5G/NR - 2Step RACH                                           Home : www.sharetechnote.com

 

 

 

 

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.

 

 

 

Overall Procedure

 

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.

 

 

 

 

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 steps
  • Allowing PUSCH transmission at the very first step

 

 

 

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 - 3GPP