Random Access R

8/11/2019 Random Access R

http://slidepdf.com/reader/full/random-access-r 1/40

Lte Random AccessRaymond Wang



Page 2

UE Power On, Then…?

› Before an LTE terminal can communicate with an LTEnetwork it has to do the following:

1. Find and acquire downlink s nchroni!ation to a cellwithin the network.

2. "ecei#e and decode the information$ also referred to asthe cell system information $ needed to communicate withand o%erate %ro%erl within the cell.

&. 'nce the s stem information has (een correctl

decoded$ the terminal can access the cell ( means of theso)called random-access procedure $ get u%links nchroni!ation.



Page &

Agenda

› *ell +earch› + stem information decoded› "andom ,ccess



Page -

Cell Search

› To assist the cell search$ two s%ecial signals aretransmitted on the LTE downlink$

1. Primary Synchronization Signal P++/2. Secondary Synchronization Signal +++/



Page 0

Cell search proced re

”Cell search procedure ”› etect P++! 0 ms timing found! *ell within *ell grou% found 1 out of &/

› etect +++! Frame timing found! *ell grou% found 1 out of 134/

› Frame timing found › Reference-signal structure

found › BCH location found

Possible to read BCH

15 ms radio frame

1 ms su(frame

65 61 62 6& 6- 6360 67 64 68

'F 9 s m(ol+econdar

s nchroni!ation signalPrimar

s nchroni!ation signal

5.0 ms slot 5.0 ms slot 5.0 ms slot 5.0 ms slot

5 1 2 & - 5 6 5 1 2 & - 0 3 7 2

s u

( c a r r i e r s

5 1 2 & - 5 6 5 1 2 & - 0 3

s s

t e m

( a n

d w

i d t h



Page 3

Cell search proced re


http://slidepdf.com/reader/full/random-access-r 7/40Page 7

PSS and sss

P++ and +++ are ma%%ed onTimeslot 65 and Timeslot 615$ in thecenter 3 "Bs. P++ and +++ aretransmitted on 32 "Es$ 0 "E on eachend used for guard (and/

P++ is ma%%ed on 'F 9 s m(ol 63;

+++ is ma%%ed on 'F 9 s m(ol 60.



PSS str ct re

› 'n a more detailed le#el$ the three P++s are three length)3& <adoff=*hu <*/ sequences e>tended with fi#e !eros at the edges andma%%ed to the center 7& su(carriers$ t should (e noted though that thecenter su(carrier is actuall not transmitted as it coincides with the *su(carrier.

› Thus onl 32 elements of the length)3& <* sequences are actualltransmitted.› The P++ thus occu%ies 72 resource elements not including the *

carrier/ in su(frames 5 and 0 F /.



SSS str ct re

› +imilar to P++$ the +++ occu%ies the center 72 resourceelements

› The two +++ +++ 1 in su(frame 5 and +++ 2 in su(frame0/ should take their #alues from sets of 134 %ossi(le #alues

corres%onding to the 134 different cell)identit grou%s.› The set of #alues a%%lica(le for +++ 2 should (e different

from the set of #alues a%%lica(le for +++ 1 to allow forframe)timing detection from the rece%tion of a single +++.



Agenda




Page 11

System "n#ormat"on

› n LTE$ s stem information is deli#ered ( two differentmechanisms rel ing on two different trans%ort channels:

› ? , limited amount of s stem information$ corres%onding tothe so)called aster !nformation Bloc" 9 B/$ is transmitted

using the B*@.› ? The main %art of the s stem information$ corres%onding

to different so)called System !nformation Bloc"s + Bs/$ istransmitted using the downlink shared channel L)+*@/.



Page 12

$%&

› Ahere is the 9 B› ) Transmitted on B*@› ) Transmission %eriod C -5 ms› ) B*@ trans%ort (lock is ma%%ed to the first su(frame of

each frame in four consecuti#e frames.› ) con#olutional coding$ suit for small trans%ort (lock› ) 9odulation: #uadrature Phase-Shift $eying DP+ /



Page 1&

$%&



Page 1-

$%&

› Ahat can get from 9 B› - !nformation a%out the do&nlin" cell %and&idth. '( %its)› - !nformation a%out the PH!CH configuration of the cell.› - *he System Frame +um%er +F / +F is 15 (its

5G152&/$ (ut 9 B onl trans%ort all (its e>ce%t the twoleast significant (its of the +F $ wh /

› ) The num(er of cell)s%ecific antenna %orts @ow /



Page 10

Why only ' ("ts o# S)*?

