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LTE Radio Overview: Downlink

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  1. 1. rfan Ali 1 LTECoreFeatures LTERadioPrimer Irfan Ali [email protected] LTE: Afeaturebasedintroduction
  2. 2. Irfan Ali 2rfan Ali 2 Overview Downlink:Howarecontrolanddatainformationsenttomultiplemobiles? OFDM DownlinkRadioFrameStructure Inasub-frame,howdoesamobileknowwheretolookfordata. LogicalChannelsandPhysicalChannels RadioProtocolstack Channelsignalstrengthmeasurement Uplink: Howiscontrolanddatainformationreceivedfrommultiplemobiles? SC-FDMA UplinkRadioFrameStructure RadioFrameSynchronization(TimingAdvance) PhysicalChannelsandLogicalChannels UplinkReferenceSignalTransmission
  3. 3. Irfan Ali 3rfan Ali 3 Whendoesthebase-stationtalkandwhendothemobilestalk? Thequestion iswhenandwhereinthetime-frequencydomain. LTEsupports twoduplexing modes: FrequencyDivisionDuplexing(FDD) TimeDivisionDuplexing(TDD) Time Frequency (MHz) 20 MHz 20 MHz 2110 2130 1920 1940 Downlink Uplink Frequency Division Duplexing
  4. 4. Irfan Ali 4rfan Ali 4 WhatifOFDM OFDM=OrthogonalFrequencyDivisionMultiplexing Whatareorthogonalfunctions? Twofunctionsh1(t)andh2(t)areorthogonaloveraninterval [0,T],if Setoffunctions{h1(t),h2(t),, hn(t)}aremutuallyorthogonal,if Ifg(t)=a1h1(t)+a2h2(t)+ +anhn(t),for[0,T],then ",$ = ' " $ * + = 0 -, . = 0, if , and -,- = - , - = --,- = - -, for = 1. .
  5. 5. Irfan Ali 5rfan Ali 5 Harmonics(multiples) ofcosinefunctionsoffrequencyf0,areorthogonaloverthebasetime-period, T =1/f0 Letf0=15kHz,T=66.67s Orthogonalcosinefunctions " = 2cos 2+ 66.67 s Time 1.4 15kHz x(t) $ = 2cos 2.2+ 66.67 s Time 1.4 30kHz B = 2cos 2.3+ 66.67 s Time 1.4 45kHz "$ = 2cos 2.12+ 66.67 s Time 1.4 180kHz = 1.5" + (1,1)$ + + (1,1)"$ ," g(t) g(t)h1(t) ,"
  6. 6. Irfan Ali 6rfan Ali 6 WhatisOFDM Orthogonal FrequencyDivisionMultiplexing T 2T 15 30 45 60 180 3T 4T 5T 6T 7T 8T 9T 10T 11T 12T 13T 14T 66.67 s Time 1.4 30kHz 66.67 s Time 1.4 45kHz 66.67 s Time 1.4 180kHz 15 30 45 60 75-15-30-45-60-75 15 30 45 60 75-15-30-45-60-75 15 30 45 60 75-15-30-45-60-75 Frequency Domain Frequency (kHz) 15 30 45 60 75-15-30-45-60-75 |X(f)| 66.67 s Time 1.4 15kHz x(t) frequency (kHz) Time 1 ms Subframe
  7. 7. Irfan Ali 7rfan Ali 7 HowisOFDMsignalgenerated? Modulate Add Cyclic Prefix Mix to RF PA Add Cyclic Prefix Digital to Analog Mix to Baseband Remove Cyclic Prefix Serial to Parallel f0 2f0 3f0 Nf0 Parallel to Serial f0 2f0 3f0 Nf0 LNA Input bit stream De-Modulate Output bit stream Inverse FFT FFT Remove Cyclic Prefix Analog to Digital
  8. 8. Irfan Ali 8rfan Ali 8 WhyOFDM? TheOFDMsymbolsdurationisrelativelylong(66.67s),which allowsonetoaddtime-gap(preamble)tohandlerelativelylong delay-spread ofthechannel(5s ~1.5km)withoutloosingmuch capacity. Reducedinter-symbolinterference Multiplesub-carriers(ratherthanasinglecarrier)overlarge bandwidths(20MHz)enabletohandlechannel-fadesoverthese largebandwidths. Increasedprocessingcapability.
