2 Step RACH

As the term implies 2 Step RACH indicates a type of RACH procedure that are complete in 2 steps. To be honest, the number of steps would look differently depending on how to split the whole procedure into smaller steps. Highlevel view of the 2step RACH can be illustrated as below.

I think the biggest difference between the two step RACH and regular (existing, conventional) rach is with step 1. In two step RACH, PUSCH can be transmitted at the very first step whereas a PUSCH can be transmitted at step 3 in regular RACH.

NOTE : Comparing the 2 Step RACH with the conventional 4 Step RACH, Step 1 (MsgA) is the combination of step 1 and step 3 and Step 2 (MsgB) is the combination of step 2 and step 4.

< Frame structure of two-step RACH and 4-step RACH >

Source : Figure 18 of Power Saving Techniques for 5G and Beyond (Ref [1])

Why 2 step RACH ?

I think the main purpose of 2 step RACH is to reduce the overall latency caused by RACH procedure. 2 Step RACH would reduce the delay in two ways.

Reduce the number of signaling steps for RACH procedure (4 steps to 2 steps) ==> reduce the control signal overhead
Reduce the latency
Allowing PUSCH transmission at the very first step
If it is used in Unlicensed spectrum, it can reduce the number of LBT(Listen Before Talk) attempt
Power Saving : By reducing the number of signaling steps, it can reduce the power consumption (For more about NR power saving techniques, refer to this note)

Possible Drawback of 2 step RACH

Some possible drawback of 2 Step RACH is mentioned in the Introduction of this paper as follows :

The demodulation performance degradation observed without time offset compensation at the base station (gNB), specially for MR(mid range) or WA(wide area) cells
In the case that all preambles from multiple users (UEs) trying to perform the initial access are mapped to the same PUSCH physical resources, the associated data parts overlap and may result in unsuccessful decoding ==> There is therefore a trade-off between the collision probability of the PUSCH part of MsgA and the resource overhead for 2SR

Overview on Configuration

In terms of RRC, you would see the huge list of parameters specified for 2step RACH as listed in RRC Parameters for Two Step RACH Process, but in terms of physical layer point of view the basic concept of each configuration almost same as existing RACH and PUSCH. I would not exaplain much of the physical layer parameters in this note. I would recommend you to refer to existing notes on PHY aspect of RACH and PUSCH linked below.

RRC Parameters for Two Step RACH Process

RACH-ConfigCommonTwoStepRA-r16 ::= SEQUENCE {

rach-ConfigGenericTwoStepRA-r16 RACH-ConfigGenericTwoStepRA-r16,

msgA-TotalNumberOfRA-Preambles-r16 INTEGER (1..63) OPTIONAL, -- Need S

msgA-SSB-PerRACH-OccasionAndCB-PreamblesPerSSB-r16 CHOICE {

oneEighth ENUMERATED {n4,n8,n12,n16,n20,n24,n28,n32,n36,n40,n44,n48,n52,n56,n60,n64},

oneFourth ENUMERATED {n4,n8,n12,n16,n20,n24,n28,n32,n36,n40,n44,n48,n52,n56,n60,n64},

oneHalf ENUMERATED {n4,n8,n12,n16,n20,n24,n28,n32,n36,n40,n44,n48,n52,n56,n60,n64},

one ENUMERATED {n4,n8,n12,n16,n20,n24,n28,n32,n36,n40,n44,n48,n52,n56,n60,n64},

two ENUMERATED {n4,n8,n12,n16,n20,n24,n28,n32},

four INTEGER (1..16),

eight INTEGER (1..8),

sixteen INTEGER (1..4)

} OPTIONAL, -- Cond 2StepOnly

msgA-CB-PreamblesPerSSB-PerSharedRO-r16 INTEGER (1..60) OPTIONAL, -- Cond SharedRO

msgA-SSB-SharedRO-MaskIndex-r16 INTEGER (1..15) OPTIONAL, -- Need S

groupB-ConfiguredTwoStepRA-r16 GroupB-ConfiguredTwoStepRA-r16 OPTIONAL, -- Need S

msgA-PRACH-RootSequenceIndex-r16 CHOICE {

l839 INTEGER (0..837),

l139 INTEGER (0..137),

l571 INTEGER (0..569),

l1151 INTEGER (0..1149)

