Understanding Call Flow In

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Understanding Call Flow In-Depth

With Long Term Evolution (LTE) comes a myriad of new and exciting attributes. One of these is the

LTE call flow itself. In fact call flow and signaling is uni!ue for LTE and is driven by "#$$

standards. %all flow is how signaling and sessions are created across an LTE network.

&o how does LTE call flow wor' exactly

ccording to Wired n Wireless the LTE call flow travels through many ste*s during its end-to-end

signalingbetween from user e!ui*ment (+E) to the evolved node , (e-,) mobility management

entity (E) home subscribe server (/&&) serving gateway (W) and $0- gateway ($#W).

It begins with &1 &etu* where the e-, is initially attached to the networ'. The e-, su**orts the

LTE air interface and includes the following functions2

Functions for Radio Resource Management: Radio Bearer Control, Radio

Admission Control, Connection
http://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.htmlhttp://www.rcrwireless.com/mobile-backhaul/lte-network-architecture-diagram.html


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Mobilit Control, !namic allocation of resources to "Es in bot# u$link and

downlink %sc#eduling&

'election of an MME at "E attac#ment w#en no routing to an MME can be

determined from t#e information $ro(ided b t#e "E

Routing of "ser )lane data towards 'er(ing *atewa

'c#eduling and transmission of $aging messages %originated from t#e MME&

'c#eduling and transmission of broadcast information %originated from t#e MME

or +M&

Measurement and measurement re$orting conguration for mobilit and

sc#eduling

s long the e-, is functioning *ro*erly the &1 setu* will stay intact. Once +E comes u* a radio

resource control (33%) connection is established for communication with the networ'. fter 33% is

established networ' attached storage (-&) signaling begins.

+E then sends an attach re!uest along with a $0- connectivity re!uest to the networ'. ttach is for

attaching to the networ'. Once E receives the attach re!uest it !ueries the /&& for

authentication details. /&& then sends the authentication vectors to E in an authentication info

answer. The next ste* in call flow for LTE has to do with authentication and security. The networ'

re!uests +E for authentication vectors. When the +E *rovides the same one E com*ares it with

what /&& has sent. If they match the +E is authenticated. E manages mobility +E identities

and security *arameters. It includes the following functions2

.on Access 'tratum %.A'& signaling and securit

/dle mode "E reac#abilit %including control and e0ecution of $aging

retransmission&

Tracking Area list management %for "E in idle and acti(e mode&

)!. *1 and 'er(ing *1 selection2MME selection for #ando(ers wit# MME

c#ange

Roaming %terminating '3a towards #ome 4''&

Aut#entication Bearer management functions including dedicated bearer

establis#ment

-ext security ta'es over and all -& messages are encry*ted using the security algorithms that

were exchanged. fter the LTE call flow moves through the security ste* the networ' creates the

E$& bearers. Then the radio bearers are created and 33% connections are modified accordingly.

Once these radio bearers are created the e-, down lin' addresses are sent to W in #T$

messages. The &erving #ateway is the node that terminates the interface towards E+T3-. 4or


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each +E associated with the E$& at a given *oint of time there is one single &erving #ateway.

4unctions include2

)acket routing and forwarding

T#e local mobilit anc#or $oint for inter e.B #ando(er

E-"TRA. idle mode downlink $acket bu5ering and initiation of network triggered

ser(ice re6uest $rocedure

E-"TRA. idle mode downlink $acket bu5ering and initiation of network triggered

ser(ice re6uest $rocedure

Accounting on user and 7o' Class /dentier %7C/& granularit for inter-o$erator

c#arging

"L and !L c#arging $er "E, )!., and 7C/

End marker #andling

)acket Filtering wit# TFTIt seems technical but LTE call flow is a technical *rocess5

To further understand this *rocess *lease see the LTE call flow diagram also created by Wired n

Wireless.

What happens while UE frst time switch on?


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Fig89 Basic Call Flow


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4ollowing is a se!uence diagram showing not only the message but also basic configurations of each

layer. ore detailed descri*tion of each layer in the context of full *rotocol stac' will be ex*lained in

6)rotocol 'tack7 section.

