WINLAB

April 3, 2017

ImpactofAsynchronousTransmissionsinNoncontiguousOFDMA

RatneshKumbhkar†,Gokul Sridharan†,NarayanB.Mandayam†,IvanSeskar†,Sastry Kompella‡

WINLAB,Rutgers,TheStateUniversityofNewJersey†

InformationTechnologyDivision,NavalResearchLaboratory ‡

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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

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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

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Interferencefromasynchronoustransmission

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Effectof0th subcarrieron(N-1)th subcarrier

Numberofsubcarriersforsamebandwidth

haseffect

KeyObservations:• Relativeinterferencepowerindependentofsubcarrierwidth(onlyafunctionofdifferenceinindices)

• MeanrelativeinterferencepowerissignificantoveralargerangeofΔm• MeanrelativeinterferencepowerdecreasesasNincreases• SubcarrierassignmentandspacingcrucialforviabilityofNC-OFDMnetworks

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Guard-bandrequirement

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Thetotalinterferencepoweratsubcarrierm

Theworst-caseguard-bandrequirement

Sumofinterferencefromsubcarriers

allocatedtoallotherlinks

Comparewithmaximumtolerable

interferencethreshold

~20%ofthesubcarriersare

wastedasguard-band

Exampleguidelinefor10links

For2links

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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

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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?