} OPTIONAL, -- Cond 2StepOnly

msgA-TransMax-r16 ENUMERATED {n1, n2, n4, n6, n8, n10, n20, n50, n100, n200} OPTIONAL, -- Need R

msgA-RSRP-Threshold-r16 RSRP-Range OPTIONAL, -- Cond 2Step4Step

msgA-RSRP-ThresholdSSB-r16 RSRP-Range OPTIONAL, -- Need R

msgA-SubcarrierSpacing-r16 SubcarrierSpacing OPTIONAL, -- Cond 2StepOnlyL139

msgA-RestrictedSetConfig-r16 ENUMERATED {unrestrictedSet, restrictedSetTypeA,

restrictedSetTypeB} OPTIONAL, -- Cond 2StepOnly

ra-PrioritizationForAccessIdentityTwoStep-r16 SEQUENCE {

ra-Prioritization-r16 RA-Prioritization,

ra-PrioritizationForAI-r16 BIT STRING (SIZE (2))

} OPTIONAL, -- Cond InitialBWP-Only

ra-ContentionResolutionTimer-r16 ENUMERATED {sf8, sf16, sf24, sf32, sf40, sf48, sf56, sf64} OPTIONAL,

--Cond 2StepOnly

...

}

GroupB-ConfiguredTwoStepRA-r16 ::= SEQUENCE {

ra-MsgA-SizeGroupA ENUMERATED {b56, b144, b208, b256, b282, b480, b640, b800,

b1000, b72, spare6, spare5, spare4, spare3, spare2, spare1},

messagePowerOffsetGroupB ENUMERATED {minusinfinity, dB0, dB5, dB8, dB10, dB12, dB15, dB18},

numberOfRA-PreamblesGroupA INTEGER (1..64)

}

RACH-ConfigGenericTwoStepRA-r16 ::= SEQUENCE {

msgA-PRACH-ConfigurationIndex-r16 INTEGER (0..262) OPTIONAL, -- Cond 2StepOnly

msgA-RO-FDM-r16 ENUMERATED {one, two, four, eight} OPTIONAL, -- Cond 2StepOnly

msgA-RO-FrequencyStart-r16 INTEGER (0..maxNrofPhysicalResourceBlocks-1) OPTIONAL, -- Cond 2StepOnly

msgA-ZeroCorrelationZoneConfig-r16 INTEGER (0..15) OPTIONAL, -- Cond 2StepOnly

msgA-PreamblePowerRampingStep-r16 ENUMERATED {dB0, dB2, dB4, dB6} OPTIONAL,

-- Cond 2StepOnlyNoCFRA

msgA-PreambleReceivedTargetPower-r16 INTEGER (-202..-60) OPTIONAL, -- Cond 2StepOnlyNoCFRA

msgB-ResponseWindow-r16 ENUMERATED {sl1, sl2, sl4, sl8, sl10, sl20, sl40, sl80, sl160, sl320}

OPTIONAL, -- Cond NoCFRA

preambleTransMax-r16 ENUMERATED {n3, n4, n5, n6, n7, n8, n10, n20, n50, n100, n200}

OPTIONAL, -- Cond 2StepOnlyNoCFRA

...

}

RACH-ConfigDedicated ::= SEQUENCE {

cfra CFRA OPTIONAL, -- Need S

ra-Prioritization RA-Prioritization OPTIONAL, -- Need N

...,

[[

ra-PrioritizationTwoStep-r16 RA-Prioritization OPTIONAL, -- Need N

cfra-TwoStep-r16 CFRA-TwoStep-r16 OPTIONAL -- Need S

]]

}

CFRA-TwoStep-r16 ::= SEQUENCE {

occasionsTwoStepRA-r16 SEQUENCE {

rach-ConfigGenericTwoStepRA-r16 RACH-ConfigGenericTwoStepRA-r16,

ssb-PerRACH-OccasionTwoStepRA-r16 ENUMERATED {oneEighth, oneFourth, oneHalf, one, two, four,

eight, sixteen}

} OPTIONAL, -- Need S

msgA-CFRA-PUSCH-r16 MsgA-PUSCH-Resource-r16,

msgA-TransMax-r16 ENUMERATED {n1, n2, n4, n6, n8, n10, n20, n50, n100, n200}

OPTIONAL, -- Need S

resourcesTwoStep-r16 SEQUENCE {

ssb-ResourceList SEQUENCE (SIZE(1..maxRA-SSB-Resources)) OF CFRA-SSB-Resource,

ra-ssb-OccasionMaskIndex INTEGER (0..15)

...

