Random Access (4-step & 2-step) in 5G NR

1. Why Random Access Exists, and What Triggers It

Uplink transmission in NR is scheduled and time-aligned. Before the gNB can grant a UE anything, two things must be true: the gNB must know the UE exists, and the UE's uplink transmissions must arrive inside the gNB's receive window despite propagation delay that varies by tens of microseconds across a cell. Random Access is the one procedure in NR that bootstraps both from nothing. It is called random because in the contention-based case the UE picks its opening move -- a preamble sequence -- at random from a pool shared with every other UE in the cell, since the network has no way to pre-assign one to a UE it does not yet know about.

TS 38.321 clause 5.1.1 lists the triggers. They matter far more than they first appear, because the same four messages carry completely different consequences depending on which trigger started them:

Table 1. Random Access triggers, TS 38.321 cl. 5.1.1, with the RRC-level consequence of exhaustion. See §12.

2. Contention-Based and Contention-Free

Table 2. CBRA and CFRA differ in exactly one thing -- who chooses the preamble -- and everything else follows from that.

CFRA is not a different procedure so much as the same procedure with the ambiguity removed. Because the gNB knows which UE owns preamble index n, detecting n is itself the identification, so the RAR can carry the real C-RNTI and the UE can go straight to using it. This is why a handover completes in roughly a third of the messages of an initial access, and why handover RACH failures show up in logs as timer expiries (T304) rather than as contention-resolution mismatches.

3. 4-Step RACH, Message by Message

3.1 MSG1 -- the preamble

The UE transmits a Zadoff-Chu-derived preamble sequence on a PRACH occasion. It carries no data at all -- the only information conveyed is which of up to 64 preamble indices was sent, and when and where it was sent. That pair is the entire content of MSG1, and both halves are used: the index becomes the RAPID echoed in the RAR, and the arrival time relative to the expected occasion becomes the timing advance.

• Sequence: generated from prach-RootSequenceIndex plus a cyclic shift determined by zeroCorrelationZoneConfig. Length 839 for long formats, 139 for short formats TS 38.211 cl. 6.3.3.1.
• Power: open-loop, from preambleReceivedTargetPower plus the UE's own pathloss estimate, ramped on each retry (§10).
• Beam: the preamble is sent using the spatial filter of the SSB the UE selected, which is how the gNB learns which beam to answer on.

3.2 MSG2 -- the Random Access Response

Within ra-ResponseWindow slots of the end of the preamble, the UE monitors Type1-PDCCH common search space for a DCI whose CRC is scrambled with the RA-RNTI it computed from its own PRACH occasion. The scheduled PDSCH carries a RAR MAC PDU, which may hold several MAC RARs (one per preamble the gNB detected in that occasion) plus an optional backoff subheader. The UE walks the subheaders looking for a RAPID equal to the preamble index it sent; anything else in the PDU belongs to other UEs and is discarded. §8 has the bit layout.

A matching RAR gives the UE three things it did not have: a timing advance command so its next transmission lands in the right place, an uplink grant for MSG3, and a Temporary C-RNTI.

3.3 MSG3 -- the first real uplink message

MSG3 is sent on PUSCH using the RAR's grant, scrambled with the TC-RNTI, and is the first transmission that is actually HARQ-protected and power-controlled in closed loop. For initial access it carries RRCSetupRequest on CCCH, whose payload includes a ue-Identity -- either a 5G-S-TMSI-Part1 if the UE has one, or a 39-bit random value. That identity is the token used for contention resolution. For other triggers MSG3 instead carries RRCResumeRequest, RRCReestablishmentRequest, RRCSystemInfoRequest, or, in RRC_CONNECTED, a C-RNTI MAC CE plus buffered data.

3.4 MSG4 -- contention resolution

The gNB echoes back the first 48 bits of the CCCH SDU it received in MSG3, inside a UE Contention Resolution Identity MAC CE TS 38.321 cl. 6.1.3.3. Every UE that sent that same preamble in that same occasion decodes the CE and compares. Exactly one match is possible. The winner considers Random Access successful and promotes its TC-RNTI to C-RNTI; the losers discard the TC-RNTI and restart from MSG1 with the power-ramping counter unchanged -- their preamble was received fine, so ramping further would only raise interference.

For a UE already in RRC_CONNECTED that sent a C-RNTI MAC CE in MSG3, contention resolution is implicit instead: successful reception of a PDCCH addressed to its own C-RNTI is the confirmation, and no CE is needed.