After Power on UE


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8& Fre6uenc 'earc# %'can t#e entire fre6uenc bands&

;& Cell 'earc# < will searc# for )'' .ormall a "E would nd multi$le cells in t#is $rocess

=& %a& M/B decoding%b& '/B decoding

>& Cell 'election? Cell Reselection

@& /nitial RAC4 )rocess

3& RRC : RRC Connection Re6uest

& RRC : RRC Connection 'etu$

& RRC : RRC Connection 'etu$ Com$lete D .A' : Attac# Re6uest

& RRC : !L /nformation Transfer D .A' : Aut#entication Re6uest

8& RRC : "L /nformation Transfer D .A' : Aut#entication Res$onse

88& RRC : !L /nformation Transfer D .A' : 'ecurit Mode Command

8;& RRC : "L /nformation Transfer D .A' : 'ecurit Mode Com$lete

8=& RRC : 'ecurit Mode Command

8>& RRC : 'ecurit Mode Com$lete

8@& RRC : RRC Connection Reconguration D .A' : Attac# Acce$t

83& RRC : RRC Connection Reconguration Com$lete D .A' : Attac# Com$lete

8& RRC : RRC Connection Release< MO or MT call > : In MT call, Paging should be

sent

8& RRC : )RAC4 )reamble

8& RRC : RAC4 Res$onse

;& RRC : RRC Connection Re6uest

;8& RRC : RRC Connection 'etu$

;;& RRC : RRC Connection 'etu$ Com$lete D .A' : 'er(ice Re6uest

;=& RRC : 'ecurit Mode Command

;>& RRC : 'ecurit Mode Com$lete

;@& RRC : RRC Connection Reconguration D .A' : Acti(ate !edicated E)' Bearer Conte0tRe6uest

;3& RRC : RRC Connection Reconguration Com$lete D .A' : Acti(ate !edicated E)' Bearer

Conte0t Acce$t

;& RRC : "L /nformation Transfer D .A' : !eacti(ate !edicated E)' Bearer Conte0t Acce$t

;& RRC : RRC Connection Release

!"#$re%uenc& 'earch !'can the entire fre%uenc& bands#When the +E *owers on it

chec's if it is *ossible to cam* on the last registered $L- ($ublic Land obile -etwor'). If no any

stored system on the +E +E will *erform the initial cell selection2

'can all RF fre6uencies band %e0 M4,8M4,8M4G9& and w#ic# band

are using b network

!etect t#e o$erating fre6uenc wit# t#e strongest R'R)
http://tweet4tutorial.com/basic-call-flow-lte/#Frequency%20Search%20(Scan%20the%20entire%20frequency%20bands)http://tweet4tutorial.com/basic-call-flow-lte/#Cell%20Searchhttp://tweet4tutorial.com/basic-call-flow-lte/#MIB%20Decodinghttp://tweet4tutorial.com/basic-call-flow-lte/#SIB%20decodinghttp://tweet4tutorial.com/basic-call-flow-lte/#Initial%20RACH%20Process%20(Random%20Access%20Procedure%20(RACH))http://tweet4tutorial.com/basic-call-flow-lte/#Initial%20RACH%20Process%20(Random%20Access%20Procedure%20(RACH))http://tweet4tutorial.com/basic-call-flow-lte/#Frequency%20Search%20(Scan%20the%20entire%20frequency%20bands)http://tweet4tutorial.com/basic-call-flow-lte/#Cell%20Searchhttp://tweet4tutorial.com/basic-call-flow-lte/#MIB%20Decodinghttp://tweet4tutorial.com/basic-call-flow-lte/#SIB%20decodinghttp://tweet4tutorial.com/basic-call-flow-lte/#Initial%20RACH%20Process%20(Random%20Access%20Procedure%20(RACH))


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Fig 898

(# )ell 'earch * will search for P'' +orall& a UE would -nd ulti.le cells in this

.rocess In LTE there are two downlin' synchroni8ation signals which are used by the +E to obtain the

cell identity and frame timing.

