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 RACH 

RACH:

The PRACH occupies 6 PRBs in the frequency domain and spans 1 or 2 or 3 sub frames in the time domain,

dependin on the specific preamb!e format" #n the frequency domain, se$era! subcarriers at both ends of the 6 PRBs

are not used to a$oid interference %ith the ad&acent P'CCH(P')CH" #n the time domain, the cyc!ic prefi* +CP and

uard time +-T are used to a$oid interference %ith the pre$ious and ne*t subframes" As it turns out, the -T

determines the ma*imum ce!! radius"

The Preamb!e format is defined in .T/ +3-PP T) 36"211, section 0""

Ts is the basic time unit in .T/, Ts 1(+10425 s 3"20021 7 18+90 ms"

Cell radius supported:

:ormat 1;<15=m,:ormat 2; <3 =m, :ormat 3; <1 =m

FDD: preamble format 0-3 used.random access configuration for preamble format 0-3.

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PRACH position in frequency domain is given by prach-FreqOset present in !"#$%.&nd in time domain is given by prach-Con'g"nde( present in !"#$%.

TDD: preamble format 0-) used."n *++ usage of preamble format depends on ,+ con'guration used. *here might bemultiple random access resources in an , sub frame depending on the ,+con'guration.

Eg: preamble format / requires $sf and format 3 requires 3sf and num of available , sf depends on ,+ con'g used.Preamble Format 4 (Short PRACH) is only used in *+-*1 2hich can be transmittedin the ,p*! part of the special sub-frame !F/ andor !F4%. 5inimum number ofsymbols needed for this is $. 6ence this can be applied only for !pecial sub frame

con'gurations 7-8 2ith normal C or con'guration )-4 2ith e(tended C 9efer: 34.$//1 *able ).$-/%.PRACH configuration indexes 48 to 57 will use Short PRACH,p*! lengths )38) .*s and 7/$0.*s only. 9&C6 con'guration inde(es )8 to 7 2ill use!hort 9&C6PRACH POSITIO:random access preamble mapping in time and frequency

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 *he random access opportunities for each 9&C6 con'guration shall be allocated intime 'rst and then in frequency if and only if time multiple(ing is not su;cient to holdall opportunities of a 9&C6 con'gurationTDD PRACH Resource allocation is defined b a !uadru"le shown below#

>etai!s of prach9Confi#nde* is i$en in Tab!e; 0""192 of 36"211""

Types of RACH preambles (Regular and restricted).

 Restricted set used in high s"eed scenarios# where $% ex"eriences higher fre!uenc offset

due to do""ler shift which causes s"urious and aliased "ea&s# These s"urious "ea&s occur at

s"ecific cclic shift 'alues and therefore( should be restricted in the "rea)ble set.

 *he parameter High!spee"!#ag provided by higher layers determines if unrestrictedset or restricted set shall be used.

*n high s"eed scenarios( there )a be RAP*D )is)atches during RACH "rocedure if high

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s"eed flag is not set +in S*,-.# *f this flag is set( $% will not use these cclic shifts while

selecting RACH "rea)bles.

 *he random access preambles are generated from <ado-Chu sequences1 generatedfrom one or several root <ado-Chu sequences

Preamble generation:

 *here are 4) preambles available in each cell. *he set of 4) preamble sequences in acell is found by including 'rst1 in the order of increasing cyclic shift1 all the availablecyclic shifts of a root <ado-Chu sequence 2ith the logical inde( root!equence"nde(12here root!equence"nde( and for preamble generation is broadcasted as part

of the !ystem "nformation. &dditional preamble sequences1 in case 4) preambles cannotbe generated from a single root <ado-Chu sequence1 are obtained from the rootsequences 2ith the consecutive logical inde(es until all the 4) sequences are found.

 *he logical root sequence order is cyclic: the logical inde( 0 is consecutive to 83total of 

8/8 0adoff1Chu se!uences..

NCS2The cclic shift can be used to ex"and the "rea)ble ca"acit# A s)all 3cs 'alue generates )ore

"rea)bles#

3u)of RA "rea)ble 839 / NCS : where 839 Is preamble sequence length.

below diagra) 2The cclic shift used b $% is and the cclic shift used b $%6 is 3cs and $%- is

-3CS# At the e3,( the obser'ed cclic shift of $% will not be but so)e 'alue x because of the

trans)ission dela# As long as x is less than 3cs( the auto1correlation between the shifted x and

shifted 3cs will be ero( and e3, will be able to distinguish between the accesses fro) $% and

$% x  +"ro"erties of 01C se!uences.

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Howe'er( the cclic shift cannot be configured s)aller than ex"ected cell radius( since that will bloc&

rando) accesses fro) the cell edge and )a cause dro"s during hando'ers#

eg23CS66 cell radius 96:;)#

  3CS4: cell radius 95#5;)#

RACH trigger:

%'ents that will trigger rando) access "rocedure#

•   *nitial access fro) RRC<*D=%>

•   *nitial access after radio lin& failure>

•   Hando'er re!uiring rando) access "rocedure>

•   D= data arri'al during RRC<C?33%CT%D when $= is @nonsnchronised>

•   $= data arri'al during RRC<C?33%CT%D when $= is @nonsnchronised 

•   or there are no P$CCH resources for SR a'ailable#

•   if $%Bs TA ti)er has ex"ired( and $% needs to send or recei'e data( it "erfor)s

rando) access

initial preamble transmit power  is set as fo!!o%s;

9&C6 = min>C5&? 1 9&5#@9C"A+@*&9B*@O9 D E d#mG.2here C5&? is the con'gured ma(imum , transmitted po2er and is the do2nlinHpathloss estimate calculated in the ,. #ased on the reference signal po2er signaled in!"#$ and the measured 9!91

Random access procedurerando) access "rocedure at the $% side#

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• The $% listens to a downlin& broadcast signal to obtain the trans)ission ti)ing# *t is

also infor)ed of the a'ailable signatures( fre!uenc bands and ti)e slots for a rando)access#+S*,-.

• ?"en1loo" "ower control can be used to obtain a suitable trans)ission "ower# The

"ath loss is esti)ated fro) a downlin& signal and the $% esti)ates the trans)ission "owerto achie'e a certain S3R target#

• The $% selects rando)l a signature( a ti)e slot and a fre!uenc band a)ong thea'ailable set#

• The 3ode, correlates the recei'ed signal in the RACH sub1fra)e with all "ossible

signature se!uences

• The $% )onitors a s"ecified downlin& channel for res"onse fro) the 3ode,#

• *n case of "ositi'e answer2

• The $% decodes the res"onse and ada"ts its trans)ission ti)ing#

• *f the $% does not recei'e an res"onse fro) the 3ode,2 A new atte)"t is

"erfor)ed b "ower ra)"ing )ethod or 2 the $% selects a new signature after a rando)bac&1off ti)e#/*amp!e;

sib2 :?""""prach9Confi?root)equence#nde* 35,prach9Confi#nfo?prach9Confi#nde* 5,hih)peed:!a :A.)/,@eroCorre!ationoneConfi 12,prach9:reqffset 2"""

prach9Confi#nde* is 5, meanin PRACH can be transmitted in any ):D and 5th sub9frame"prach9:reqffset is 2, meanin PRACH starts from 1st RB in frequency domain@eroCorre!ationoneConfi 12 means DC) is 11E"

UE Logs on RACHVersion = 2

Current SFN SF = 10184

Preamble Sequence = 2

Cycle Shift = 52

Preamble Format = 0

First RB for PRACH Opportunity = 17

Start RB = 17

PRACH Transmit Power = -19 dBm

Logical Root (q) = 129