4. 2-Step RACH and the Fallback Mechanism

2-step RACH, added in Rel-16, removes a full round trip by bundling the preamble and the MSG3-equivalent payload into a single transmission. The trade is that the payload is sent before any timing advance has been applied and before any grant has been received, so it only works where the UE's uplink timing error is already within the cyclic prefix and its power estimate is good enough -- in practice, in good coverage, which is why selection is gated on msgA-RSRP-Threshold.

msgB-ResponseWindow

msgA-TransMax

Table 3. MSGA and MSGB contents, TS 38.321 cl. 5.1.3a and 5.1.4a.

Three outcomes are possible after MSGA, and the distinction is exactly what makes 2-step deployable:

1. Preamble and payload both decoded -> successRAR. The UE has a C-RNTI and the procedure is over in two messages.

2. Preamble decoded, payload not -> fallbackRAR. The UE retransmits just the payload on the granted PUSCH, i.e. it completes as 4-step from MSG3 onward. No new preamble, no power re-ramp, no restart.

3. Nothing decoded -> no MSGB inside msgB-ResponseWindow. The UE ramps power and retries MSGA, up to msgA-TransMax, after which it switches to 4-step for the remaining attempts if 4-step is also configured.

5. Before MSG1: Resource and Preamble Selection

By the time a preamble goes out, the UE has already made four decisions that the gNB will never see directly. Understanding them is what turns an unexplained preamble index in a log into a readable statement about what the UE believed.

5.1 SSB and beam selection

The UE measures SSB RSRP and picks an SSB whose RSRP exceeds rsrp-ThresholdSSB. If none does, it may pick any SSB TS 38.321 cl. 5.1.2. The chosen SSB determines both the spatial filter used for the preamble and, through the SSB-to-RO association, the set of PRACH occasions and preamble indices the UE is allowed to use. This is the mechanism by which a beamformed cell steers uplink access without any uplink measurement having happened yet.

5.2 SSB-to-RO association

ssb-perRACH-OccasionAndCB-PreamblesPerSSB specifies how many SSBs share one RO and how many contention-based preambles each SSB gets. The choice branch names are the ratio: oneEighth means one SSB is mapped across eight consecutive ROs (many occasions per beam, useful for a cell with few beams and heavy load), while sixteen means sixteen SSBs share a single RO (few occasions, many beams). The association repeats over an association period of 1, 2, 4, 8 or 16 PRACH configuration periods, chosen as the shortest that covers all actual SSBs TS 38.213 cl. 8.1.

5.3 Preamble group A and group B

When groupBconfigured is present, the 64 preambles are split. Choosing a group B preamble is the UE's way of saying "my MSG3 will be larger than ra-Msg3SizeGroupA and I have enough power headroom to send it well", letting the gNB size the MSG3 grant appropriately from the preamble alone. The UE picks group B only if both conditions hold; if it needs the bigger MSG3 but lacks headroom, it falls back to group A and accepts segmentation. messagePowerOffsetGroupB is the headroom test's threshold offset.

5.4 4-step or 2-step

If both are configured on the selected carrier and the measured SSB RSRP of the selected SSB is above msgA-RSRP-Threshold, the UE uses 2-step; otherwise 4-step. If only one is configured, that one is used. A dedicated CFRA configuration overrides all of this.

6. PRACH Configuration: Formats, Occasions, Root Sequences

6.1 Preamble formats

The format sets sequence length, subcarrier spacing, number of repeated sequences and cyclic prefix length -- and through the CP, the maximum round-trip delay the gNB can absorb, which is the cell radius limit. Long formats use a 839-length sequence at 1.25 or 5 kHz PRACH SCS; short formats use 139 at the same SCS family as the uplink data.

Table 4. Preamble formats, TS 38.211 Tables 6.3.3.1-1 and 6.3.3.1-2. k = 64; durations and radii are at 15 kHz and halve per SCS doubling. Radii are the CP-implied limits, not link-budget limits.

6.2 PRACH occasions in time and frequency

prach-ConfigurationIndex selects a row of a large table TS 38.211 Tables 6.3.3.2-2 to 6.3.3.2-4 giving the format, the system-frame periodicity x and offset y, which subframes or slots within those frames carry ROs, the starting symbol, and how many time-domain ROs per slot. msg1-FDM (one/two/four/eight) then multiplies that in frequency, and msg1-FrequencyStart places the block in the uplink BWP.

