LTE Radio Overview: Downlink

İrfan Ali 1

LTECoreFeatures

LTERadioPrimer

Irfan Ali

[email protected]

LTE:Afeaturebasedintroduction

Irfan Ali 2İrfan Ali 2

Overview

• Downlink:Howarecontrolanddatainformationsenttomultiplemobiles?Ø OFDMØ DownlinkRadioFrameStructureØ Inasub-frame,howdoesamobileknowwheretolookfordata.Ø LogicalChannelsandPhysicalChannelsØ RadioProtocolstackØ Channelsignalstrengthmeasurement

• Uplink:Howiscontrolanddatainformationreceivedfrommultiplemobiles?Ø SC-FDMAØ UplinkRadioFrameStructureØ RadioFrameSynchronization(TimingAdvance)Ø PhysicalChannelsandLogicalChannelsØ UplinkReferenceSignalTransmission


Whendoesthebase-stationtalkandwhendothemobilestalk?• Thequestion iswhenand“where”inthetime-frequencydomain.• LTEsupports twoduplexingmodes:

Ø FrequencyDivisionDuplexing (FDD)Ø TimeDivisionDuplexing (TDD)

Time

Frequency (MHz)

20 MHz

20 MHz

2110

2130

1920

1940

Downlink

Uplink

Frequency Division Duplexing


WhatifOFDM

• OFDM=OrthogonalFrequencyDivisionMultiplexing• Whatareorthogonalfunctions?

Ø Twofunctionsh1(t)andh2(t)areorthogonaloveraninterval[0,T],if

Ø Setoffunctions{h1(t),h2(t),…, hn(t)}aremutuallyorthogonal,if

Ø Ifg(t)=a1h1(t)+a2h2(t)+… +anhn(t),for[0,T],then

ℎ",ℎ$ = ' ℎ" 𝑡 ℎ$ 𝑡 𝑑𝑡*

+= 0

ℎ-, ℎ. = 0, if𝑖 ≠ 𝑘, and ℎ-,ℎ- = 𝑚-

𝑔, ℎ- = 𝛼-ℎ-, ℎ- = 𝛼- 𝑚-, for𝑖 = 1. . 𝑛


• Harmonics(multiples) ofcosine functions offrequencyf0,areorthogonal overthebasetime-period, T =1/f0• Letf0=15kHz,T=66.67µs

Orthogonalcosinefunctions

ℎ" = 2� cos 2𝜋𝑓+𝑡66.67 µs

Time

1.4 15kHz

x(t)

ℎ$ = 2� cos 2𝜋.2𝑓+ 𝑡66.67 µs

Time

1.4 30kHz

ℎB = 2� cos 2𝜋.3𝑓+ 𝑡66.67 µs

Time

1.4 45kHz

ℎ"$ = 2� cos 2𝜋.12𝑓+𝑡66.67 µs

Time

1.4 180kHz

𝑔 = −1.5ℎ"+ 𝑟𝑎𝑛𝑑(1,−1)ℎ$+ ⋯+ 𝑟𝑎𝑛𝑑(1,−1)ℎ"$

𝑔,ℎ"

g(t)

g(t)h1(t)

𝑔,ℎ"


WhatisOFDM• Orthogonal FrequencyDivisionMultiplexing

T 2T

15

30

45

60

180

3T 4T 5T 6T 7T 8T 9T 10T 11T 12T 13T 14T

66.67 µsTime

1.4 30kHz

66.67 µsTime

1.4 45kHz

66.67 µsTime

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 µsTime

1.4 15kHz

x(t)

frequency(kHz)

Time1 ms

Subframe


HowisOFDMsignalgenerated?

ModulateAdd

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-ModulateOutput bit stream

Inverse FFT

FFTRemoveCyclicPrefix

Analog to Digital


WhyOFDM?

• TheOFDMsymbolsdurationisrelativelylong(66.67µs),whichallowsonetoaddtime-gap(preamble)tohandlerelativelylongdelay-spread ofthechannel(5µs ~1.5km)withoutloosingmuchcapacity.Ø Reducedinter-symbolinterference

• Multiplesub-carriers(ratherthanasinglecarrier)overlargebandwidths(20MHz)enabletohandlechannel-fadesovertheselargebandwidths.