  9. 9. Irfan Ali 9rfan Ali 9 LTEDownlinkFrameStructure Channel Bandwidth MHz # Resource Blocks in Frequency Domain Total Subcarrier Bandwidth MHz 1.4 MHz 6 1.095 3 MHz 15 2.715 5 MHz 25 4.515 10 Mhz 50 9.015 15 Mhz 75 13.515 20 Mhz 100 18.015 1ms Subframe Radio Frame 10 ms Radio Frame 10 ms SystemFrameNumber SFNn SystemFrameNumber SFNn+1 Time Frequency
  10. 10. Irfan Ali 10rfan Ali 10 SynchronizingtoDLRadioFrame Radio Frame 10 ms Radio Frame 10 ms SystemFrameNumber SFNn SystemFrameNumber SFNn+1 Frequency Time Secondary Synchronization Signal (SSS) Physical Broadcast Channel (PBCH) Primary Synchronization Signal (PSS) 62subcarriers 72subcarriers 3MHz Cell Reference Signal MIB Downlink Bandwidth System Frame Number PHICH Configuration PHICH Physical Hybrid ARQ Indicator Channel
  11. 11. Irfan Ali 11rfan Ali 11 SynchronizationSignals ThePrimarySynchronizationSignalisallocatedtothecentral62subcarriersinthe1st and6th subframe ofeveryRadioFrame.Itsinthe7th symbolinthesubframe.Bothtransmissionsare identical. ThePrimarySynchronizationSignalisusedto: Achievesymbol, slotandsubframe synchronization DeterminethefirstpartofPhysicalLayerCellIdentity(PCI):3values. TheSecondarySynchronizationSignalisallocatedtothecentral62subcarriersinthe1st and6th subframe ofeveryRadioFrame.Itsinthe6th symbolinthesubframe. The2SSStransmissionswithineachradioframeusedifferentsequencestoallowtheUEto differentiatebetweenthe1st and2nd transmission,i.e.allowingtheUEtoachieveframe synchronization. TheSecondarySynchronizationSignalisusedto: Achieveframesynchronization DeterminethesecondpartofPhysicalLayerCellIdentity:168differentvalues.ThiswaytheUEdeterminesthe PCIofthecell,whichis1of504differentvalues
  12. 12. Irfan Ali 12rfan Ali 12 DownlinkPhysicalChannels Radio Frame 10 ms Radio Frame 10 ms SystemFrameNumber SFNn SystemFrameNumber SFNn+1 Frequency Time Physical Broadcast Channel (PBCH) 62subcarriers 72subcarriers Physical Downlink Control Channel (PDCCH) Physical Downlink Shared Channel (PDSCH) 3MHz Physical Control Format Indicator Channel (PCFICH) Physical Hybrid ARQ Indicator Channel (PHICH)
  13. 13. Irfan Ali 13rfan Ali 13 PhysicalDownlinkSharedChannel(PDSCH) Intheremaining11symbolsofthe subframe Transfers: SystemInformationBlocks(SIBs). PagingRRCmessage OtherRRCmessages Applicationdata. QPSK(2bits/RE),16QAM(4bits/RE)or 64QAM(6bits/RE)modulationisused. SIB-2 Paging UE-1 UE-2 RRC Message UE-3 Data Radio Bearer UE-3 Data Radio Bearer 1.4MHz Subframe (1ms)
  14. 14. Irfan Ali 14rfan Ali 14 PhysicalDownlinkControlChannel(PDCCH) Inthefirst1-to-3(configurable)symbols ofeverysubframe (1ms) TransfersDownlinkControlInformation (DCI). DCImessagesconsistsofmultiples (i=1,2,3,4)of36resourceelements. Threegoals: Downlinkresourceallocationfor samesubframe. AllocatedasResourceBlockGroup RBGSize=1,for1.4MHz, RBGSize=4,for20MHz BitmapusedtoindicatewhichRBGis allocatedtoUE Uplinkresourceallocation TransmitPowerControl QPSKmodulationisused(2bits/RE) DCIhasa16bitCRCUplink Resource Allocation for UE 2 1.4MHz Subframe (1ms) SIB-2 Paging UE-1 UE-2 RRC Message UE-3 Data Radio Bearer UE-3 Data Radio Bearer