› From the initial cell search$ the terminal has found onl thecell frame timing. Thus$ when recei#ing B*@$ the terminaldoes not know to what set of four frames a certain B*@trans%ort (lock is ma%%ed to. nstead$ a terminal must tr todecode the B*@ at four %ossi(le timing %ositions. ,nd +Fwill onl change in the edge of e#er -5ms. e%ending onhow man PB*@ frame decoding is successful$ theterminal can know the two least significant (its of the +F .This is the reason these (its do not need to (e e>%licitlincluded in the 9 B.



Page 13

+ow to now the Antenna * m(er

› ,ctuall $ if two antenna %orts are a#aila(le within the cell$+FB* must (e used for B*@. +imilarl $ if four antenna%orts are a#aila(le$ com(ined +FB*HF+T must (e used.Thus$ ( (lindl detecting what transmit)di#ersit scheme isused for B*@$ a terminal can$ indirectl $ determine thenum(er of cell)s%ecific antenna %orts.

› +FB*: +%ace) Frequenc Block *oding

› F+T : Frequenc +witched Transmit i#ersit



Page 17

System %n#ormat"on

› + B1 ,ccess related %arameters e.g. Ahether IE is %ermitted to cam%on the cell/; +cheduling details for other + BJs; *ell dentit = uniqueidentit 24 (its/

› + B2 ,(out the u%link cell (andwidth$ randomaccess› %arameters$ and %arameters related to u%link %ower control+ B& *ell

reselection information

› + B- ntra frequenc LTE neigh(ors ) non standard configurations› + B0 nter frequenc LTE neigh(ors› + B3 ",T cell reselection to IT",› + B7 ",T cell reselection to K+9› + B4 ",T cell reselection to * 9,2555

› + B8 @ome e ode B information› + B15 ETA+ Primar otification› + B11 ETA+ +econdar otification› + B12 *9,+ notification› + B1& 9B9+ control information





Page 18

*oted a(o t S%

› ifferent + s ha#e different non)o#erla%%ing time windows.Thus$ a terminal knows what + is (eing recei#ed withoutthe need for an s%ecific identifier for each + .



Page 25

Agenda




Page 21

Random Access

› n the LTE network$ the IE uses the ", %rocess to gainaccess to cells for the following reasons:1. nitial access to the network from the idle state2. "egaining access to the network after a radio link failure

&. ,s %art of the hando#er %rocess to gain timings nchroni!ation with a new cell

-. "egaining u%link time alignment.0. "equest u%link grant for IE without dedicated scheduling

request resource.



Page 22

$o("le Term"nated Call

Random ,ccess Pream%le

Random ,ccess Response

Scheduled *ransmission ' S ()

Contention Resolution ' S )

1

2

3

4

H,R#

H,R#

*B",



Page 2&

Step . ! Pream(le Transm"ss"on

› "andoml select %ream(le from %ream(le su(set = Pream(le e>%licitl %ro#ided for contention)free random access› 3- %ream(le sequences a#aila(le in each cell = Two su(sets configured$ su(set 6, and 6B wh / = "emaining sequences used for contention)free random access› +u(set selected (ased on amount of data to transmit in second ste%



Page 2-

Why /ro p A 0 &?

› ) f "andom ,ccess Pream(les grou% B e>ists and ifthe msg& message si!e data a#aila(le for transmission%lus 9,* header and$ where required$ 9,* controlelements/ is greater than messageSize roup,

› and if the %athloss is less than P*9, = pream%le!nitialRecei/ed*argetPo&er = deltaPream%le sg( = messagePo&er0ffset roupB $

› then:› ) select the "andom ,ccess Pream(les grou% B;

) else:› ) select the "andom ,ccess Pream(les grou% ,.