6.3 Root sequences and cyclic shifts

The 64 preambles a cell offers are generated from consecutive logical root sequences starting at prach-RootSequenceIndex, each root yielding several preambles through cyclic shifts. How many shifts fit per root is set by zeroCorrelationZoneConfig (N_CS): a larger zero-correlation zone tolerates more timing uncertainty -- necessary in a big cell -- but yields fewer preambles per root, so more roots are consumed to reach 64. restrictedSetConfig (type A or B) removes shifts that alias under high Doppler, at further cost in preambles per root.

7. RA-RNTI, MSGB-RNTI, and the Worked Addressing Example

The gNB must address the RAR to a UE whose identity it does not know. It solves this by addressing the occasion instead: both sides can compute the same value from the time and frequency position of the preamble transmission TS 38.321 cl. 5.1.4.

RA-RNTI   = 1 + s_id + 14 x t_id + 14 x 80 x f_id + 14 x 80 x 8 x ul_carrier_id
MSGB-RNTI = RA-RNTI + 14 x 80 x 8 x 2

  s_id           first OFDM symbol of the PRACH occasion   0 .. 13
  t_id           first slot of the PRACH occasion in a system frame   0 .. 79
  f_id           frequency-domain index of the RO among the FDM'd ones   0 .. 7
  ul_carrier_id  0 = normal uplink (NUL),  1 = supplementary uplink (SUL)

7.1 Worked example

A UE transmits its preamble in the RO at symbol 4 of slot 19, frequency-domain index 1, on the normal uplink carrier:

The gNB scrambles the DCI CRC for that RAR with 1391, and every UE that used that occasion -- regardless of preamble index or beam -- descrambles it successfully and reads the PDU. Disambiguation happens one layer up, at the RAPID subheader, and one layer up again at contention resolution.

8. Inside MSG2: RAR MAC PDU Bit Layout

A RAR MAC PDU is one or more MAC subPDUs. Each subPDU is a subheader, optionally followed by a payload. There are two subheader shapes, distinguished by the T bit:

• T = 0, backoff subheader: E | T | R | R | BI(4). Carries no payload. If present it must be first, and it applies to every UE reading the PDU.
• T = 1, RAPID subheader: E | T | RAPID(6). Followed by a 7-octet MAC RAR, except when the RAPID corresponds to an on-demand SI request, in which case there is no payload -- the acknowledgement is the RAPID.
• E is the extension bit: 1 means another subPDU follows, 0 means this is the last.

R

Table 5. MAC RAR fields, TS 38.321 cl. 6.2.3. Total 56 bits = 7 octets, fixed.

8.1 The MSGB variant

A MSGB MAC PDU TS 38.321 cl. 6.1.5a can mix successRAR and fallbackRAR subPDUs plus a backoff subheader, because one MSGB answers every MSGA the gNB detected in that occasion and some payloads will have decoded while others did not. A successRAR is larger than a MAC RAR -- it carries the 48-bit contention resolution identity and a C-RNTI, since it must both identify the winner and equip it.

9. Timing Advance: What the 12-Bit Command Actually Means

The RAR's TA command is an absolute value, unlike the 6-bit relative adjustments carried later in Timing Advance Command MAC CEs. It sets:

N_TA = T_A x 16 x 64 / (2^mu)            [in units of T_c]
  T_c = 1 / (480000 x 4096) s  ~ 0.509 ns
  mu  = numerology: 0 = 15 kHz, 1 = 30 kHz, 2 = 60 kHz, 3 = 120 kHz
  T_A = the 12-bit RAR field, 0 .. 3846

The maximum T_A of 3846 corresponds to roughly 1 ms of advance at 15 kHz -- about 150 km of one-way path -- which is the ceiling on terrestrial cell size regardless of preamble format. Rel-17 NTN raises this with a separate common TA (ta-Common) applied on top, precisely because the 12-bit field cannot express a satellite's delay.

10. Power Control and Ramping

PRACH power is open loop -- there is no feedback yet -- so the UE computes it from a target the network broadcasts plus its own downlink pathloss estimate, and increases it on each failed attempt TS 38.213 cl. 7.4:

P_PRACH = min { P_CMAX ,
                preambleReceivedTargetPower
                + PL                                (DL pathloss estimate)
                + DELTA_preamble                    (format-dependent offset)
                + (COUNTER - 1) x powerRampingStep }

COUNTER = PREAMBLE_POWER_RAMPING_COUNTER, starts at 1

11. Timers, Counters, and Backoff

ra-ResponseWindow

msgB-ResponseWindow

ra-ContentionResolutionTimer

preambleTransMax

msgA-TransMax

powerRampingStep

preambleReceivedTargetPower

backoffIndicator

T300

T304

T319

Table 6. RACH-relevant timers and counters, TS 38.331 RACH-ConfigGeneric and UE-TimersAndConstants.