}

CFRA-SSB-Resource ::= SEQUENCE {

ssb SSB-Index,

ra-PreambleIndex INTEGER (0..63),

...,

[[

msgA-PUSCH-Resource-Index-r16 INTEGER (0..3071) OPTIONAL -- Cond 2StepCFRA

]]

}

CFRA-CSIRS-Resource ::= SEQUENCE {

csi-RS CSI-RS-Index,

ra-OccasionList SEQUENCE (SIZE(1..maxRA-OccasionsPerCSIRS)) OF INTEGER (0..maxRA-Occasions-1),

ra-PreambleIndex INTEGER (0..63),

...

}

MsgA-PUSCH-Config-r16 ::= SEQUENCE {

msgA-PUSCH-ResourceGroupA-r16 MsgA-PUSCH-Resource-r16 OPTIONAL, -- Cond InitialBWPConfig

msgA-PUSCH-ResourceGroupB-r16 MsgA-PUSCH-Resource-r16 OPTIONAL, -- Cond GroupBConfigured

msgA-TransformPrecoder-r16 ENUMERATED {enabled, disabled} OPTIONAL, -- Need R

msgA-DataScramblingIndex-r16 INTEGER (0..1023) OPTIONAL, -- Need S

msgA-DeltaPreamble-r16 INTEGER (-1..6) OPTIONAL -- Need R

}

MsgA-PUSCH-Resource-r16 ::= SEQUENCE {

msgA-MCS-r16 INTEGER (0..15),

nrofSlotsMsgA-PUSCH-r16 INTEGER (1..4),

nrofMsgA-PO-PerSlot-r16 ENUMERATED {one, two, three, six},

msgA-PUSCH-TimeDomainOffset-r16 INTEGER (1..32),

msgA-PUSCH-TimeDomainAllocation-r16 INTEGER (1..maxNrofUL-Allocations) OPTIONAL, -- Need S

startSymbolAndLengthMsgA-PO-r16 INTEGER (0..127) OPTIONAL, -- Need S

mappingTypeMsgA-PUSCH-r16 ENUMERATED {typeA, typeB} OPTIONAL, -- Need S

guardPeriodMsgA-PUSCH-r16 INTEGER (0..3) OPTIONAL, -- Need R

guardBandMsgA-PUSCH-r16 INTEGER (0..1),

frequencyStartMsgA-PUSCH-r16 INTEGER (0..maxNrofPhysicalResourceBlocks-1),

nrofPRBs-PerMsgA-PO-r16 INTEGER (1..32),

nrofMsgA-PO-FDM-r16 ENUMERATED {one, two, four, eight},

msgA-IntraSlotFrequencyHopping-r16 ENUMERATED {enabled} OPTIONAL, -- Need R

msgA-HoppingBits-r16 BIT STRING (SIZE(2)) OPTIONAL, -- Need R

msgA-DMRS-Config-r16 MsgA-DMRS-Config-r16,

nrofDMRS-Sequences-r16 INTEGER (1..2),

msgA-Alpha-r16 ENUMERATED {alpha0, alpha04, alpha05, alpha06,

alpha07, alpha08, alpha09, alpha1} OPTIONAL, -- Need S

interlaceIndexFirstPO-MsgA-PUSCH-r16 INTEGER (1..10) OPTIONAL, -- Need R

nrofInterlacesPerMsgA-PO-r16 INTEGER (1..10) OPTIONAL, -- Need R

...

