PRACH summary
• Ke Bihui, Aug, 2018,
1
© Nokia 2015
PRACH procedure
PRACH use cases(red-RRC_IDLE, others for RRC_CONNECTED)
•
Initial access for UE originated or terminated call(CB)
•
Tracking area update(CB)
•
New UL data/report without synchronization(CB)
•
New DL data and UL ACK without synchronization(CB&CF)
•
Beam failure recovery(CB/CF)
•
Recovery from RL failure(CB)
•
Cell handover(CB/CF)
•
Scheduling request without valid PUCCH resource(CB)
•
Request for other SI(CB/CF)
UE
BS
TX
PBCH(MIB)
PDCCH/PDSCH(SIB1)
Select preamble
RX
PRACH preamble(MSG1)
TX
RX
RAR(MSG2)
L3 message(MSG3)
CB RA and CF RA
•
•
Contention-free(CF) random access

Dedicated preamble

Only MSG1 and MSG2
Contention-base(CB) random access

Share preambles

MSG1,MSG2,MSG3 and MSG4
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TX
Contention resolution(MSG4)
PRACH Resources-1
•
•
•
•
FDM of PRACH blocks can be 1,2,4 or 8 as defined by “msg1-FDM”
PRACH blocks are continuous and starts from “msg1-FrequencyStart ”
PRACH format and time domain positions are decided by “prach-ConfigurationIndex”
Maximum PRACH configuration period is 16 frames
msg1-FDM(1,2,4,8)
msg1-FrequencyStart
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SSB07
SSB15
SSB23
SSB31
SSB39
SSB47
SSB06
SSB14
SSB22
SSB30
SSB38
SSB46
SSB05
SSB13
SSB21
SSB29
SSB37
SSB45
SSB04
SSB12
SSB20
SSB28
SSB36
SSB44
SSB03
SSB11
SSB19
SSB27
SSB35
SSB43
SSB02
SSB10
SSB18
SSB26
SSB34
SSB42
SSB01
SSB09
SSB17
SSB25
SSB33
SSB41
SSB00
SSB08
SSB16
SSB24
SSB32
SSB40
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decided by prach-ConfigurationIndex
PRACH Resources-2
•
•
•
PRB occupation is expressed in number of RBs for PUSCH
RA
It is the least RB number which has more sub-carriers( f ) than PRACH length
The bandwidth of RACH is:
863
862
1.25kHz
860
859
1 PUSCH SC
-
SCS*144 for short PRACH, e.g. 2.16MHz when SCS=15kHz
-
SCS*864 for short PRACH, e.g. 1.08MHz when SCS=15kHz and 4.32MHz when SCS=5kHz
861
858
857
856
855
854
LRA
f
RA for PRACH
f
for PUSCH
N
RA
RB
, allocation expressed in
number of RBs for PUSCH
853
852
k
851
850
839
1.25
15
6
7
839
839
839
839
839
139
139
139
139
139
139
139
139
139
139
1.25
1.25
5
5
5
15
15
15
30
30
30
60
60
120
120
30
60
15
30
60
15
30
60
15
30
60
60
120
60
120
3
2
24
12
6
12
6
3
24
12
6
12
6
24
12
1
133
12
10
7
2
2
2
2
2
2
2
2
2
2
849
848
847
846
6 PUSCH RB
839
k
...
6
5
4
3
2
1
0
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SSB indices mapping to RACH
occasions(ROs)
3GPP NR supports
o
Mapping between one SSB index to only one PRACH transmission occasion
o
Mapping between many SSB indices to only one PRACH transmission occasion
o
Mapping between one SSB index to many PRACH transmission occasions
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3GPP configuration for mapping
 N: ssb-perRACH-Occasion, R: CB-PreamblesPerSSB
 CB preambles per RO is R* max(1, N)
 If N<=1, nth RO has R preambles starting from 0


E.g. N=1/4 & R=16, then preambles for each RO is [0, 15]
E.g. N=1 & R=64, then preambles for each RO is [0, 64]
 If N>1, nth RO has R preambles staring from n*64/N

