IN tsUI,],ETS

I,ONG TERM EVOLUTION (LTE)

2]..1 RANDOM ACCESS

Thr: random access procedure is required when:

o making the transilion from RRC ldle modc 1o RRC Connected mode

o complcting an intra-system handovcr

o uplinl or downlinl data arrjves whilc the UE is in the 'non-synchroniscd' RRC Connccled mode state

o re-esl.ablishing an RRC connection

The random access procedure can be eithcr contention based or non-contcntion bascd. The contention hased procedure

involvcs the UE selecting a random access resource, whereas the non-contention based procedure involves thc eNode Ballocating the random access resource. The conlention based procedure can bc used for all random acccss reasons. J he

non-contention based procedure can be used for intra-systcm handover and the anival ofdownlink data

Th€re are two possibilities for the contention based random access procedure. These arc illustrated in Figure 84.

o RRC connection establishment and RRC corurection re-establishment procedures: the initial layer 3 message is

transfe[ed on the CCCII logical channel, conlenlion resolution is based upon the receplion of a ContenlionResolution Identity (CRI) MAC control clenent, and a new C-RNTI is allocated

o intra-system handover and the arrival of uplink or downlink data while non-synohronised: thc initial laycr 3 mcssage

is translerrcd on the DCCH Jogical channel and contention resolution is based upon the reception ofa PDCCIJ whoscCRC bits havc been scramblcd by the ahcady allocated C-RN II

lntra system handoverArrival of uplink ot dawnlink data while non synchranised

RRC connection establishmenl

Figurc 84 Signalling for contcntion based random acccss proccdurc

'l he contention based procedurc starts with the UFI selecting a set ofresources for the PRACII in tcrms ola preambJc

scquence. and the next available subliame for PRACH trarsmission

The preamble sequence is uscd to differcntiate between multiple LrE using th(] same subframe. There is a total of 64preamble sequences which can be divided into groups A and B. Seclion l3.l describes the rules lor selectjng a prcamble

sequence from the two groups

The next available subframe is defincd by the I'I{ACH configuration index broadcast wilhin SlB2, or signallcd to the Ullwithin an RRC Conncction Reconljguration message. Table 86 within scction l3.l presents the relationship bclween thePRACH configuration indcx ard the sct of subframes which can be used for PRACH transmission

CCCH/UL.SCH/PUSCH

/ PDCCH i i

3 Messaqe DCCH / LJL-SCH / PUSCH

i

1u6

t$-

IN BULLETS

* The UE procceds to transmit the PRACI I preamblc using a transmit powcr defincd by:

PRACH Preamble 'l ransmit Power = min{p6yay, pl, + preambicRrT argctpower) )

where,

P6y,11 is the UE maximum transmit power according to thc tJE powcr cla-ss, c.g. 23 dBm for power class 3

PL is thc downiink path loss calculated by the UE using a combination ofRSRP measurcments and knowlcdse ofthcReference Signal transmil power, i.c. PL : Reference Signal transmit powcr - RSRp measurcment

PreambleRxTargelPower: InitialRx'IargctPower + DeltaPreamble .i (preambleCounter l) x RampingSteprr here.

InitialR\TargctPower is broadcast wjthin SlB2, or sent to the UII within an RRC Connection Reconfisuralr'onmessage. Its value can range lrom -120 dllm to -90 dBm

DehaPreamble defines a power ofiset which is dependent upon the random access preamble format. The preambieformat is identified by the PRACH configuration index presented in Table 86. Prcamble lbrmats 0 and 1 use a 0 dBpower offset, whereas prcamble formats 2 and 3 use a -3 dts power offset

PrcambleCounter is a counter maintaincd by the UE. Its value is initialised 1o I a1 thc start ofthe rartlom accessprocedure. lt is incrementcd by I if no response is received aflcr transmitling a PRACH prcamble. It is used as amuitiplying factor to increasc thc preantble transmil power after receiving no response. It is also used to delerminewhen thc miuimum allowed number of preamble fansmissions has becn reached

