lte physical and mac layer procedures
Post on 13-Nov-2014
94 Views
Preview:
DESCRIPTION
TRANSCRIPT
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
top related