/symbol)8-QAM (3 bits/symbol)
Set of ModulationSchemes
![Page 56: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/56.jpg)
+Adaptive Modulation on Parallel Channels
56
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Number of Subcarriers
SNR (dB) BW Efficiency
1 bit
2 bits
3 bits
4 bits
16
![Page 57: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/57.jpg)
+Operation of Adaptive Algorithms
57
ChannelQuality
Estimator
Adaptive Algorithm
BitsAnd
Power
Subcarrier 1
Subcarrier 2
Subcarrier 3
Subcarrier N
Channel QualityInformation, e.g. SNR
time
Allocation
Based on optimal“Water-Filling”Power Distribution
+
Modulation Scheme Selection
![Page 58: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/58.jpg)
+OFDM Based Wireless LANs – IEEE 802.11an OperatesintheU-NIIBand
n 5.15–5.25,5.25–5.35,and5.725–5.825GHz
n Providesmultipletransmissionmodes/ratesdependingonchannelconditions.n 6,9,12,18,24,36,48,and54Mbps
n 4digitalmodulations:BPSK,QPSK,16-QAM,64QAM
n Radiospectrumisdividedinto8separatesegments/channels,20MHzeach
n 52carriers(subchannels)perchanneln Eachsubcarrierhasbandwidthof~300kHzn 48fordatamodulation,and4forpilotsignal
58
![Page 59: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/59.jpg)
+Recent Trends
nMIMOwithOFDMn IEEE802.11n,802.11acnDataratesgreaterthan100Mbps
nOFDMforwideareadataservicesn LTEandWiMax
nOtherPHYtechnologiesnUWBwithOFDMnMC-CDMA
![Page 60: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/60.jpg)
+Revisiting “Data Rates” in Wireless
nHomeA/Vnetworksareexpectedtoneed1-10GbpsnAssumingaspectralefficiencyof1bps/Hz,weneedatleast1GHzofspectrumnHaveignoredtheeffectsofmultipathfading
nBruteforceapproachnMaynotmeetthetechnology,regulatoryandcostrequirements
nCanweincreasethebps/Hzinwirelesssystems?
60
![Page 61: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/61.jpg)
+
Source:IEEESpectrum- July2004
Even
tualcon
vergen
ce
Edholm’s Law
n PhilEdholmn Nortel’sCTO
n ThreeTelecomCategoriesn Wirelinen Nomadic(Portable)n Wireless(Mobile)
n Dataratesincreaseexponentiallyn Thereisapredictabletimelagbetweenwirelessandwirelinesystems
61
![Page 62: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/62.jpg)
+How can we increase data rates?
n Traditionalwaysn Reducethesymbolduration
n Needslargerbandwidthn Leadstoawidebandchannelandfrequencyselectivity-irreducibleerrorrates
n Increasethenumberofbits/symboln ErrorratesincreasewithM forthesameEb/N0
n MIMOsystemsn Thereisnoneedtoincreasethebandwidthorpower
n Butwhatarethelimitations?n Usemultipletransmit(Tx)andreceive(Rx)antennasn Increasesspectralefficiencytoseveraltensofbps/Hz
62
![Page 63: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/63.jpg)
+What is MIMO?
n SofarwehaveconsideredSingleInputSingleOutputorSISOsystemsn Bothtransmitterandreceiverhave
oneantennaeachn Simplestformoftransceiver
architecture
n Singleinputmultiple-output(SIMO)systemsn Receiverhasmultipleantennas
n Multipleinputmultipleoutput(MIMO)systemsn Bothtransmitterandreceiverhave
multipleantennasn Strictly:EachantennahasitsownRF
chain(modulator,encoderandsoon)
63
![Page 64: Lecture 8 - University of Pittsburgh School of …nCell breathing n The boundary of a CDMA cell is not fixed and depends on where the E b/I t is reached n Capacity must be offloaded](https://reader033.vdocuments.us/reader033/viewer/2022042011/5e722ac958433711bb41f4a4/html5/thumbnails/64.jpg)
+Performance enhancements due to MIMOnDiversitygainnAbilitytoreceivemultiplecopiesofthesignalwithindependentfading
nSpatialmultiplexinggainnSenddifferentinformationbitsoverdifferentantennasandrecovertheinformation
nInterferencereductionnReducetheregionofinterferencetherebyincreasingcapacity
64