Ranpingstep is broadcast within SIB2. or senl to thc UE within an RRC Conncction Reconfiguration message. Itdetermines the ratc a1 which the preamble transmit power is increascd afler receiving no response. 1he step sizc canbe configured wilh a vaiue of0, 2, 4 or 6 dB

* l-he structure oI'the PRACII preamble is prescnted in scction 13. L It occupies I, 2 or 3 subframes jn thc time domain (1, 2or 3 ms), and 839 subcariers in the liequency domain ( I .05 MI lz). There is a I 5 kllz guard band either side ol thepreamble so a total of 1.08 MHz is used (6 Rcsource Blocks)

* After transmjtting the PRACH preambJe, the UE searches for a rcsponsc during thc time domain window defined by rheRandom Access (RA) responsc window. The RA response window s1afts during the third sublrame after the preamble, andhas a length defined by the responsc window size whicl is broadcast in SIts2. or car bc signallcd ro the Ull wjthjn an I{RCConncction Reconfiguration mcssagc. The response windorv size can be configured as 2. j,4. 5. 6.7. 8 or l0 subfiames.An exampie response window is shown in Figurc 85

Random AccessPreamble Transmrssion'

Random AccessResponse Window

e.g. l suorrames eg 5 Subirames

Iigure 85 Random acccss response window

* The UE checks cach suhftame within the search window lor a PDCCH whosc CRC bits have bcen scralnbled by therelevanl RA-RNTL There is a one-to-one mapping bel$,ccn RA-RNTI and the time/frequency resources used by thePRACIJ preamble. The RA-RNTI associated with a specific preamble is defined by:

RA-RNTI- l+t id+ (l0xf id)

Wlere t id is the index ofthc subframe within which the start olthe preamble was transmitted (0 < t id < l0). and f id isthc lrequency domain indcx ofthe PRACH within thal subJiame (0 :, f id < 61. ln the case of fDlJ, there is amaximum ofone set of Resource Illocks allocated lo the PRACIJ within each subframe so f id alrvays equals 0 ind the cquationsimplifies to:

RA-RNI r (fDD)= I +r id

i.c FDD PRACII preambles can be associaled wilh R \-RNTI values of I to 10. All IJE using the same subframes forPRACI I preambie transmission share the same IL{-RNTL These UD are difl'erentiatcd by thcir preamble sequcnce.Contenlion oocun if multiplc UE havc selected the same preamble sequencc fbr transmission during the samc se1 ofsubframe s

e.g.2 subframes 5 subframes

187

I,ONG TERM EVOLUTION (LTE)

Figurc 86 illustrates 3 UH transmilting prcambles slading jn subftame 2 (sharing ILA-RNTI : 3), and a f'urther ? I )l'l

transnitrinS preambles starting in subframe 7 (sharing RA-RN'I l = 8). [ach tJE can bg dillcrentiated by its RA-RNTI ani]

prcamble sequence combination. f'rcamble sequcnccs arc idenlified by their Randorn Access I'reamblc Idcnlity (RAPID).

l here is rro contcntion because all Ull sharing the Same subfiamcs use different prcamble sequenccs

Iijgurc 86 iiiustratcs that the rcsponsc windorvs lor thc 2 scts ofUI ovcrlap but cach scl of IJE attempts to find its own

RA-RNTI within a PDCCI I. Response windows only overlap when thc1, are associatcd with diflcrent RA-RNTI. l he

miuimum windou, size of l0 subframes means thal search wjndows associalcd with the same RA-RN I I never overJap

Radio FramePreamble

TransmissionRandom AccessUPI'nI Pr"ambles

uE4 3 1

uE5rym:#r:;8

Response window for UE 1 to 3

Response window for UE 4 and 5

PDCCHDownlink

PDSCH

RandomAccessResponses *""0"Il;ET""lo":o-*"

l'igurc 86 - Random arccss responsts frrr 5 Llf using 2 \ets of random access subframcs

lfrhe IJE docs no1find a PDCCII rvith its IdA-RN]'l rvilhin the rcsponsc windorv, or ifthc UE finds a I'DCCII with i1s