11.1 Backoff indicator values

A gNB under load sets a backoff subheader in the RAR. Every UE reading that PDU -- including UEs whose RAPID is not present -- waits a uniformly random time in [0, BI] before its next attempt, which spreads a thundering herd without any per-UE signalling.

Table 7. Backoff parameter values, TS 38.321 Table 7.2-1.

12. Failure Modes and What Each One Means

Table 8. RACH failure modes, what the UE does, and what each one points at.

13. Configuration Reference (ASN.1 and Ranges)

The broadcast configuration lives at SIB1 -> servingCellConfigCommon -> uplinkConfigCommon -> initialUplinkBWP -> rach-ConfigCommon. Abridged to the fields that matter for the procedure:

RACH-ConfigCommon ::= SEQUENCE {
    rach-ConfigGeneric                RACH-ConfigGeneric,
    totalNumberOfRA-Preambles         INTEGER (1..63)              OPTIONAL,
    ssb-perRACH-OccasionAndCB-PreamblesPerSSB CHOICE {
        oneEighth   ENUMERATED {n4,n8,n12,...,n64},
        oneFourth   ENUMERATED {n4,n8,n12,...,n64},
        oneHalf     ENUMERATED {n4,n8,n12,...,n64},
        one         ENUMERATED {n4,n8,n12,...,n64},
        two         ENUMERATED {n4,n8,...,n32},
        four        INTEGER (1..16),
        eight       INTEGER (1..8),
        sixteen     INTEGER (1..4)
    }                                                              OPTIONAL,
    groupBconfigured SEQUENCE {
        ra-Msg3SizeGroupA          ENUMERATED {b56,b144,b208,b256,...,b1000},
        messagePowerOffsetGroupB   ENUMERATED {minusinfinity,dB0,dB5,...,dB18},
        numberOfRA-PreamblesGroupA INTEGER (1..64)
    }                                                              OPTIONAL,
    ra-ContentionResolutionTimer      ENUMERATED {sf8,sf16,...,sf64},
    rsrp-ThresholdSSB                 RSRP-Range                   OPTIONAL,
    prach-RootSequenceIndex           CHOICE {
        l839  INTEGER (0..837),
        l139  INTEGER (0..137)
    },
    msg1-SubcarrierSpacing            SubcarrierSpacing            OPTIONAL,
    restrictedSetConfig               ENUMERATED {unrestrictedSet,
                                                 restrictedSetTypeA,
                                                 restrictedSetTypeB},
    msg3-transformPrecoder            ENUMERATED {enabled}         OPTIONAL
}

RACH-ConfigGeneric ::= SEQUENCE {
    prach-ConfigurationIndex          INTEGER (0..255),
    msg1-FDM                          ENUMERATED {one,two,four,eight},
    msg1-FrequencyStart               INTEGER (0..maxNrofPhysicalResourceBlocks-1),
    zeroCorrelationZoneConfig         INTEGER (0..15),
    preambleReceivedTargetPower       INTEGER (-202..-60),
    preambleTransMax                  ENUMERATED {n3,n4,n5,n6,n7,n8,n10,
                                                 n20,n50,n100,n200},
    powerRampingStep                  ENUMERATED {dB0,dB2,dB4,dB6},
    ra-ResponseWindow                 ENUMERATED {sl1,sl2,sl4,sl8,sl10,
                                                 sl20,sl40,sl80}
}

Listing 1. Abridged from TS 38.331. ... marks omitted enumeration members and extension markers.

14. Illustrative Message Traces

14.1 The broadcast configuration the UE starts from

SIB1
 servingCellConfigCommon
  uplinkConfigCommon
   initialUplinkBWP
    rach-ConfigCommon
     rach-ConfigGeneric
      prach-ConfigurationIndex ....... 159        -- format B4, every 10 ms
      msg1-FDM ....................... four       -- 4 ROs side by side
      msg1-FrequencyStart ............ 12         -- PRB offset in the BWP
      zeroCorrelationZoneConfig ...... 11
      preambleReceivedTargetPower .... -104       -- dBm
      preambleTransMax ............... n10
      powerRampingStep ............... dB4
      ra-ResponseWindow .............. sl10
     totalNumberOfRA-Preambles ....... 56         -- 8 reserved for CFRA
     ssb-perRACH-OccasionAndCB-PreamblesPerSSB
      one ........................... n56
     groupBconfigured
      ra-Msg3SizeGroupA ............. b56
      messagePowerOffsetGroupB ...... dB10
      numberOfRA-PreamblesGroupA .... 48
     ra-ContentionResolutionTimer .... sf64
     rsrp-ThresholdSSB ............... 42         -- RSRP-Range -> -114 dBm
     prach-RootSequenceIndex
      l139 .......................... 22
     restrictedSetConfig ............. unrestrictedSet

Listing 2. rach-ConfigCommon as it would appear in a decoded SIB1.