  15. 15. Irfan Ali 15rfan Ali 15 Howdoesamobileknowifthereisamessageforitinasubframe? Therearefouridentitiesthatamobile searchesforintheDownlinkControl Information(DCI)inthePDCCH: UEsuniquecell-radionetwork temporaryidentity(C-RNTI) Paging-RNTI,P-RNTI (0xFFFE)and SystemInformation-RNTI,SI-RNTI (0xFFFF). P-RNTIandSI-RNTIarethesameforall mobiles. ThecheckforP-RNTIandSI-RNTIarenot performedineverysubframe,buton selected/paging-occasionsubframes, (onceeveryDRXcycle). DuringRandomaccess 1. RandomAccess-RNTI(RA-RNTI):ForRandom accessresponse message. 2. TemporaryC-RNTI:ForRRCConnection Setup message InthePDSCH,theMACheadertellsthe mobile,ifthemessageisanRRCmessageora datapacket LogicalChannelID=0..2->SRB0..2 LogicalChannelID=3..10->DRBs Uplink Resource Allocation for UE 2 1.4MHz Subframe (1ms) SIB-2 Paging UE-1 UE-2 RRC Message UE-3 Data Radio Bearer UE-3 Data Radio Bearer C-RNTI 2 C-RNTI 3 C-RNTI 2 P-RNTI SI-RNTI
  16. 16. Irfan Ali 16rfan Ali 16 Howdoesthemobilefindoutwhatinformationisbeingsenttoit? IfLogicalChannelID==0(SRB0)CommonControlChannel(CCCH) DL-CCCH-MessageType ::=CHOICE{ rrcConnectionReestablishment rrcConnectionReestablishmentReject rrcConnectionReject rrcConnectionSetup } IfLogicalChannelID==1,2(SRB1andSRB2)DedicatedControlChannel(DCCH) DL-DCCH-MessageType ::=CHOICE{ csfbParametersResponseCDMA2000 dlInformationTransfer handoverFromEUTRAPreparationRequest mobilityFromEUTRACommand rrcConnectionReconfiguration rrcConnectionRelease securityModeCommand ueCapabilityEnquiry counterCheck ueInformationRequest-r9 spare6NULL,spare5NULL,spare4NULL, spare3NULL,spare2NULL,spare1NULL} IfLogicalChannelID==3-10(DRBs):DedicatedTrafficChanel(DTCH).Datatraffic SentbeforeRRCChannelissetup SentafterRRCChannelissetup
  17. 17. Irfan Ali 17rfan Ali 17
  18. 18. Irfan Ali 18rfan Ali 18 DownlinkProtocolLayersandChannelMappingineNB RRC ROHC ROHC Encryption Encryption Sequence Number Sequence Number PDCP RLC MAC PHY RRC/ Data Segmentation Acknowledged Mode (ARQ) Segmentation Acknowledged Mode (ARQ) DCCH LCID1 LCID2 Integrity Protection Encryption Sequence Number Integrity Protection Encryption Sequence Number Segmentation Segmentation UnAck Mode UnAck Mode LCID3 LCID4 DTCH MIB SIB SRB0 SRB1 SRB2 DRB1 DRB2Page PDSCH, C-RNTI Transparent Mode (Buffer) BCCH PDSCH, SI-RNTIPBCH Transparent Mode (Buffer) PCCH PDSCH, P-RNTI Logical Channels Transport Channels/ Physical Channels Transparent Mode (Buffer) CCCH LCID0 PDSCH, Temporary C-RNTI RRC Radio Resource Control PDCP Packet Data Convergence Protocol RLC Radio Link Control MAC Medium Access Control Scheduling Priority Handling HARQ Multiplexing of MAC SDUs
  19. 19. Irfan Ali 19rfan Ali 19 DownlinkProtocolLayersandChannelMappingineNB RRC ROHC ROHC Encryption Encryption Sequence Number Sequence Number PDCP RLC MAC PHY RRC/ Data Segmentation Acknowledged Mode (ARQ) Segmentation Acknowledged Mode (ARQ) DCCH LCID1 LCID2 Integrity Protection Encryption Sequence Number Integrity Protection Encryption Sequence Number Segmentation Segmentation UnAck Mode UnAck Mode LCID3 LCID4 DTCH MIB SIB SRB0 SRB1 SRB2 DRB1 DRB2Page PDSCH, C-RNTI Transparent Mode (Buffer) BCCH PDSCH, SI-RNTIPBCH Transparent Mode (Buffer) PCCH PDSCH, P-RNTI Logical Channels Transport Channels/ Physical Channels Transparent Mode (Buffer) CCCH LCID0 PDSCH, Temporary C-RNTI RRC Radio Resource Control PDCP Packet Data Convergence Protocol RLC Radio Link Control MAC Medium Access Control Scheduling Priority Handling HARQ Multiplexing of MAC SDUs Physical Downlink Control Channel (PDCCH) Physical Downlink Shared Channel (PDSCH) Physical Broadcast Channel (PBCH) Source: Netmanias