)rimar snc#roniation signal %)''&

'econdar snc#roniation signal %'''&

The division into two signals is aimed to reduce the com*lexity of the cell search *rocess

Priar& and 'econdar& 'econdar& '&nc:%ell synchroni8ation is the very first ste* when +E

wants to cam* on any cell. 4rom this +E ac!uires *hysical cell id ($%I) time slot and frame

synchroni8ation which will enable +E to read system information bloc's from a *articular networ'.4irst

the +E has to synchroni8e with the networ' at thefraeandslotlevel.

i#/etect P'' !Priar& '&nchroni0ation 'ignal# for s&bol tiing

T#e Priar& '&nchroni0ation 'ignalis res$onsible for slotsnc#roniation9


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T#e 'nc#roniation 'ignal are sent onl on tie slots%8st and 88t#& of eac#

frame9

)'' is send at t#e last +F!M smbol of t#e 8st and 88t# slot of eac# frame9

ii# Identi-cation of cell I/ !1,",(#2 4rom $&& +E is also able to obtain *hysical layer identity (9

to :).

Query_1: How does UE know to look or the P synchroni!ation si"nal?

1ell, "E doesnHt need to worr muc# for t#is9 As, t#e snc#roniation signal are alwas sent

onl on t#e center 3; sub carriers irres$ecti(e of t#e c#annel bandwidt# %89;@,=,@,8,;&9

T#erefore, "E will look for t#e central sub carriers, i9e at t#e last +F!M smbol of t#e 8st

time slot and again at t#e last +F!M smbol of t#e 88t# slot9 1it# t#is "E snc#ronies at

t#e slot le(el9

/etect ''' !'econdar& '&nchroni0ation 'ignal# for radio frae tiing:

T#e 'econdar& '&nchroni0ation 'ignalis res$onsible

for fraesnc#roniation9

"sing bot# t#ese signals, t#e "E deri(es t#e )#sical Cell /! of

t#e corres$onding cell9

Question_# : How does UE know to look or the synchroni!ation si"nal?

+nce, w#en t#e )'' is identied, ''' is alwas send at t#e slot before t#e )'' is $resent9 /n

ot#er words, ''' immediatel $recedes t#e )''9

2uestion34 : 5et6s see how the UE deri7es the )ell I/ using these two signals:

From )'': )4I'/CAL LAIER CELL /!E.T/TI is deri(ed9 /t carries t#e (alue of , 8 and ;9

From ''': )4I'/CAL LAIER CELL /!E.T/TI *R+") is deri(ed9 /t can take t#e (alue to to

839

i# /etection of cell I/ grou. !1, ,"89#2 4rom &&& +E is able to obtain *hysical layer cell

identity grou* number (9 to 1;


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)ell Id

Badio and )ore nCw ca.abilities%for itHs n?w selection&

The above information is being broadcast by the networ' in order to hel* the +E for itBs selection *rocess.

This broadcast is 'nown as 'D'TEM I+$OBMATIO+which is MIand 'I6s.

The call flow for the +E to read the &ystem information is mentioned below2

4#!a#MI /ecoding

i) 0L ,andwidth -umber of Transmit ntenna 3eference &ignal Transmit $ower

ii) &ystem 4rame -umber (&4-)

iii) $/I%/ %onfigurationiv) Transmit every >9 ms(used for /3C %DA-%D)

iv)The I, uses a fixed schedule with a *eriodicity of >9 ms and re*etitions made within >9 ms.

v)The first transmission ofthe I, is scheduled in subframe 9 of radio frames for which the &4- mod > @

9 and re*etitions are scheduled insubframe 9 of all other radio frames.

MasterInformationBlock ::= SEQUENCE {

dl-Bandwidth ENUMER!E" { n#$ n%&$ n'&$ n&($ n)&$ n%((*$

+hich-Confi, .IC.-Confi,$

s/stem0rameN1m2er BI! S!RIN3 4SI5E 4677$

s+are BI! S!RIN3 4SI5E 4%(77

*

Query: How does the UE read $%&?

T#e M/B is transmitted on $#sical c#annel %BCC4-BC4-)BC4& and it alwas

occu$ies t#e central ; sub carriers in t#e Fre6uenc domain irres$ecti(e of t#e

c#annel bandwidt#9

T#e rst transmission of t#e M/B is sc#eduled in sub-frame number of radio

frames for w#ic# t#e 'F. mod > J

re$etitions are sc#eduled in sub-frame of all ot#er radio frames

!b#'I decoding

i& Cell Access Related /nformation < )LM. /dentit List, )LM. /dentit, TA Code, Cell identit