RA-RN]'l bul the associated Random Access Itesponsc (RAR) within th€ PDSCI I does not include any informalion

addressed to the UE, tben the UE:

o incrcmcnts the value of PrcambleCounter by 1

o chccks whethcr or not thc maximum allowed number of preamble transmissions has been reachcd. Thc UE cxils the

random access procedurc if the maximum numbcr of tra.nsmissions has been reached. Tbe maximum allowcd nurnber

oflransmissions is broadcast within S1ts2. or can bc signalled to the Ull wilhin an RRC Corurection Reconliguration

message. Its value can bc 3, 4, 5, 6, 7, 8, 10, 20, 50 100 or 20t)

o waits until lhc backolftimer has cxpired. The backolltimer is set to a valuc of0 ms at the start oflhc random access

procedurc, so by default a llE docs not have to wail before identilling the next subframc for PRACIJ lransmission.'l he eNodc R can increase the valuc ofthe back-ofltimer by attaching an En /R/lt/Bl subheader to a Random Acccss

Responsc (RAR) message on the PDSCH. This may be done during pcriods olcongestion. lhe backofftimer is

signalled as a Backoff lndicator (BI) which hm a lcngth of 4 bits. Thc rclationship between the signalled BI and thc

actual backol'f timer is prcsentod in Table 1 1?

o selecls another PRACII resourcc (preamble scquence and subframc) lor the next PRACH transmission

uE13 5

u,,Qm:4,:;,uE3{fr H!,!'!;'

RI Backoff

0 0ms

1 l0 ms

). 20 ms

-l 30 ms

BI Rackoff

4 40 ms

5 60 ms

6 80 ms

7 120 ms

BI Backoff

8 160 ms

9 240 ms

t0 320 ms

I] 480 s

BI Backoff

1?, 960 ms

l3 rcscrved

14 rcsen'cd

t5 resened

t..1

3.?

.d,]?I';I:$

1#'I

:{ll

!i:!;

't,,:

':-.;

ii

a

l

d

'l'able ll7 Relationship berw-een signallcd Backoff Irdicator (Bl) and hackofftimcr

188

IN BULLETS

lfthe UE finds a PDCCH whosc CRC bits havc been scrambled by its RA-RNTI then i1 proo(rsds to read thc content oftheDownlink Control Inlonnation (DCl) within the PDCCll. I)Cl Iormats lA and lC can have thcjr CIIC bits scramblecl byan RA-RN'fl. I'he downlink resource aliocation inlorrnation is rcad to identif) the position ofthe Random AcccssResponse (RAR) wilhin the I'DSCH of that subliame

The structure ofthe subhcaders and payload belonging to a RAR on the PI)SCH is illustrated in |igure 87

E/I/R/R/BJsubheader(8 bits)

E/T/MPIDsubheader(8 bits)

RandomAccessResponsepayload(48 bits)

0 or 1 instance

E T RAPID 0 to n instances

Figure 87 - Format of subheaders and paylord for Random Acccss Response (RAR) message on I'DSCll

l'he E,{-,&-4VBI subheader is included ifthe eNode B wishes to dcfinc a backolftimer. It is appJicable to all UII with therelevant RA-RN'I l. I'here can be a maximum of I jnstance ofthc E/T/R {l,4fl subheader within a RAR messagc

The E/l /RAI'ID subhcader is used 10 address specilic tJE. I'herc is I instancc olthis subheader for each LJE bcingaddressed. I-lvery instancc ofthe E/l'lRAPID subheader has its own instance ofthe RAR payload

The various fields within thc RAR subheaders are:

o the Extension (E) ficld indicates whether or not there are any further subheadcrs

o thc Type (T) field indicates the lilrmat of rhe su bhcader (E/ l /R/R/BI or E/l /RAPID)

o thc Rcserved (R) field is not uscd

o the Backoff I ndicator (BI) ficld is used to signal the backofltimer

o the Random Acccss Preamblc ldeDtiry (RAPID) field addresses a UE by thc index of its prcamble sequcnce