  20. 20. Irfan Ali 20rfan Ali 20 DownlinkProtocolLayersandChannelMappingineNB RRC ROHC ROHC Encryption Encryption Sequence Number Sequence Number PDCP RLC MAC PHY RRC/ Data Segmentation Acknowledged Mode (ARQ) Segmentation Acknowledged Mode (ARQ) DCCH LCID1 LCID2 Integrity Protection Encryption Sequence Number Integrity Protection Encryption Sequence Number Segmentation Segmentation UnAck Mode UnAck Mode LCID3 LCID4 DTCH MIB SIB SRB0 SRB1 SRB2 DRB1 DRB2Page PDSCH, C-RNTI Transparent Mode (Buffer) BCCH PDSCH, SI-RNTIPBCH Transparent Mode (Buffer) PCCH PDSCH, P-RNTI Logical Channels Transport Channels/ Physical Channels Transparent Mode (Buffer) CCCH LCID0 PDSCH, Temporary C-RNTI RRC Radio Resource Control PDCP Packet Data Convergence Protocol RLC Radio Link Control MAC Medium Access Control Scheduling Priority Handling HARQ Multiplexing of MAC SDUs RLC Header n n+1 n+2 n+3 RLC Header RLC SDU RLC PDU
  21. 21. Irfan Ali 21rfan Ali 21 DownlinkProtocolLayersandChannelMappingineNB RRC ROHC ROHC Encryption Encryption Sequence Number Sequence Number PDCP RLC MAC PHY RRC/ Data Segmentation Acknowledged Mode (ARQ) Segmentation Acknowledged Mode (ARQ) DCCH LCID1 LCID2 Integrity Protection Encryption Sequence Number Integrity Protection Encryption Sequence Number Segmentation Segmentation UnAck Mode UnAck Mode LCID3 LCID4 DTCH MIB SIB SRB0 SRB1 SRB2 DRB1 DRB2Page PDSCH, C-RNTI Transparent Mode (Buffer) BCCH PDSCH, SI-RNTIPBCH Transparent Mode (Buffer) PCCH PDSCH, P-RNTI Logical Channels Transport Channels/ Physical Channels Transparent Mode (Buffer) CCCH LCID0 PDSCH, Temporary C-RNTI RRC Radio Resource Control PDCP Packet Data Convergence Protocol RLC Radio Link Control MAC Medium Access Control Scheduling Priority Handling HARQ Multiplexing of MAC SDUs
  22. 22. Irfan Ali 22rfan Ali 22 ExampleofIDsinthePDCCHandmessageinPDSCH MIB SIB-1 SIB-2 SIB-5 Random Access Preamble Random Access Response RRC Connection Request RRC Connection Setup RRC Connection SetupComplete DL Info Transfer (NAS: Authn Req) Physical Downlink Control Channel (PDCCH) Physical Downlink Shared Channel (PDSCH) Physical Broadcast Channel (PBCH) ID in PDCCH MAC Packet SI-RNTI SIB RA-RNTI RAR C-RNTI LCID 1 LCID 29 Timing Advance NAS Message Authn Request RAPID, Uplink Grant, TC-RNTI TC-RNTI LCID 0 LCID 31 LCID 28 UE Contention Resolution ID RRC Connection Setup Pad RRC Connection Req Msg RRC Connection Request UE-Identity rand Establish-cause mo-data, mo-signaling, mt-Access, Info Content MIB Downlink Channel Bandwidth, PHICH Configuration, SFN SIB 1 PLMN ID, Tracking Area Code, Cell Selection Parameters, Frequency band, cell barring, Schedulinginfo for other SIBs SIB2 Access Class Barring, Channel (RACH, BCCH, ..) parameters, UE timers, UL Carrier Frequency SIB3 Cell Selection Parameters SIB4 Inter Frequency neighbour cell info SIB5 Intra Frequency neighbour cell info