• Increasedprocessingcapability.


LTEDownlinkFrameStructure

Channel Bandwidth

MHz

# ResourceBlocks 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 Frame10 ms

Radio Frame10 ms

SystemFrameNumberSFNn

SystemFrameNumberSFNn+1

Time

Frequency


SynchronizingtoDLRadioFrame

Radio Frame

10 ms

Radio Frame10 ms



Frequency

Time

Secondary Synchronization Signal (SSS)

Physical Broadcast Channel (PBCH)

Primary Synchronization Signal (PSS)

62 s

ubca

rrie

rs

72 s

ubca

rrie

rs3

MHz

Cell Reference Signal

MIB• Downlink Bandwidth• System Frame Number• PHICH Configuration

PHICH Physical Hybrid ARQ Indicator Channel


SynchronizationSignals

• ThePrimarySynchronizationSignalisallocatedtothecentral62subcarriersinthe1st and6thsubframe ofeveryRadioFrame.It’sinthe7th symbolinthesubframe.Bothtransmissionsareidentical.

• ThePrimarySynchronizationSignalisusedto:

Ø Achievesymbol, slotandsubframe synchronization

Ø DeterminethefirstpartofPhysical LayerCellIdentity(PCI):3values.

• TheSecondarySynchronization Signalisallocatedtothecentral62subcarriersinthe1st and6thsubframe ofeveryRadioFrame.It’sinthe6th symbolinthesubframe.

• The2SSStransmissionswithineachradioframeusedifferentsequencestoallowtheUEtodifferentiatebetween the1st and2nd transmission,i.e.allowingtheUEtoachieveframesynchronization.

• TheSecondarySynchronization Signalisusedto:

Ø Achieveframesynchronization

Ø DeterminethesecondpartofPhysicalLayerCell Identity:168different values.ThiswaytheUEdeterminesthePCIofthecell,whichis1of504differentvalues


DownlinkPhysicalChannels

Radio Frame

10 ms

Radio Frame10 ms



Frequency

Time


62 s

ubca

rrie

rs

72 s

ubca

rrie

rs

Physical Downlink Control Channel (PDCCH)

Physical Downlink Shared Channel (PDSCH)

3 M

HzPhysical Control Format Indicator Channel (PCFICH)

Physical Hybrid ARQ Indicator Channel (PHICH)


PhysicalDownlinkSharedChannel(PDSCH)

• Intheremaining11symbolsofthesubframe

• Transfers:Ø SystemInformationBlocks(SIBs).Ø PagingRRCmessageØ OtherRRCmessagesØ Applicationdata.

• QPSK(2bits/RE),16QAM(4bits/RE)or64QAM(6bits/RE)modulationisused.

SIB-2

Paging UE-1

UE-2 RRC Message

UE-3 Data Radio Bearer


1.4

MH

z

Subframe (1ms)


PhysicalDownlinkControlChannel(PDCCH)

• Inthefirst1-to-3(configurable)symbolsofeverysubframe (1ms)

• TransfersDownlinkControlInformation(DCI).

• DCImessagesconsistsofmultiples(i=1,2,3,4)of36resourceelements.

• Threegoals:Ø Downlinkresourceallocationfor

samesubframe.• AllocatedasResourceBlockGroup

• RBGSize=1,for1.4MHz,• RBGSize=4,for20MHz

• BitmapusedtoindicatewhichRBGisallocatedtoUE

Ø UplinkresourceallocationØ TransmitPowerControl

• QPSKmodulationisused(2bits/RE)• DCIhasa16bitCRCUplink

Resource Allocationfor UE 2

1.4

MH

z

Subframe (1ms)

SIB-2

Paging UE-1

UE-2 RRC Message




Howdoesamobileknowifthereisamessageforitinasubframe?

• Therearefouridentities thatamobilesearchesforintheDownlinkControlInformation(DCI)inthePDCCH:Ø UE’sunique cell-radionetwork

temporaryidentity(C-RNTI)Ø Paging-RNTI,P-RNTI (0xFFFE) andØ SystemInformation-RNTI,SI-RNTI

(0xFFFF).Ø P-RNTIandSI-RNTIarethesameforall

mobiles.Ø ThecheckforP-RNTIandSI-RNTIarenot

performedineverysubframe, butonselected/“paging-occasion” subframes,(onceeveryDRXcycle).