Cell 'tatus

ii& Cell 'election /nformation < Minimum Recei(er Le(el

iii& 'c#eduling /nformation < '/ message t$e )eriodicit, '/B ma$$ing /nfo, '/ 1indow

lengt#

Till now the +E has synchroni8ed itself with the %ell with the hel* of $&& and &&&F followed by reading

the I,. -ow +E will *roceed to read &I,1 and &I,: which carriers im*ortant information for the +E to

select a %ell. 'o know more a(out %&)s and it)s workin"* click here
http://tweet4tutorial.com/tutorial/system-information/http://tweet4tutorial.com/tutorial/system-information/


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fter the above *rocess the +E is synchroni8ed with the networ' in the/ownlin direction and have

read &I,1 and &I, :. -ow it needs to synchroni8e in the +*lin' direction

4or this to ha**en BAP(Random Access Procedure) is initiated

!?# Initial BA)F Process !Bando access .rocedure#

In order to 2e s/nchroni8ed with the network$ RC. +roced1re is 1sed9S1++ose a UE wants

to access the network$ so first it will tr/ to attach or s/ncronise with the network9 In !E a

se+arate channel RC. 4 h/sical Random ccess Channel7 is +ro;ided for initial access to

the network9

Ghen does UE need to .erfor Bando Access Procedure:-s +er s+ecifications$

RC. can 2e +erformed in 2elow mentioned scenarios :-

Initial ccess < UE is tr/in, to access the network in RRC idle state9

"1rin, RRC connection re-esta2lishment +roced1re 9

.ando;er

hen 1+link s/nchroni8ation is lost :- It means from a certain d1ration network

has not recei;ed an/thin, from UE in 1+link9

hen UE does not ha;e an/ UCC. reso1rces a;aila2le for SR4Sched1lin,

Re>1est7 9

hen timin, ad;ance is needed for +ositionin, +1r+ose in RRC connected state

for UE9

!o initiate the +roced1re UE lower la/es will send a Random ccess Re>1est4RC. Re>1est7

after recei;in, re>1est from UE RRC9UE recei;es a lot of information from eNodeB in

S/stem Information 4SIB'79 It?s im+ortant to 1nderstand those terminolo,ies first9@o1 can

also refer !E SIB' +arameters section9

RC. Confi,1rations

RC. Confi,1rations

Fow UE decides about when and where it needs to send BA)F Be%uest:HIt isdecided on the 2asis of +arameters recei;ed in SIB' RC. confi,1rations9UE can 1se #

Reso1rce 2locks for sendin, the RAC4Re>1est9

$ro PBA)F )on-g Inde< !here is a ma++in, in s+ec A#'%% ta2le &9)9%-' from which

UE can find the a;aila2le s12frames where it can send a RC.9For e0am$le:-rach Confi,

inde ;al1e = )$ 0rom ta2le it is clear that UE can send a RC. Re> in an/ s/stem frame

21t s12frame sho1ld 2e ' and )9

$ro PBA)F $re%uenc& Oset < !his ;al1e is also recei;ed in SIB' 2/ UE9 It ,o;erns

which fre>1enc/ reso1rce UE can 1se for the RC. Re>9E0am$le:-If the ;al1e of RC.

0re> ffset is %(9 UE can 1se the # RB?s startin, from RB %( for RC. Re>1est9

Ghat eactl& BA)F Be%uest contains:H

" Preable Inde :H !here are total of #D +ream2les a;aila2le which are di;ided into two

,ro1+s 3ro1+ and 3ro1+ B9 UE decides the +ream2le inde from a ,ro1+ on the 2asis of

+arameters recei;ed in SIB':-

num(ero+aPream(les:H eNodeB sends this ;al1e in SIB' which denotes the total

n1m2er of +ream2les a;aila2le for UE to send a Rach Re>1est9

si!e,+aPream(les-roup.:- It re+resents the n1m2er of +ream2les a;aila2le within

3ro1+ 9


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So n1m2er of +ream2le in 3ro1+ B

= num(ero+aPream(les* si!e,+aPream(les-roup.

