Post on 19-Mar-2018
transcript
WINLAB
April 3, 2017
ImpactofAsynchronousTransmissionsinNoncontiguousOFDMA
RatneshKumbhkar†,Gokul Sridharan†,NarayanB.Mandayam†,IvanSeskar†,Sastry Kompella‡
WINLAB,Rutgers,TheStateUniversityofNewJersey†
InformationTechnologyDivision,NavalResearchLaboratory ‡
WINLAB
Overview1. IntroductiontoNC-OFDM2. Systemsetup3. Problemsfromasynchronoustransmission4. Guard-bandrequirement5. Enablingsynchronization6. Experimentalresults7. Summary
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• Amethodtoenableaccesstoaspectrumfragmentedbyeitherpresenceofaprimarytransmissionorothersecondarytransmissions
• NC-OFDM(Non-contiguousOFDM):Transmissionononlysomeofthesubcarriersbasedonavailabilityofspectrum
OccupiedbyPUorunavailableAvailablespectrum
Allocatedtolink1Allocatedtolink2
Whatisnon-contiguousOFDMA?
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WhyuseNC-OFDM?• HasadvantagesassociatedwiththeOFDMsuch
as– EfficientFFT-baseddesign– One-tapequalization
• Fromaneconomicperspective,NC-OFDMbenefitsfromthewell-establishedecosystemforreadily-availableOFDM-basedtransceivers
• Wellsuitedtomakeuseofnon-contiguousspectrum
• Dynamicallyadapttochangingspectrumconditions
• Enablesmultipleaccesssystem
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Subcarriermapping
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Networksetup
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• Multiplepoint-to-pointlinksinoperatingoverabandwidth,B
• Bandwidthdividedinmultiplesubcarriers• Somesubcarriersareusedbyprimarytransmissions
• Linksareallocateddisjointsetofsubcarriersnotusedbyprimary
• LinksareusingNC-OFDMforcommunication
• Transmissionsareasynchronous
B
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AsynchronoustransmissioninNC-OFDM
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Designconsiderations• ICImitigation/avoidancecrucial• Needsnontrivialguardbanddesign• Impactssubcarrierallocation• Enablesynchronization
PartialoverlapofsymbolLossoforthogonality• StartandendofNC-OFDMframenot
synchronized• Observationwindowcapturesfragmentsof
NC-OFDMframe• SubcarriersnolongerorthogonalConsequence• Interference(ICI)betweenlinks
• NotanissueinregularOFDMA• ControlledbyeNodeB
WINLAB
Interferencefromasynchronoustransmission
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Nointerferencewithincyclicprefix
Case1:0≤τµ ≤Ncp :(timeoffsetwithinCP)OnlyasingleNC-OFDMsymbolcontributestointerference
Case2:Ncp <τµ ≤N+Ncp −1:(timeoffset>CP)TwoNC-OFDMsymbolscontributetointerference,and
• Interferencebetweentwosubcarriersarecalculated
• ∆m=µ−m,(subcarriersallocatedtodifferentlinks)
CP
Numberofsamples
WINLAB
Interferencefromasynchronoustransmission
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Effectof0th subcarrieron(N-1)th subcarrier
Numberofsubcarriersforsamebandwidth
haseffect
KeyObservations:• Relativeinterferencepowerindependentofsubcarrierwidth(onlyafunctionofdifferenceinindices)
• MeanrelativeinterferencepowerissignificantoveralargerangeofΔm• MeanrelativeinterferencepowerdecreasesasNincreases• SubcarrierassignmentandspacingcrucialforviabilityofNC-OFDMnetworks
WINLAB
Guard-bandrequirement
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Thetotalinterferencepoweratsubcarrierm
Theworst-caseguard-bandrequirement
Sumofinterferencefromsubcarriers
allocatedtoallotherlinks
Comparewithmaximumtolerable
interferencethreshold
~20%ofthesubcarriersare
wastedasguard-band
Exampleguidelinefor10links
For2links
WINLAB
Enablingsynchronization
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[CISS16]R.Kumbhkar,G.Sridharan,N.Mandayam,I.Seskar,andS.Kompella,“DesignandImplementationofanUnderlayControlChannelforNC-OFDM-Based
Networks,”inCISS2016,Princeton
Usedforfrequencyoffsetandtimeoffsetestimation[CISS16]
• Alow-powerunderlayPN-sequenceisusedasabeaconsignal
• Underlaytransmissionisacrossallbandwidth
• Linkssearchforbeaconbeforetransmission
• Forexperimentalpurposesbeaconsaretransmittedintimesharedmanner
• Similartransmissionwasusedforfrequencyandtimeoffsetforasinglelink[CISS16]
Lowpowerunderlay
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Resultsfromexperiments
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• OperatingSNRforbothlinksaresame• IncreasingSNRresultsinhigherdegradationduetoasynchronoustransmission
Setup:• USRPN210radioswithSBXdaughterboards• Numberofsubcarriers=64• Noprimarytransmissions
WINLAB
ResultsforasymmetricSNR
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LinksoperatingatSNR,ρ
LinksoperatingatSNR,ρ-3dB
• AssumelinksareoperatingatdifferentSNRs(differenttransmissionpower)
• LinkoperatingathigherSNRisnotaffectedmuchbylowerSNRlink
LinkwithlowerSNRhasadvantageinsynchronizingitstransmissionwithlinkwithhigherSNR
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Keytakeaways• Thetimeoffsetacrossdifferentlinkscancausesubstantialinterference
evenwithorthogonalsubcarrierallocationacrossthelinks• Guard-bandrequirementsaresignificant• Guidelinesonthechoiceofbasictransmissionparameterssuchas
lengthofcontiguoussubcarriersassignedtoalinkandoperatingSNRs• Adecentralizedsynchronizationschemeusingbeaconsappendedto
NC-OFDMsymbolsispresented• Enablingtimingcoordinationisparticularlybeneficialinasymmetric
transmission
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Questions?