E.g. N=4 & R=12, then preambles for each RO is [0, 11],[16,27],[32,43] and [48,59]
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SSB indices to ROs mapping order for CB
 First, in increasing order of preamble indexes within a single PRACH occasion.
preambles
 Second, in increasing order of frequency resource indexes for frequency multiplexed
PRACH occasions.
 Third, in increasing order of time resource indexes for time multiplexed PRACH
occasions within a PRACH slot.
 Fourth, in increasing order of indexes for PRACH slots.
 Detail one to one mapping can be found in the previous slide
 Detail many to one mapping(SSB per RO = 4) can be found below
12~ 15
8~ 11
4~ 7
0~ 3
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28~ 31
24~ 27
20~ 23
16~ 19
© Nokia 2016
44~ 47
40~ 43
36~ 39
32~ 35
60~ 63
56~ 59
52~ 55
48~ 51
Numbering of ROs for PDCCH order
 PDCCH order is DCI format 1_0 scrambled by C-RNTI and the
"Frequency domain resource assignment" field are of all ones




Random Access Preamble index: dedicated index to be used
UL/SUL indicator: Non-SUL(0) and SUL(1)
SS/PBCH index: SSB index to be used for association
PRACH Mask index: index of RO associated to the SSB index
 Numbering the PRACH occasions associated to the SSB index





First, in increasing order of frequency resource indexes for frequency
multiplexed PRACH occasions
Second, in increasing order of time resource indexes for time
multiplexed PRACH occasions within a PRACH slot
Third, in increasing order of indexes for PRACH slots
Maximum one SSB index can be associated to 8 ROs
For contention free RA or BFR, the same table is used to decide
which RO is used for random access
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PRACH Mask
Index
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Allowed PRACH occasion(s) of SSB
All
PRACH occasion index 1
PRACH occasion index 2
PRACH occasion index 3
PRACH occasion index 4
PRACH occasion index 5
PRACH occasion index 6
PRACH occasion index 7
PRACH occasion index 8
Every even PRACH occasion
Every odd PRACH occasion
Reserved
Reserved
Reserved
Reserved
Reserved
SSB indices to RO associated period
 An association period, starting from frame 0, for mapping SSB indices to ROs is the smallest
value in the set determined by the PRACH configuration period according to following table,
such that every SSB index is at least mapped to one RO within the period
PRACH configuration period (msec)
10
20
40
80
160
Association period (number of PRACH
configuration periods)
{1, 2, 4, 8, 16}
{1, 2, 4, 8}
{1, 2, 4}
{1, 2}
{1}
 An example below with
 PRACH configuration period = 20ms
SSB
 N Tx
= 8 , SSB per RO=1,
 A2A2A2 in each RACH slot
 Associated period = 4*20ms=80ms
………
SSB0
SSB1
SSB2
………
SSB3
SSB4
SSB5
………
PRACH configuration period = 20ms
Associated period = 4*20ms =80ms
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SSB6
SSB7
Not Used
………
Not Used
Not Used
Not Used
Preparation for random access
 Select the carrier to be used for random access
 If the carrier is signal(e.g. in PDCCH order), then use the signaled carrier for random access
 If not,
 select SUL for random access if RSRP < “rsrp-ThresholdSSB-SUL”
 Select NUL for random access if RSRP >= “rsrp-ThresholdSSB-SUL”
 Setting power ramping step
 Set default power ramping step to “preamblePowerRampingStep”
 If “powerRampingStepHighPriority” is configured, use that for BFR and handover random access
 Setting SCALING_FACTOR_BI
 Set default scaling factor for BI timer to 1
 If “scalingFactorBI” is configured, use that for BFR and handover random access
 It can have different “powerRampingStepHighPriority” and “scalingFactorBI” for handover and BFR.
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Preamble split