$essa"ei!e-roup.:- It is 1sed for selectin, a +ream2le from a ,ro1+9 Its ;al1e is in

2its9

Now UE needs to decide the ,ro1+ from which it needs the +ream2le9 3ro1+ is decided on

the 2asis of si8e of MS3A4 RRC connection re>1est 79

If Msg4 si0e > essage'i0eJrou.A$ +ream2le will 2e selected from 3ro1+Belse

+ream2le will 2e selected from 3ro1+ 9

!he act1al form1la for selectin, a +ream2le is ,i;en in s+ec A#A'% sec &9%9'9 !hat contain

factors other than MS3A si8e as well9 B1t mainl/ this is how UE decides the 3ro1+90rom the

selected ,ro1+$ randoml/ UE selects a +ream2le inde9

Fow UE decides the Power used for Bach re%uest Transission:HNow UE need to

decide the +ower which will 2e 1sed for RC. Re>1est transmission9

Power is decided on the factors recei7ed in 'I( as:-

pream(le%nitial+ecei/edtar"etPower2G $ower factor which will be used for first transmission of

3ach 3e!uest.Halue varies from G1:9d,m to G9 d,m .

power+ampin"tep2G This is mainly used when e-ode, is not able to detect the 3ach 3e!uest then

+E will re transmit the 3%/ 3e!uest by increasing the *ower topowerRampingStep factor.

.ower used for Bach Be%uest transission

pream(le%nitial+ecei/ed'ar"etPower /E5TA3PBEAM5E

!PBEAM5E3TBA+'MI''IO+3)OU+TEB * "# ;power+ampin"tep

/E5TA3PBEAM5E@ This is *reamble format based delta offset. There are four formats available for

*reamble which are called as *reamble formats. We will be ex*laining them in detail later. ost of the

time *reamble format 9 is used.

0or Eam+le:-

In &ib:preambleInitialReceivedtargetPower = -100powerRampingStep =

4irst Transmission of 3ach 3e!uest2G

$3E,LEJT3-&I&&IO-J%O+-TE3 @ 1

4or *reamble format 9 0ELTJ$3E,LE @ 9

$ower used @ G199 K 9 K (1G 1) : @ G199

&u**ose e-ode, is not able to receive it

&econd transmission of 3ach re!uest2G

$3E,LEJT3-&I&&IO-J%O+-TE3 @ : $ower used @ G199 K 9 K (:G1) : @ G?

+E will send the 3%/ 3e!uest containing these values in the s*ecified subframe by using the s*ecified

resources.

Ghat is ste.s followed b& UE for sending a Bach re%uest:H


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Fig 89; 'te$ followed b "E for sending RAC4 re6uest

How eNodeB behaves after receiving Random Access Request:-

10 .llocation o temporary +'% 2+adio etwork temporary %dentifer3:4fter

recei;in, RC. Re>1est$ enodeB allocates a tem+orar/ identit/ to the UE which is made

+ermanent after s1ccessf1l RC. +roced1re9 !he +ermanent identit/ is called as CRN!I 4Cell

RN!I79 !his !C-RN!I is transmitted to the UE as +art of RC. Res+onse which will 2e 1sed

for f1rther comm1nication 2etween the UE and the network9

( 'imin" .d/ance: 4fter recei;in, RC. Re>1est$ eNodeB .@ 4h/sical la/er7

calc1lates the timin, ad;ance which is transmitted to the UE as +art of res+onse messa,e9

s +art of Rach res+onse$ a2sol1te !imin, ad;ance ;al1e is transmitted9

Timing advance calculation for RACH Response message:-

s +er A3 s+ec$

!s= %4%&((( F '(D67 seconds

NTA= !AF %# where !A= %% 2it ;al1e for random access res+onse ;ar/in, from ($%$' G9

%'6'

NTA= mo1nt of time ali,nmentSo transmission of 1+link radio frame from the UE shall start = 4NTAH NTA o5set7 F !s

NTA o5set= ( 4 0or t/+e % frame str1ct1re7= 44NTAH (7 F %7 4%&((( F

'(D67

= 4!AF %#74%&((( F '(D6 7seconds

Mainly this timing advance fundamental is used for eNodeB PH to receive the !" message

at correct timing# $t is possi%le that !" is far a&ay from the eNodeB ' so for eNodeB to

receive the data from !" at correct timing this timing ad(ustments are done#


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If tiing ad7ance is recei7ed in subfrae n, UE should do the adKustent fro the

starting of subfrae !n 8# fter the Random access res+onse messa,e$ enodeB does

not send a2sol1te timin, ad;ance ;al1e9 It will send a ;al1e which UE will 1se to ad1st the

alread/ stored timin, ad;ance ;al1e9 e will e+lain this in !imin, ad;ance section later9