Ø DuringRandomaccess1. RandomAccess-RNTI(RA-RNTI):ForRandom

accessresponse message.2. TemporaryC-RNTI:ForRRCConnection Setup

message

• InthePDSCH,theMACheadertellsthemobile, ifthemessageisanRRCmessageoradatapacketØ LogicalChannel ID=0..2->SRB0..2Ø LogicalChannel ID=3..10->DRBs

Uplink Resource Allocationfor UE 2

1.4

MH

z

Subframe (1ms)

SIB-2

Paging UE-1

UE-2 RRC Message



C-RNTI 2

C-RNTI 3

C-RNTI 2

P-RNTI

SI-RNTI


Howdoesthemobilefindoutwhatinformationisbeingsenttoit?

• IfLogicalChannelID==0(SRB0)CommonControlChannel(CCCH)DL-CCCH-MessageType ::=CHOICE {

rrcConnectionReestablishmentrrcConnectionReestablishmentRejectrrcConnectionRejectrrcConnectionSetup }

• IfLogicalChannelID==1,2(SRB1andSRB2)DedicatedControlChannel(DCCH)DL-DCCH-MessageType ::=CHOICE {

csfbParametersResponseCDMA2000dlInformationTransferhandoverFromEUTRAPreparationRequestmobilityFromEUTRACommandrrcConnectionReconfigurationrrcConnectionReleasesecurityModeCommandueCapabilityEnquirycounterCheckueInformationRequest-r9spare6NULL,spare5NULL,spare4NULL,spare3NULL,spare2NULL,spare1NULL}

• IfLogicalChannelID==3-10(DRBs):DedicatedTrafficChanel(DTCH).Datatraffic

SentbeforeRRCChannelissetup

SentafterRRCChannelissetup



DownlinkProtocolLayersandChannelMappingineNBRRC

ROHC ROHC

Encryption Encryption

Sequence Number

Sequence Number

PDCP

RLC

MAC

PHY

RRC/Data

Segmentation

Acknowledged Mode (ARQ)

Segmentation


DCCH

LCID1 LCID2

Integrity Protection

Encryption

Sequence Number


Encryption

Sequence Number

Segmentation Segmentation

UnAckMode

UnAckMode

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)

CCCHLCID0

PDSCH, Temporary C-RNTI

RRC Radio Resource ControlPDCP Packet Data Convergence ProtocolRLC Radio Link ControlMAC Medium Access Control

Scheduling

Priority Handling

HARQ

Multiplexing of MAC SDUs



ROHC ROHC


Sequence Number

Sequence Number

PDCP

RLC

MAC

PHY

RRC/Data

Segmentation


Segmentation


DCCH

LCID1 LCID2


Encryption

Sequence Number


Encryption

Sequence Number


UnAckMode

UnAckMode

LCID3 LCID4

DTCH


PDSCH, C-RNTI

Transparent Mode

(Buffer)

BCCH

PDSCH, SI-RNTIPBCH

Transparent Mode

(Buffer)

PCCH

PDSCH, P-RNTI

Logical Channels


Transparent Mode

(Buffer)

CCCHLCID0



Scheduling

Priority Handling

HARQ





Source: Netmanias



ROHC ROHC


Sequence Number

Sequence Number

PDCP

RLC

MAC

PHY

RRC/Data

Segmentation


Segmentation


DCCH

LCID1 LCID2


Encryption

Sequence Number


Encryption

Sequence Number


UnAckMode

UnAckMode

LCID3 LCID4

DTCH


PDSCH, C-RNTI

Transparent Mode

(Buffer)

BCCH

PDSCH, SI-RNTIPBCH

Transparent Mode

(Buffer)

PCCH

PDSCH, P-RNTI

Logical Channels


Transparent Mode

(Buffer)

CCCHLCID0



Scheduling

Priority Handling

HARQ


RLC Header

n n+1 n+2 n+3

RLC Header

RLC SDU

RLC PDU



ROHC ROHC


Sequence Number

Sequence Number

PDCP

RLC

MAC

PHY

RRC/Data

Segmentation


Segmentation


DCCH

LCID1 LCID2


Encryption

Sequence Number


Encryption

Sequence Number


UnAckMode

UnAckMode

LCID3 LCID4

DTCH


PDSCH, C-RNTI

Transparent Mode

(Buffer)