Page 20

System %n#ormat"on 1S%& 23

IE reads which "andom ,ccessPream(le s ",P s/ area#aila(le.

n this e>am%le all 3- are a#aila(le/



Page 23

PRAC+ t"me-#re4 ency reso rces

› Frequenc : 3 "B %rach)Freq'ffset in + B2/› Time: 1G&ms› Each su(frame at most one ", region



Page 27

Pream(le #ormat



Page 24

Pream(le #ormat

› The random access signaling is one of the factors limitingthe cell range. Pream(le format 5 is default and %ro#ides ama>imum cell radius of a%%ro>imatel 10 km. The feature9a>imum *ell "ange introduces %ream(le format 1. Aiththe feature$ the cell radius can (e e>tended u% toa%%ro>imatel 155 km with some reduced u%link ca%acit .



Page 28

RAC+ Sched l"ng

1 "B C 145k@!M5.0ms 12 su(carriers M 1 slot$ 12M7 s m(ols/

%rach)Freq'ffset

"adio Frame 15ms

start %oint and s%acing( %rach)*onfig nde>

,lwa s 3 "Bs

Length ( %rach)*onfig nde>

ra)"es%onseAindow+i!e2 9a> tries (efore (ackoff$ %ream(leTrans9a>

Slot allocation

UE RA example



Page &5

Random Access Pream(le 1Step .3

",P -0used for%ream(le



Page &1

Step 25 Random-access response

› Pream(le detected res%onse on L)+*@ = +ent using ",)" T ",)" T C 1 H tNidH15MfNid Ahere tNid is the inde> of the first su(frame of the s%ecified P",*@

5O tNid 15/$ and fNid is the inde> of the s%ecified P",*@ within thatsu(frame$ in ascending order of frequenc domain 5O fNid 3/. For

F fNidC5. = Transmitted within configura(le time window = I%on rece%tion$ IE adQusts its u%link timing› "es%onse contains 1. nde> of detected %ream(le

2. Timing correction calculated from estimated P",*@ timing &. I%link scheduling grant for IL)+*@ -. Tem%orar identifier Tem%orar *)" T /



Page &2

Random Access Response 1Step 23

9,* La er uses ",)" T

IE is assigned Tem%orar *)" T

Timing ,d#ance and schedulinginformation for +cheduledTransmission 9+K &/

",P used returned to IE in 9,* "andom ,ccess"es%onse ","/ control header +ote more than 1 3 could ha/e used the same R,P!4





Page &-

Sched led Transm"ss"on 1Step 63

*)" T 7- used ( the P **@ isunfortunatel not correctl decodedcorrectl here ( Aireshark (ut can(e seen here in the Packets B tesdis%la (elow.



Page &0

RRC Connect"on Re4 est 1Step 63

n the ""* *onnection "equest messagethe IE identifies itself with 99E code

mmec/ which uniquel identifies the 99E ina %ool and 99E T9+ m)T9+ / whichuniquel identifies the IE in the 99E. f theIE has not (een assigned a T9+ et it will

use a -5)(it random num(er see ne>t slide/.

Tem%orar *)" T

9+K& also carries ""* *onnection "equest message



Page &3

RRC Connect"on Re4 est1)rom UE w"tho t T$S%3

Using a random number avoids sending IMSI over the air



Page &7

Content"on Reso lt"on 1Step 73

9,* la er sends (ack m)T9+ recei#ed

from IE to resol#e %ossi(le contentionas more then one IE ma (e using thesame *)" T

*)" T no longer tem%orar /



Page &4

RRC Connect"on Set p 1Step 73

""* *onnection +etu% message

*)" T

9+K- also carries ""* *onnection "equest +etu%message



Page &8

re#erence

› &KPP &3.151$#4.3.5$IE radio transmission and rece%tion› &KPP &3.211$#4.7.5$%h sical channels and modulation› &KPP &3.212$#4.7.5$multi%le>ing and channel coding› &KPP &3.21&$#4.7.5$%h sical la er %rocedure

› &KPP &3.&&1$#4.3.5$""*



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Random Access R