Refer s+ec A#'%A sec D9'9A and s+ec A#'%% sec 69% for details a2o1t timin, ad1stments

4 Allocate u.lin Besources ! U5 JBA+T #:H eNodeB will +ro;ide the re>1ired

information in Random ccess Res+onse4RR7 messa,e for UE to send the MS3A4 RRC

Connection Re>1est7 9

U 3rant is a '( 2it messa,ewith contents !rom "S# to $S# as:-

%opping &lag :- !his is % 2it of information which ,o;erns whether fre>1enc/

ho++in, is ena2led or not9 If the ;al1e of the 2it is % and reso1rce 2lock

assi,nment is t/+e ( $ UE sho1ld +erform USC. fre>1enc/ ho++in,9

What is 5re6uency Hoppin":4

s we know that RB allocation is conti,1o1s in 1+link which means eNodeB

allocates contin1o1s RB?s in fre>1enc/ domain9 s in downlink we ha;e m1lti+le

reso1rce allocation +olicies to achie;e fre>1enc/ di;ersit/9So in 1+link$ to achie;e

fre>1enc/ di;ersit/ 0re>1enc/ .o++in, is 1sed9 eNodeB comm1nicates re>1ired

+arameters to UE?s in SIB' which are needed for +erformin, fre>1enc/ ho++in,

at UE9

For Exampe:-

'u$$ose e.odeB #as allocated @ RBHs to a $articular "E in u$link as s#own below9


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Fig 89= 8subframe time domain

It is clear that the allocation in 1+link in terms of RB is conti,1o1s9 B1t eNodeB

has ena2led fre>1enc/ ho++in, for this UE9

e are a2le to achie;e fre>1enc/ di;ersit/ in conti,1o1s allocation as well d1e to

ho++in,9 In the fi,1re a2o;e$ same color re+resents the ho++in, of UE data in

different slots for a +artic1lar RB9

!here is a lot more to disc1ss in fre>1enc/ ho++in,9 we will 2e takin, it later in

that +artic1lar section9

s of now$we can s1mmari8e it as :-)re*uency hopping is used to provide

fre*uency diversity in uplin+# $ts main advantage is' suppose in fre*uency

domain at RB, channel conditions are %ad %ut at RB conditions are good# .o in

that case' !" can decode the data of RB, after com%ining the same colored data#

Resource #lock Assignment :- It is %( 2its of information em2edded in U 3rant

messa,e9

How !E decodes the "# bit RB Assignment :-

ccordin, to s+ec A#'%A sec #9%9%$UE decodes it as:-

If NURB J= DD

!r1ncate the fied si8e reso1rce 2lock assi,nment to its 2 least si,nificant 2its where$

b = ceiling ' log'()$R#* '()$R#+ 1,, ,


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and inter$ret t#e truncated resource block assignment according to t#e rules for a regular

!C/ format

else/nsert b most signicant bits wit# (alue ero w#ere, b = 'ceiling ' log'()$R#* '()$R#+ 1,,

,, . 10

and inter$ret t#e e0$anded resource block assignment according to t#e rules for a regular

!C/ format

For E0am$le :-

ets ass1me the %( 2its of the information as :- % % % % % % % % % %

ss1me !otal n1m2er of 1+link RB?s in s/stem = %((

2 = 'ceiling ' log'()$R#* '()$R#+ 1,, ,, . 10

= 'ceiling ' log'100 *101,,, . 10

='ceiling '1/,, . 10

= %A -%(

= A

So$ RB assi,nment will 2e :- ( ( ( % % % % % % %

RB assi,nment 4 ;al1e in decimal7 = %')

Now we need to inter+ret it accordin, to the r1les of "CI format (9

N1m2er of RB?s allocated = 0loor4 %') %(( 7 H %

= % H %

= '

RB start inde = %') mod %((

= ')