BCCH

PDSCH, SI-RNTIPBCH

Transparent Mode

(Buffer)

PCCH

PDSCH, P-RNTI

Logical Channels


Transparent Mode

(Buffer)

CCCHLCID0



Scheduling

Priority Handling

HARQ



ExampleofIDsinthePDCCHandmessageinPDSCH

MIB

SIB-1 SIB-2

SIB-5

Random Access Preamble

Random Access Response

RRC Connection Request

RRC Connection Setup

RRC Connection Setup Complete

DL Info Transfer (NAS: Authn Req)




ID in PDCCH MAC Packet

SI-RNTI SIB

RA-RNTI RAR

C-RNTI LCID1

LCID29

Timing Advance

NAS Message Authn Request

RAPID, Uplink Grant, TC-RNTI

TC-RNTI LCID0

LCID31

LCID28

UE Contention Resolution ID

RRC Connection Setup Pad

RRC Connection Req Msg

…

RRC Connection Request UE-Identity randEstablish-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, Scheduling info 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


CellReferenceSignals(CRS)• CellReferenceSignals

Ø Knownreferencesignalsareinsertedatregular intervalswithintheOFDMtime-frequencygrid.

Ø Therearefour resourceelementsperresourceblockthatarededicatedtoReferenceSignal.

7 symbols = 0.5 ms(Slot)

12 s

ubca

rrier

s =

180

kHz

Æ The location of Reference Signals depends on the Physical layer cell identity of the cell.

Æ The Primary and Secondary Synchronization Signals the Physical Layer Cell Identity Resource Elements used

for Reference Signal


ReferenceSignalReceivedPower(RSRP)

• TheRSRPistheaveragepower(inwatts)receivedfromasingle ReferenceSignal(RS)resourceelement

• RSRPmeasuresonlytheRSpowerandexcludesallnoiseandinterferencepower.• Knowledge ofabsoluteRSRPenablesmobile tocalculatedownlinkpath-loss.• ThemaximumRSRPisbasedonmaximuminputpowertoUEof-25dBm(0.0032

mWatts).In1.4MHzBWwith6RBs(72ResourceElements),maxRSRPis-44dBm.• Theminimum valueis-140dBm (has6dBm ofmargin fromminimumpossible

receivedpoweratUE).


12 s

ubca

rrier

s =

180

kHz

Resource Elements used for Reference Signal𝑅𝑆𝑅𝑃 =

1𝐾P𝑃QR,.

S

.T"

where,Prs,k istheestimatedreceivedpower(inWatts)ofthekthReferenceSignalResourceelement


Measurement2:ReferenceSignalReceivedQuality(RSRQ)• RSRPdoesnotgiveanindicationofsignalquality,i.e.thestrengthofthereferencesignalcompared

tooverallenergyinthechannel(akareceivedsignalstrengthindicator(RSSI))• TheRSSIparameterrepresentstheentirereceivedpower includingthewantedpowerfromthe

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)


12 s

ubca

rrier

s =

180

kHz

Resource Elements used for Reference Signal

RSSI is measured only in OFDM symbol containing the RS


Summary:DLRadioFrame

Radio Frame

10 ms

Radio Frame10 ms



Frequency

Time




62 s

ubca

rrie

rs

72 s

ubca

rrie

rs



3 M

Hz


Physical Control Format Indicator Channel (PCFICH)



OFDMinmulti-colorFrequency

Time




62 s

ubca

rrie

rs

72 s

ubca

rrie

rs



3 M

Hz


Physical Control Format Indicator Channel (PCFICH)


Radio Frame

10 ms

Radio Frame10 ms


References

• Specifications:Ø TS36.300:RANArchitectureØ TS36.331:RRCØ TS36.323:PDCPØ TS36.322:RLCØ TS36.321:MAC

• OtherReferences:Ø LTEinBulletsØ www.sharetechnote.comØ www.youtube.com/lte4g

Technology

LTE Radio Overview: Downlink