}

MsgA-DMRS-Config-r16 ::= SEQUENCE {

msgA-DMRS-AdditionalPosition-r16 ENUMERATED {pos0, pos1, pos3} OPTIONAL, -- Need S

msgA-MaxLength-r16 ENUMERATED {len2} OPTIONAL, -- Need S

msgA-PUSCH-DMRS-CDM-Group-r16 INTEGER (0..1) OPTIONAL, -- Need S

msgA-PUSCH-NrofPorts-r16 INTEGER (0..1) OPTIONAL, -- Need S

msgA-ScramblingID0-r16 INTEGER (0..65535) OPTIONAL, -- Need S

msgA-ScramblingID1-r16 INTEGER (0..65535) OPTIONAL -- Need S

}

Example

Example 01 : FDD, UL 2x2

Thus example is from the test with Amarisoft Callbox and Amarisoft UEsim.

[1] SIB1 (For the full SIB1 message, check here)

message c1: systemInformationBlockType1: {

....

uplinkConfigCommon {

...

initialUplinkBWP {

genericParameters {

locationAndBandwidth 28875,

subcarrierSpacing kHz15

rach-ConfigCommon setup: {

rach-ConfigGeneric {

prach-ConfigurationIndex 16,

msg1-FDM one,

msg1-FrequencyStart 7,

zeroCorrelationZoneConfig 15,

preambleReceivedTargetPower -110,

preambleTransMax n7,

powerRampingStep dB4,

ra-ResponseWindow sl10

ssb-perRACH-OccasionAndCB-PreamblesPerSSB one: n8,

ra-ContentionResolutionTimer sf64,

prach-RootSequenceIndex l839: 1,

restrictedSetConfig unrestrictedSet

pusch-ConfigCommon setup: {

pusch-TimeDomainAllocationList {

{

k2 4,

mappingType typeA,

startSymbolAndLength 41

{

k2 4,

mappingType typeA,

startSymbolAndLength 27

}

p0-NominalWithGrant -84

...

msgA-ConfigCommon-r16 setup: {

rach-ConfigCommonTwoStepRA-r16 {

rach-ConfigGenericTwoStepRA-r16 {

msgB-ResponseWindow-r16 sl40

msgA-CB-PreamblesPerSSB-PerSharedRO-r16 16,

msgA-RSRP-Threshold-r16 56

msgA-PUSCH-Config-r16 {

msgA-PUSCH-ResourceGroupA-r16 {

msgA-MCS-r16 5,

nrofSlotsMsgA-PUSCH-r16 1,

nrofMsgA-PO-PerSlot-r16 one,

msgA-PUSCH-TimeDomainOffset-r16 4,

startSymbolAndLengthMsgA-PO-r16 27,

mappingTypeMsgA-PUSCH-r16 typeA,

guardBandMsgA-PUSCH-r16 0,

frequencyStartMsgA-PUSCH-r16 7,

nrofPRBs-PerMsgA-PO-r16 1,

nrofMsgA-PO-FDM-r16 four,

msgA-DMRS-Config-r16 {

msgA-PUSCH-NrofPorts-r16 1

nrofDMRS-Sequences-r16 1

msgA-TransformPrecoder-r16 disabled

}

timeAlignmentTimerCommon infinity

...

}

[2] MsgA : PRACH @ SFN 929.1

Message: sequence_index=20 ta=1 prb=7:6 two_steps=1 snr=29.2

[3] MsgA : PUSCH @ SFN 929.5

Message: harq=0 prb=7 symb=0:14 CW0: tb_len=12 mod=2 rv_idx=0 cr=0.37 retx=0 crc=OK snr=35.4 epre=-76.1 ta=0.9

[4] MsgB : RAR

Message: MSGB: uecri=0x1453233e84e6 mac_sdu=1

uecri=0x1453233e84e6

ta=1

harq_feedback_timing_ind=1

pucch_rsc_ind=0

pucch_tpc_command=0

c-rnti=0x8c01

mac_sdu=1

[5] PDSCH @ SFN 929.9 - PDSCH carrying MsgB

Message: harq=si prb=2:16 symb=1:13 CW0: tb_len=317 mod=2 rv_idx=0 cr=0.66

[6] PDCCH @ SFN 929.9 - PDCCH/DCI Scheduling the PDSCH carrying the RAR

Message: ss_id=1 cce_index=0 al=4 dci=1_0

rb_alloc=0x5a0

time_domain_rsc=0

vrb_to_prb_map=0

mcs=9

tb_scaling=0

lsb_sfn=1

Reference

[1] Power Saving Techniques for 5G and Beyond