There is an instance ofthe LAR payload for every instance ofthe IllT,{tAPID subheader. 1'he payload includcs:

o Timing Advance inlormation to ensure that subsequent UE lransmissions are synchronised wilh othcr UE whenarriving at the eNode B. Tining Advance is describcd in seclion 21.2

o tJpiink Grant infbrmation to allocate upJink Resourcc Blocks to the UII for transmission on the PUSCH. For exarnple,when establishing an RRC connection, this allocalion allows the UD 1() transmit thc RRC Conncction Requestmessage. 1'he Resource Block allocation is signalled using the approach describcd in section 9.3

o a Tcmporary C-RNTI which becomcs thr: C-RNTI after successful oompletion ofthc random acccss procedure lorRRC conneclion establishment and RRC-' conncction rc-establishment

l he overall structure of an exampie RAR mcssage is illuslrated in Irigure 88. This example includes an E/T/R/R/B]subheader to define a backoff timer for all tJE with th€ rclevanl RA-RNTL lt also inoludes 3 instances ofthe E/T/RAPIDsubheader and a corresponding 3 instances ofthe RAR payload

Figure 88 liramplc Randon Acccss Rcsponse (RAR) message

E T R R BI

189

I,ONG TERM F]VOLUIION (LTI])

lfthe lJIl identifics an E/T/RAPIl) subhcader u'ith the appropriate random access prcamble idcnlity. thcn il rcads ti'rc

corresponding instancc of th(] RAI.L mcssagc pa-vload and identifies the allocated uplink llcsource lllocks for transmission

on the I,tJSCil. 'l hc UH then procccds to transmil its init;al laycr 3 ntcssage using a transmit powcr dcfined by thc

equa(ion prcsented in seclion 2I.4.I withk=2

The initial layer 3 mcssage can bc transmitted using eitlier the cccH or DCCI I logical channels:

o RRC connection establishmcnt: RRC Conneclion Requesl mcssage is senl using thc CCCH

o RRC connection rc-cstablishment: RRC (lonnection Re-eslablishmenl Request mcssago is scnt using lhe CCICII

o intla-Syslem handover: RRC Connection Reconfiguration Complete mcssagc iS Sent using 1ho DCCII

o arrival ofuplink or downlink data whilc non-synchronised: Uplink lnformation 1'ransfcr mcssage using the DCCII

Thc tJplink Information Translbr message contains a Non-Access Stralum (l'JAS) Servjcc Request mcssage

Whcn usjng th€ CCCH, the Temporary C-RNTI is used as an inpul when scrambling the PUSCI I physical laycr bits p or

1o modulation. Whcn using the DCCH, the existing CI-RNTI is used as an input

'l'he C-RNl'l MAC control elemcnt is included as pafl of thc MAC headcr *'hen using the DCCH to transfer the initial

Iayer 3 messagc. I'his control elemenl uses l6 bils to specili thc existing C-RNTI associated with the tJE

The final stage ofths random acccss procedure is contentjon resolution. lhis stage is used to determinc whethcr or not

multiple UE-used tle same combinalion of l{A-RNl l and preamblc scquence. Ifcontention occuned then multiple UIi

would have read the same section of payloacl within the ILAR message and would have transmittcd on the same set oluplink Rcsource Blocks

'l he UE slarts a oontention resoiution timer after transmitling thc initial laycr 3 messagc. 'I he contention resoiution timer is

broadcasl in SlB2. or can be signalled directly to the UE \\,ithjn an RRC Connecljon Reoonfiguration message. It can be

configured with values of8, 16, 24- 32, 40,48, 5 6 and 64 subframcs. If the UII does nol receive a responsc from thc eNode

Il belirre thc contention resolution limer cxpircs thcn the UE retums to lransmitting I'li-ACH preanbles

lfthe UE senl lhc initial layer 3 message using the CCCIII then contention rcsolution is bised upon thc eNodc B

rcsponding with a UE Contcntion Rcsolution ldentily (CRl) MAC conlrol element. In this case, the LJII scarches lor a

pOtCg wtrose CnC bits have been scrambled by thc l cmporary C-RNTI. l'his I'DCClll directs thc UE 1o a dou'nlink