If ssb-perRACH-Occasion <=1, 64 preambles are use only for the SSB mapped to this RO
If ssb-perRACH-Occasion>1, then 64 preambles are shared by those ssb-perRACH-Occasion SSBs
All preambles for each SSB are divided to 3 parts, preamble for CB RA, CF RA and SI request
CB preambles are further divided to group A and group B based on MSG3 size.
For beam failure recovery, a separate root sequence-- “rootSequenceIndex-BFR” is used for generating
preambles used for BFR.
totalNumberOfRA-Preambles/ssb-perRACH-Occasion
Preamble_0
numberOfRA-PreamblesGroupA
CB-PreamblesPerSSB
ra-PreambleStartIndex
Preambles for 1st SSB
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n*64/ssb-perRACH-Occasion
Preambles for 2nd SSB
Decide SSB and preamble index-1
 The SSB /preamble decision is done as followings
Decide the SSB/CSIRS and preamble from
BFR configuration
Decide the SSB/CSIRS and preamble from
CFRA configuration
Decide the SSB and
preamble from
CBRA configuration
Decide the RO
associated to selected
SSB or CSI-RS or
None
 For BFR, the preamble used can be from BFR or CFRA or CBRA configuration
 For CFRA, the preamble used can be from CFRA or CBRA configuration
BFR CFRA resource
associated to SSB configured
At least one SSB RSRP higher
than rsrp-ThresholdSSB
SSB selected
Yes
Select an
SSB or
CSI-RS
BFR CFRA resource
associated to CSI-RS
configured
At least one CSI-RS RSRP
higher than rsrp-ThresholdCSIRS
No BFR CFRA resource or no
good RSRP SSB or CSI-RS
Decide the preamble according
to normal CFRA resources
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Select the QCL SSB
Yes
Preamble decision for BFR
Select the associated
preamble of the SSB
CSI-RS
selected
No
With
associated
preamble
Yes
Select the associated
preamble of the CSI-RS
Decide SSB and preamble index-2
Select the signaled
preamble
Preamble signal in PDCCH
or RRC but no CFRA
resources
Yes
At least one SSB RSRP
higher than rsrpThresholdSSB
CFRA resource associated to
SSB configured
CFRA resource associated to
CSI-RS configured
At least one CSI-RS RSRP higher
than rsrp-ThresholdCSI-RS
All others
Decide the preamble according to
CBRA resources
Yes
Select
an SSB
Select the associated
preamble of the SSB
Select
an CSIRS
Select the associated
preamble of the CSIRS
Preamble decision for CFRA
At least one SSB RSRP higher
than rsrp-ThresholdSSB
Yes
Select
an
SSB
Randomly select the preambles in
the group associated to the SSB
No
Yes
Select
any SSB
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Select the group A or group
B according to MSG3 size
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RA resources
associated with
SSB
Preamble decision for CBRA
No
Randomly select the
preambles in the group
Decide SSB and preamble index-3
Select original group
No
MSG3 not
transmitted
Initiated by
CCCH
Yes
Yes
No
Yes
Group B
configured
Power not
limited
MSG3 size >raMsg3SizeGroupA
No
Yes
Yes
Select group
B
No
No
Select group
A
Group decision for CBRA
An SSB
is
selected
Yes
An CSIRS is
selected
Yes
SSB associated to
RO is configured
RA for BFR and no BFR
CFRA resource
configured
Yes
Yes
determine the next available PRACH occasion according to rassb-OccasionMaskIndex with equal probability if more at same
time
Select
the QCL
SSB
Yes
CSI-RS associated to
RO is configured
Yes
determine the next available PRACH occasion according
to raNo
OccasionList with equal probability if more at same time
All others
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RO determination
determine the next available PRACH with equal probability if
more at same time
Power for preambles and MSG3
 “preambleReceivedTargetPower” is the expected preamble received power in range [-220,-60] dBm
for PRACH format 0, which should be decided by the mis-detection and false alarm requirements.
 DELTA_PREAMBLE is the pre-defined value for each preamble format considering its preamble
period in time domain comparing to PRACH format 0.
 Pathloss = referenceSignalPower – higher layer filtered RSRP of selected SSB to compensate the
pathloss on UL by assuming same value as that of DL.
 PREAMBLE_POWER_RAMPING_STEP is the step of preamble power ramp up, it can be higher
for random access for BFR or handover. It can be configured to {0,2,4,6}dB.
 “msg3-DeltaPreamble” is the target power difference of MSG3 to preamble and is configured in
“PUSCH-ConfigCommon” with range [-1,6] dB.
POWER_RAMPING_STEP
POWER_RAMPING_STEP
TPC from MSG2
POWER_RAMPING_STEP
POWER_RAMPING_STEP
Pathloss
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DELTA_PREAMBLE
values (dB)
0
0 dB
1
-3 dB
2
-6 dB
3
0 dB
A1
8+3×μ
A2
5+3×μ
A3
3+3×μ
B1
8+3×μ
B2
5+3×μ
B3
3+3×μ
B4
3×μ
C0
11 + 3 × μ
C2
5+3×μ
deltaTF
msg3-Alpha*Pathloss
(2^u )*10log10(M)
DELTA_PREAMBLE
msg3-DeltaPreamble
preambleReceivedTargetPower
preambleReceivedTargetPower
Power for preambles and MSG3
Preamble
Format
TPC Command
0
1
2
3
4
5
6
7
Value (in dB)
-6
-4
-2
0
2
4
6
8
MSG2 timing