"odulation and oding scheme to be used b2 )3:- It is D 2its of information in

U ,rant messa,e whose ;al1e ;aries from ( to %&9 !he mod1lation and codin, is

determined 2/ UE from MCS indices ( to %& in ta2le 69#9%9% from s+ec A#'%A9

Power used b2 )3 !or P)S% :-It is indicated in A 2its as a !C command whose;al1e ;aries from ( to )9 !C command to +ower ma++in, is mentioned in ta2le

2elow

Fig 89>

)$ dela2 :-% 2it of information which can 2e set to ( or %9 It indicates whether

the dela/ of USC. is introd1ced or not9


16/19

SI 4eld :-!his % 2it of information determines whether an a+eriodic CQI$ MI

and RI re+ort can 2e incl1ded in the USC. transmission9 0or contention 2ased

Random access CSI field is reser;ed9

e+ode will .re.are a res.onse ! Bando Access Bes.onse# with abo7e discussed

-elds and send it to UE ut we need to understand the tiing of res.onse

essage corres.onding to Bach Be%uestTiing wrt to Bach Be%uest and

Bando Access Bes.onse:H +.+ +esponse Windowis the ;al1e transmitted in SIB'9

3%/(%ontentionA-onG%ontention)

T#ere are two t$es of RA)%RAC4%Contention?.on-Contention&&:

Contention based RA)

.on-contention based RA)

)ontention based BAP

/n contention based, multi$le "EHs attem$t to connect to t#e network at t#e same time9 T#e

e.B is intelligent enoug# to tackle t#is situation because e(er "E s#ould be uni6ue to t#e

network9

T#e "EHs can alwas send t#e same )reamble /! to t#e network, t#ereb resulting on

collisions9 T#is kind of collision is called KContention and is known as KContention based

RAC4 )rocess9 T#e network would go t#roug# additional $rocess to resol(e t#ese contention

and #ence t#is $rocess is called KContention Resolution ste$9

T#e below mentioned call ow would e0$lain elaboratel:

89 /n t#e rst message t#e "E $ro(ides an indication to t#e network about itHs resource

re6uirement9 T#is carries t#e )reamble /!, RA-R.T/

Query_7: How does UE "ets or selects these parameters:

a9 Most of t#e information is $assed on to t#e "E t#roug# '/B;2click here* to know more

a(out % parameters3

i9 "E MAC laer #as to select t#e )reamble se6uence %*rou$ A or *rou$ B&

ii9 "E will congure itself wit# t#e ma0 retires it will tr for sending RA) %if it doesnHt

recei(e RAR&
http://tweet4tutorial.com/rach-contention-noncontention/#Non-contention%20based%20RAPhttp://tweet4tutorial.com/rach-contention-noncontention/#Non-contention%20based%20RAPhttp://lteinwireless.blogspot.in/2012/12/sib2-in-lte.htmlhttp://tweet4tutorial.com/rach-contention-noncontention/#Non-contention%20based%20RAPhttp://tweet4tutorial.com/rach-contention-noncontention/#Non-contention%20based%20RAPhttp://lteinwireless.blogspot.in/2012/12/sib2-in-lte.html


17/19

iii9 Also, after e(er retr, #ow muc# $ower le(el #as to be increased for transmitting t#e

RA)

i(9 "E MAC laer constructs t#e RA) message and $asses it to t#e "E )4I laer9 "E )4I

laer will transmit t#is message t#roug# )RAC4

(9 +nce t#e "E #as transmiited t#e RA) on )RAC4, it will start looking for RAR immediatel

after = sub-frames9 T#is number i9e9 = sub-frame is s$ecied b =*))9

Query_8: How lon" should UE monitor the rames or +.+?

T#is sub-frame number is again s$ecied in '/B; and is known as window lengt#2 so, after

t#e = sub-frames as mentioned abo(e, "E will start looking for RAR in t#e sub-frames as

mentioned b t#e 1indow lengt#9 /f b t#at time "E doesnHt recei(e RAR, it will go back to

transmit RA)

;9 T#e e.B con(es t#e resources reser(ed for t#is "E along wit# t#e Timing Ad(ance %TA&,