  23. 23. Irfan Ali 23rfan Ali 23 CellReferenceSignals(CRS) CellReferenceSignals KnownreferencesignalsareinsertedatregularintervalswithintheOFDMtime- frequencygrid. Therearefourresourceelementsperresourceblockthatarededicatedto ReferenceSignal. 7 symbols = 0.5 ms (Slot) 12subcarriers=180kHz The location of Reference Signals depends on the Physical layer cell identity of the cell. The Primary and Secondary Synchronization Signals the Physical Layer Cell Identity Resource Elements used for Reference Signal
  24. 24. Irfan Ali 24rfan Ali 24 ReferenceSignalReceivedPower(RSRP) TheRSRPistheaveragepower(inwatts)receivedfromasingle ReferenceSignal(RS) resourceelement RSRPmeasuresonlytheRSpowerandexcludesallnoiseandinterferencepower. Knowledge ofabsoluteRSRPenablesmobiletocalculatedownlinkpath-loss. ThemaximumRSRPisbasedonmaximuminputpowertoUEof-25dBm(0.0032 mWatts).In1.4MHzBWwith6RBs(72ResourceElements),maxRSRPis-44dBm. Theminimum valueis-140dBm (has6dBm ofmarginfromminimumpossible receivedpoweratUE). 7 symbols = 0.5 ms (Slot) 12subcarriers=180kHz Resource Elements used for Reference Signal = 1 P QR,. S .T" where,Prs,k istheestimatedreceivedpower(inWatts)of thekthReferenceSignalResourceelement
  25. 25. Irfan Ali 25rfan Ali 25 Measurement2:ReferenceSignalReceivedQuality(RSRQ) RSRPdoesnotgiveanindicationofsignalquality,i.e.thestrengthofthereferencesignalcompared tooverallenergyinthechannel(akareceivedsignalstrengthindicator(RSSI)) TheRSSIparameterrepresentstheentirereceivedpowerincludingthewantedpowerfromthe servingcellaswellasallco-channelpowerandothersourcesofnoise. MeasuringRSRQbecomesparticularlyimportantnearthecelledgewhendecisionsneedtobe made,regardlessofabsoluteRSRP,toperformahandovertothenextcell. ThemaximumvalueofRSRQis-3dB. (Onereferencesignalhas50%energyintheRB) TheminimumvalueofreportedRSRQis-19.5dB. (OnereferencesignalREhasonly1%ofenergyin RB) where NRB is the number of Resource blocks (NRB= 6 for 1.4MHz Bandwidth) RSRQ = RSRP (RSSI / NRB) 7 symbols = 0.5 ms (Slot) 12subcarriers=180kHz Resource Elements used for Reference Signal RSSI is measured only in OFDM symbol containing the RS
  26. 26. Irfan Ali 26rfan Ali 26 Summary:DLRadioFrame Radio Frame 10 ms Radio Frame 10 ms SystemFrameNumber SFNn SystemFrameNumber SFNn+1 Frequency Time Secondary Synchronization Signal (SSS) Physical Broadcast Channel (PBCH) Primary Synchronization Signal (PSS) 62subcarriers 72subcarriers Physical Downlink Control Channel (PDCCH) Physical Downlink Shared Channel (PDSCH) 3MHz Cell Reference Signal Physical Control Format Indicator Channel (PCFICH) Physical Hybrid ARQ Indicator Channel (PHICH)
  27. 27. Irfan Ali 27rfan Ali 27 OFDMinmulti-color Frequency Time Secondary Synchronization Signal (SSS) Physical Broadcast Channel (PBCH) Primary Synchronization Signal (PSS) 62subcarriers 72subcarriers Physical Downlink Control Channel (PDCCH) Physical Downlink Shared Channel (PDSCH) 3MHz Cell Reference Signal Physical Control Format Indicator Channel (PCFICH) Physical Hybrid ARQ Indicator Channel (PHICH) Radio Frame 10 ms Radio Frame 10 ms
  28. 28. Irfan Ali 28rfan Ali 28 References Specifications: TS36.300:RANArchitecture TS36.331:RRC TS36.323:PDCP TS36.322:RLC TS36.321:MAC OtherReferences: LTEinBullets www.sharetechnote.com www.youtube.com/lte4g