RAR window start from the 1st symbol of PDCCH for Type1-PDCCH common search space.
subframe,.μ
slot
RAR window should start at least   N slot
after last symbol of sent preamble
 N sy
mb Tsf 
Possible RAR window is { sl1, sl2, sl4, sl8, sl10, sl20, sl40, sl80}
RAR window can be different for CB RA, CF RA and also for BFR RA
MSG3 is decided by the K2 value of UL grant in RAR and minimum time between last symbol of
PDSCH for RAR and first symbol for MSG3 is equal to 𝑁1 + 𝑁2 + 0.5 ms
RAR
Preamble
*
*
MSG3
K2
PDSCH decoding time N1 [symbols]

dmrs-AdditionalPosition = pos0 in
DMRS-DownlinkConfig in either of
dmrs-DownlinkForPDSCH-MappingTypeA,
dmrs-DownlinkForPDSCH-MappingTypeB
0
8
1
10
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20
dmrs-AdditionalPosition ≠ pos0 in
DMRS-DownlinkConfig in either of
dmrs-DownlinkForPDSCH-MappingTypeA,
dmrs-DownlinkForPDSCH-MappingTypeB
13
13
20
24
PUSCH preparation
time N2 [symbols]
0
10
1
12
2
23
3
36
MSG2 contents
 A MAC PDU consists of one or more MAC sub-PDUs and optionally padding. Each
MAC sub-PDU consists one of the following:
 A MAC sub-header with Back-off Indicator only;
 A MAC sub-header with RAPID only (i.e. acknowledgment for SI request);
 A MAC sub-header with RAPID and MAC RAR.
 HARQ operation is not applicable to the Random Access Response transmission
E/T/R/R/BI
subheader
MAC subPDU 1
(BI only)
E/T/RAPID
subheader
MAC subPDU 2
(RAPID only)
E/T/RAPID
subheader
MAC subPDU 3
(RAPID and RAR)
MAC RAR
MAC subPDU 4
RA type
BFR CFRA
SI request
CFRA
Address to
C-RNTI
RA-RNTI
RA-RNTI
subheader
NA
RAPID
RAPID
CBRA
RA-RNTI
RAPID
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...
MAC subPDU n
Payload
NA
NA
TAC
TAC, UL grant and TCRNTI
Padding (opt)
R
R
R
Timing Advance Command
Timing Advance Command
UL
Grant
Oct 1
Oct 2
UL Grant
Oct 3
UL Grant
Oct 4
UL Grant
Oct 5
Temporary C-RNTI
Oct 6
Temporary C-RNTI
Oct 7
MAC RAR
MSG2 Backoff indicator
 If MSG2 include a BI, then:
 set the PREAMBLE_BACKOFF to value of the BI field of the MAC sub-PDU according to BI table and
multiplied with SCALING_FACTOR_BI
 SCALING_FACTOR_BI is “scalingFactorBI” if configured for BFR or handover, and is 1 for other cases
 PREAMBLE_BACKOFF is used to control the new RA dealy for those UEs whose RAs are not completed
successfully, i.e. UEs who are not ACKed in MSG2 or whose contention resolution is failed or time expired
Index
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0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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Backoff Parameter value (ms)
5
10
20
30
40
60
80
120
160
240
320
480
960
1920
Reserved
Reserved
MSG2 UL Grant
 The fields in UL grant is listed in the table below




RAT1 is used for frequency resource allocation, N UL,hop bits used for hopping information if enabled
Time resource allocation provides an index to pusch-AllocationList in pusch-ConfigCommon if configured
or a default time resource allocation table
The MCS is determined from the first sixteen indices of the applicable MCS index table for PUSCH
CSI request is used for CFRA to request an aperiodic CSI together with MSG3 PUSCH
RAR grant field
Frequency hopping flag
Msg3 PUSCH frequency resource allocation
Msg3 PUSCH time resource allocation
MCS
TPC command for Msg3 PUSCH
CSI request
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Number of bits
1
14
4
4
3
1
MSG3
 Detail time domain allocation can be referred to 38.214 6.1.2.1
 Here is the example of default time domain allocation table with normal CP
 For MSG3 scheduled by RAR, an additional Δ value is added to K2
 Redundancy version number 0 is used and (re)transmission is addressed to TC-RNTI
 C-RNTI is included if have or initial UE identity is included
Default PUSCH time domain resource allocation A for normal CP
Row index
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
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PUSCH
mapping type
Type A
Type A
Type A
Type B
Type B
Type B
Type B
Type A
Type A
Type A
Type A
Type A
Type A
Type B
Type A
Type A
© Nokia 2016
𝑲𝟐
j
j
j
j
j
j
j
j+1
j+1
j+1
j+2
j+2
j+2
j
j+3
j+3
S
L
0
0
0
2
4
4
4
0
0
0
0
0
0
8
0
0
14
12
10
10
10
8
6
14
12
10
14
12
10
6
14
10
Definition of value j
µPUSCH
0
1
2
3
j
1
1
2
3
Definition of value Δ
µPUSCH
0
1
2
3
Δ
2
3
4
6
MSG4
 It is addressed to C-RNTI(if included in MSG3) or TC-RNTI
 It is based on C-RNTI on PDCCH or UE Contention Resolution Identity on DL-SCH
 If PDCCH is addressed to C-RNTI, consider the Contention Resolution is successful
and discard the TC-RNTI
 If PDCCH is addressed to TC-RNTI:

If UE Contention Resolution Identity is matched to initial UE identity in MSG3:
‾ For SI request, indicate the reception of an acknowledgement to upper layers
‾ For initial access, promote TC-RNTI to C-RNTI

Else consider this Contention Resolution not successful and discard TC-RNTI
 Three possibilities after UE receives the contention resolution message



The UE correctly decodes the message and detects its own identity: it sends back an ‘ACK’
The UE correctly decodes the message and discovers that it contains another UE’s identity
(contention resolution): it sends nothing back (‘DTX’).
The UE fails to decode the message or misses the DL grant: it sends nothing back (‘DTX’).
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RA failure
 RA failed when no MSG2 is received within the RAR window
 Or ra-ContentionResolutionTimer expired or contention resolution failed


Increment PREAMBLE_TRANSMISSION_COUNTER by 1
If PREAMBLE_TRANSMISSION_COUNTER = preambleTransMax + 1
‾ Indicate a Random Access problem to upper layers
‾ Consider the Random Access unsuccessfully completed if triggered by SI request

If the Random Access procedure is not completed
‾ Select a random backoff time according to a uniform distribution between 0 and the
PREAMBLE_BACKOFF;
‾ delay the subsequent Random Access Preamble transmission by the backoff time
‾ perform a new Random Access procedure
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SCS for RA MSGs
 Type1-PDCCH common search space set is configured by ra-SearchSpace in
PDCCH-ConfigCommon for a DCI format with CRC scrambled by a RARNTI, or a TC-RNTI on a primary cell
 DMRS of PDCCH or PDSCH of MSG2/MSG4 is QCL with the SSB or CSIRS selected by UE for PRACH association
PRACH related SCS
Configuration
Field
Message 1
RACH-ConfigCommon
msg1-SubcarrierSpacing
Message 2
initialDownlinkBWP
subcarrierSpacing
Message 3
BWP-UplinkCommon
SubcarrierSpacing
Message 4
Same as MSG2
PUCCH on MSG4 ACK
Same as MSG3
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Timing advance
 Timing Advance Group(TAG): A group of Serving Cells using the same timing reference cell and the same
Timing Advance value(𝑇𝐴 ). 𝑇𝐴 is in 16*Ts unit when Ts is the basic time unit for LTE.
 𝑇𝐴 in MSG2 has 12 bits which has values of 0, 1, 2, ..., 3846 with NTA  TA  16  64 2

 𝑇𝐴 in MAC CE has 6 bits which has values of 0, 1, 2,..., 63 with N TA_new  N TA_old  TA  31  16  64 2
 When a 𝑇𝐴 is received on slot n, the timing is adjust at UL slot n+k where
subframe,
k  Nslot
 NT,1  NT,2  NTA,max  0.5 Tsf 



𝑁𝑇,1 is a time duration of N1 symbols corresponding to PDSCH reception time for PDSCH processing capability 1
when additional PDSCH DM-RS is configured
𝑁𝑇,1 is a time duration of N2 symbols corresponding to PUSCH preparation time for PUSCH processing capability 1
𝑁𝑇𝐴,𝑚𝑎𝑥 is the maximum TA value for 12 bits TA command
 If two adjacent slots overlap due to a TA command, the latter slot is reduced in duration
 UL and DL timing relation is shown below:
Downlink frame i
Uplink frame i
 𝑁𝑇𝐴,𝑜𝑓𝑓𝑠𝑒𝑡 : 0 for FDD, 39936 or 25600 for TDD FR1 and 13792 for FR2 with also unit Tc
24
06/07/2021
Confidential
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