Resourcc Block allocalion within the same subframe. Thcse PDSCH Resourcc Blocks include a combination ofthc

R/tt/ll,,T.ClD MAC subheaclcr and CRI MAC oontrol elemcnt.'lhe structure of this PDil is illustraled in Figure 89

RlRlEl LclD(11100)

UE Contentior Resolution ldentity

UE Cofltention'Resolut'icn ldentity

UE Contention Resolution ldentity

UE Contenhon Resolution ldenlity

UE Coitention Resolution ldenlity

UE Contention Resolutbn ldentity

l'igurc 89, R/R/En,CID It C subheader with a L'E Contcntion Resolution ldcntity }{-AC control clcment

The Logical Channel ldcntily (LCID) value of I I 100 idcntifies thc subsequent CRI MAC controi elemenl. l'he control

olemeniitself includes a reflection ofthe initial iayer 3 message sent by the l,lE using the CCCH

lfthe UE nanagos 1o successfully dccode the MAC PDti and identify its own inilial laycr 3 messag€ thcn rontention

resolution ancl the ovcrall random access procedure is successful and the Temporary C-RNl') becomes thc C-RNTl. Ifthe

UE fails to decode the lr4AC I,DU, or docs not find i1s own initial layer 3 Inessage within the MAC cortrol elcmcnl tbcn

conlention rcsolution lails and the UB returns to transmitling PRACII prcambles

If the LlE sent the initial laycr 3 mcssage using the DCCH thcn contention resolution is based upon the cNode B

responding with a pDCCH whose CRC bils have bcen scrambled by thc C--RN']1 specjficd by the UII within thc prcceding

C-flN'f f fr,tlC control element. If thc UE manages 1() successfully decode a PDCCIT insociatcd with its C-RN'I l then

contention resolution ard t11e overall random access procedurc is successf'ul and the lemporary C-RN'I l is discarded lfthe t.JFl fails to idcntifl a PDCCI I associated with i1s c-RNTI bclorc the contcnlion resolution timer expires, then

contcn{ion resolution fails and thc UL returns to lransmitling PRACIl preamblcs

MAC subheader(8 bits)

MAC ControlElement(48 blts)

190

IN BULLETS

* Thc non-contcntjon bascd random access procedure avoids the possibility of multiple UE selecting the samc PRACIIresourcc (subframe and preamble sequence). This is achieved by thc eNode B instructing the UE to usc a specific resourccwhich is outsidc the pooi available to UE compl€ting thr; contention based random aoccsi procedure, i.e. some ofthe 64preamble sequenccs can be reserved for allocalion by the cNode B

* There are two possibililies for fie non-contention based random acccss procedure. Thcse are illustratcd in Figure 90.

o intra-system handover: the PRACH resource is signalied 10 thc UE within an RRC Connection Reconfigurationmessage. The initial layer 3 message is an RRC Corurection R€configuration Complete message on the bCCII logjcalchannel

o arrival ofdownlink data while non-synchronr'scd: the PRACH resource is signalled to the UE withit a DownlinkControl Inlormation (DCI) lormal I A PDCCtl. The initial iayer 3 message is an Uplink Information Transfermessage on thc DCCH Iogical channel

lntra-system handover Arrival af downlink data while non,synchrcnised

]UEUE eNode B

ri Randon

PRACH Preamble

PRACH Preamble PRACH Preamble

Resource allocation for RAR / PDCCH

I j Layer 3 Message DCCH / UL-SCH / PUSCH ii-: 'RRC Connectton Recontlquraltor CompJetei

i Layer 3 Message DCCH / UL-SCH i PUSCH l

i I Uplink lnformation Transfer - Service Request

Iigure 90 Signalling for non_contentioD bascd random access procedure

Once the UE has been inslructcd to initiate thc random access procedure using a specific pRACH resource- pRACHpreambles are transmilted in thc same way as lbr the contention based procedurc. Also similar to the contention basedprocedure, the UE searches for a PDCCH whosc cRC bits have bccn scrambied by the relevant RA-RN.l-l

l'he non-contention bascd random acocss procedure is successfui as soon as the IJE receivcs a Random Access Rcsponse(RAR) message which inciudes an E/T/R PID subheadcr with the appropriate Random Access Prcamblc ldcnlily(RAIIII) The []E can then proceed to transf€r its layer 3 message using thc uplink Resource Block allocation signalJedwithin the RAR

