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.
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 be multiple 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 transmitted in the ,p*! part of the special sub-frame !F/ andor !F4%. 5inimum number of symbols 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/$0.*s only. 9&C6 con'guration inde(es )8 to 7 2ill use !hort 9&C6 PRACH POSITIO: random access preamble mapping in time and frequency
*he random access opportunities for each 9&C6 con'guration shall be allocated in time 'rst and then in frequency if and only if time multiple(ing is not su;cient to hold all opportunities of a 9&C6 con'guration TDD PRACH Resource allocation is defined b a !uadru"le shown below#
>etai!s of prach9Confi#nde* is i$en in Tab!e; 0""192 of 36"211" "
Regul arandr es t r i c t ed) . Ty pesofRACHpr eambl es( Restricted set used in high s"eed scenarios# $here %& 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 se t .
*he parameter High!spee"!#ag provided by higher layers determines if unrestricted set or restricted set shall be used. +n high s"eed scenarios) there *a be RAP+D *is*atches during RACH "rocedure if high
s"eed flag is not set ,in S+-./# +f this flag is set) %& will not use these cclic shifts while selecting RACH "rea* bl es .
*he random access preambles are generated from
*here are 4) preambles available in each cell. *he set of 4) preamble sequences in a cell is found by including 'rst1 in the order of increasing cyclic shift1 all the available cyclic shifts of a root
-elow diagra*3 The cclic shift used b %&6 is 6 and the cclic shift used b %& is cs and %&. is .CS# At the e-) the obser(ed cclic shift of %&6 will not be 6 but so*e (alue x because of the trans*ission dela# As long as x is less than cs) the auto2correlation between the shifted x and shifted cs will be ero) and e- will be able to distinguish between the accesses fro* %&6 and %& x ,"ro"erties of 12C se!uences/
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# eg3CS9 cell radius :;<*# CS4; cell radius :5#5<*# RACH trigger:
&(ents that will trigger rando* access "rocedure# • • • • • • •
rando*
+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@&CT&D when %> is nonsnchronisedB? %> data arri(al during RRC=C@&CT&D when %> is nonsnchronisedB @r there are no P%CCH resources for SR a(ailable# if %&s TA ti*er has ex"ired) and %& needs to send or recei(e data) it "erfor*s 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. here C5&? is the con'gured ma(imum , transmitted po2er and is the do2nlinH pathloss estimate calculated in the ,. #ased on the reference signal po2er signaled in !"#$ and the measured 9!91 Random access procedure Rando* access "rocedure at the %& side#
•
•
•
•
• • • •
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+-./ @"en2loo" "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 "ower to achie(e a certain SR target# The %& selects rando*l a signature) a ti*e slot and a fre!uenc band a*ong the a(ailable set# The ode- correlates the recei(ed signal in the RACH sub2fra*e with all "ossible signature se!uences The %& *onitors a s"ecified downlin' channel for res"onse fro* the ode-# +n case of "ositi(e answer3 The %& decodes the res"onse and ada"ts its trans*ission ti*ing# +f the %& does not recei(e an res"onse fro* the ode-3 A new atte*"t is "erfor*ed b "ower ra*"ing *ethod or3 the %& selects a new signature after a rando* bac'2off 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"
UELogsonRACH Ve r s i o n=2 Cur r entSFNSF=10184 Pr eambl eSequence=2 Cy c l eSh i f t=5 2 Pr e a mb l eF o r ma t=0 Fi r s tRBf o rPRACHOp p or t u ni t y=1 7 St a r t RB=1 7 PRACHTr ansmi tPower=19dBm L og i c al Ro ot( q )=1 29