)reamble /! and T-CR.T/ %a number generated b e.B and asks t#e "E to send t#e RRC

connection&

=9 "E sends t#e RRC connection Re6uest using resources gi(en b t#e e.B9 /t also sends t#e

identier %CR/& to t#e e.B w#ic# is used to resol(e t#e Contention9

>9 T#e e.B runs an algorit#m and generates C-R.T/ w#ic# will be a $ermanent /! for t#e "E

till t#e connection is ali(e9 T#e e.B sends t#e "E identier9 /n t#is ste$, t#e "E w#ic# #as

recei(ed t#e /! continues w#ile ot#er "EHs will back o5 and tr again9

cenario:

Multi$le "EHs attem$t to access t#e network:

89 'o, t#e "Es initiates RAC4 wit# same )reamble se6uence, RA-R.T/

;9 T#erefore, t#e "Es will recei(e t#e same T-C-R.T/ and resource allocation from e.B

=9 All "Es would send msg = %RRconnectionRe5uest& message t#roug# t#e same resource

allocation to t#e .etwork

>9 +nce, w#en msg= is transmitted, two Timers are started:a9 T= : Transmission of RRconnectionRe5uest

b9 Contention Resolution Timer: broadcasted in '/B;9 /f t#e "E doesnHt recei(e msg>

%Contention Resolution message& wit#in t#is timer, t#en it go back to 'te$ 8 i9e9 transmitting

RA)9 /f t#ere is a 4AR7 .ACN for msg= %RRconnectionRe5uest& and it #as to be re-

transmitted t#en t#is Contention Resolution Timer will be re-started

Query_9: ow the (i" 6uestion: How should the e& (eha/e?

" One: T#e signals act as interference to eac# ot#er and e.B decode neit#er of t#em9 /n

t#is case, none of t#e "E would #a(e an res$onse %4AR7 ACN& from e.B and all "E will go

back to 'te$ 89

( 'econd: T#e e.B would successfull decode t#e message from onl one "E and fail to

decode from ot#ers9 T#e decoded "E will get 4AR7 ACN from e.B

4 Third: e.B recei(es msg= %RRconnectionRe5uest& from bot# t#e "EHs9 4ere, e.B will

send msg> %Contention Resolution& wit# MAC CR/ %Contention Resolution /dentit& to bot# t#e

"EHs9 T#is CR/ will carr a reection of t#e RRCconnectionRe6uest as generated b one of t#e

"E9 T#e MAC laer of t#e "E will matc# t#e CR/ %as recei(ed from msg>& wit# t#e CR/

embedded in t#e RRconnectionRe5uest/ /f it matc#es, t#en t#e "E will $roceed to


18/19

decode RRconnectionSetupand t#e ot#er "EHs will back o5 and return to 'te$8, i9e start t#e

RA $rocedure again9

Contention Resolution $rocess is again of two t$es:

89 MAC based Contention Resolution

> )HB+TI on P/))F

> uses the /))F logical channel

> used in FO scenarios

JJT#e rule is: if t#e "E #as a (alid C-R.T/ and is going for RA $rocedure t#en it will be a

MAC based Contention Resolution $rocedure

;9 L8 based Contention Resolution

> )BI !)ontention Besolution Identit on /5H')F based

> )ontention Besolution is addressed to TH)B+TI

> uses )))F logical channel

JJT#e rule is: if t#e "E doesnHt #as a (alid C-R.T/ and is going for RA $rocedure t#en it

will be L8 based Contention Resolution $rocedure

+onHcontention based BAP

T#is $rocedure is alwas initiated from network in case of a #ando(er9 For t#is $rocedure, t#e

e.B reser(es a set of $reamble se6uence9 1#en t#is t$e of scenario is encountered t#e

e.B allocates t#e set from t#is reser(ed $ool9T#is entire $rocedure is controlled b t#e e.B9

4ence9 no 6uestion of collision9

T#e call ow is mentioned below:

on ontention &ased +.P


19/19

E-UTRA

band

FDL_Low

(MH!

"DL_#$$setdownlin%

EARFC"("DL! FUL_Low(MH! "UL_#$$set

&plin% EARFC"

("UL!

1 :119 9 9G= 1:9 1?999 1?999G1?=

: 1"9 ;99 ;99G11 1?=9 1?;99 1?;99G11

" 1?9= 1:99 1:99G1> 1

> :119 1=9 1=9G:" 1=9 ">=9G"=9 :1>=9G:1>. "?99 "?99G>1> 1. :1?99 :1?99G::1>

19 :119 >1=9 >1=9G> 1 1>:

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