3GPP References: 1'S 36.213. 1S 36.321

Reso{Jrce €location t$ RAR I pDCcH

Rendom Acess Response (FIAR) / DL-SCH / p&SCi

191

LONG TERM EVOLI]TION (LTE)

2L.2 TIMING ADVANCE'l iming advancc is used to control thc uplinJ< transmission tirning of individuai UFl. It helps to cnsurc thal transmissionsfrom al1 Uli are synohroniscd whcn receivcd by the cNode 13

Thc gencral concept oltiming advance is shown in Figurc 91. This ligure illusl.ates 2 tJE. The UFI furthest from thc eNodeB requires a larger timing advance to compcnsate for thc larger propagation delay

firI

I

r*p

Tining Advance = Na x I" seconds i

0 < NrAa2A512

Large timing advanceTs-1/30720ns

liigure 9l - Conc€pts of timing advance and uplink / downlink radio framc timing

i' Figure 92 iilusfrates the radio framc timing lbr the 2 UE shown in Figurc 91. The downlink radio frame arrives at UE lrelatively latc as a rcsult ofths larger propagation delay. This figure illusfates that the timing advance equals 2 xpropagation deiay

UE 1 wilh largepropagation delay

UE 2 with smallprcpagation delay

3ffi,il

Timing advance =2 x prcpaaatian delay

at eNode B

atUEl

atUEl

at eNade B

atUE2

Radio franessynchronisedat eNode B

Timing advance =2 x propagation delay

Uplink Radlo Fnne- UE 2

Uplink Radio Fmme- UE 2 at eNode B

liigure 92 - Concepts of timing advancc and uplink /downlink radio frame timing

Thc maximun liming advaxce supporlcd by the 3GPP specifications is 20i12 /30720:0.6677 ms. Based upon thc speedoflight (3 x 108 ms r), this allows a maximum propagalion distance of 100 km

UE iLrc first provided rvilh timing advancc inlbrmation during the Random Acccss procedure uscd to make the transilionfrom RRC Idlc mode to RRC Connected mode. An I I bit timing advancc oommand is included within the Random n ccessIiesponslj

]'hese I I bits arc used 1o signal a value betwecn 0 and I282. Thc variablc N11 : signallcd valuc x 16. while the actualtiming advance : NrA x Ts. where the value ofT, is givcn b1, I / 30720 ms

A1ler the Random Acccss procedure, timing advance commands are provided using the fiming Advance MAC Controllllement which can bc included as part oflhe MAC headcr. 1he Timing Advance MAC Control Elemcnt includes a 6 hirtiming advancc command which provide a rangc from 0 to 63

Uplink Radio Frame- UE 2

synchronised al the eNode B

Uptink Radio Fnme- UE 2

Smalltining advance

192

IN BTILLETS

* The signalied vaiue ofthe timing advance command within the MAC Control Element corresponds to IA within theequation:

NrA,,"* = NrAdd + (To - 31) x l6Subtracting 3l from the value ofTa allows the eNode B to shift the timing advance in both positive and negativedirections, i.€. the timing advance command provided during the Randomlccess procedure is an absolute timing advance,whereas the subsequent timing advance commands provided within the MAC Co#ol Elements are Jative and definechanges to the existing timing advance

* Timing advance commands received during downlink subframe 'n' are applied to uplink subfram€ ,n+6'

* when a timing advance command causes subfiame 'm+l' to overlap with sublrame 'm' the l,rB fansmits all of subframe'm' but does not transmit the overlapping part of subfiame.m+l'

* 3GPP References: TS 36.21 l, TS 36.213, TS 36.321

193