14.2 MSG1 and MSG2

12:04:31.882  [MAC-UL] RA procedure initiated
              trigger ................ initialAccess (RRC_IDLE)
              ra-Type ................ 4-step   (2-step not configured)
              selected SSB ........... idx 2, RSRP -96 dBm  (> -114 threshold)
              preamble group ......... A        (Msg3 56 bits <= b56)
              ra-PreambleIndex ....... 23       (random, 0..47 in group A)
              PRACH occasion ......... SFN 132, slot 19, symbol 4, f_id 1
              RA-RNTI ................ 1391     (see the worked example, S7)
              PREAMBLE_POWER_RAMPING_COUNTER 1
              P_PRACH ................ 14.0 dBm (target -104 + PL 118)

12:04:31.887  [PHY-DL] PDCCH found, DCI 1_0, CRC scrambled by RA-RNTI 1391
              -> PDSCH  32 PRB, MCS 4

12:04:31.888  [MAC-DL] RAR MAC PDU, 9 octets
              subPDU 1  E=1 T=0  BI=3            -- 30 ms backoff advertised
              subPDU 2  E=0 T=1  RAPID=23        -- our preamble, match
                MAC RAR
                  R ...................... 0
                  Timing Advance Command . 245     -> N_TA 125440 T_c ~ 63.8 us
                  UL Grant ............... 0x0A3C81
                    freq hopping ......... 0
                    PUSCH freq alloc ..... 6 PRB from PRB 18
                    PUSCH time alloc ..... k2=3, S=0, L=14
                    MCS .................. 2
                    TPC for MSG3 ......... 0 dB
                    CSI request .......... 0
                  Temporary C-RNTI ....... 0x4601
              -- RAPID matched: stop ra-ResponseWindow, apply TA, send MSG3

Listing 3. Preamble transmission and a matching RAR, including the backoff subheader that a loaded cell would set.

14.3 MSG3 and MSG4

12:04:31.892  [MAC-UL] MSG3 on PUSCH, TC-RNTI 0x4601, 8 octets
              CCCH SDU (48 bits carried onward for contention resolution)
              RRCSetupRequest
                ue-Identity
                  randomValue .......... 0x3F2A19C4B7   -- 39-bit random
                establishmentCause ..... mo-Signalling
                spare .................. 0

12:04:31.902  [MAC-DL] PDSCH to TC-RNTI 0x4601, 42 octets
              MAC subheader  LCID=62  (UE Contention Resolution Identity)
                UE Contention Resolution Identity  48 bits
                  = 0x1F3F2A19C4B7  -- first 48 bits of our MSG3 CCCH SDU
              MAC subheader  LCID=0   (CCCH)
                RRCSetup
                  rrc-TransactionIdentifier .. 0
                  radioBearerConfig
                    srb-ToAddModList  srb-Identity 1
                  masterCellGroup ............ (SEQUENCE, 214 bytes)

12:04:31.902  [MAC] Contention Resolution Identity matches own MSG3
              -> RA procedure successfully completed
              -> TC-RNTI 0x4601 promoted to C-RNTI
              -> stop ra-ContentionResolutionTimer

12:04:31.910  [RRC-UL] RRCSetupComplete on SRB1 (DCCH)
                selectedPLMN-Identity ...... 1
                dedicatedNAS-Message ....... (NAS Registration Request)

Listing 4. Contention resolution succeeding: the echoed 48-bit identity is the whole mechanism.

14.4 A failing procedure

12:07:14.201  [MAC-UL] RA initiated  trigger=initialAccess  SSB idx 5 RSRP -119 dBm
12:07:14.201  [MAC-UL] preamble 31  counter 1   P_PRACH 19.0 dBm
12:07:14.211  [MAC]    ra-ResponseWindow expired, no RAR
12:07:14.229  [MAC-UL] preamble 07  counter 2   P_PRACH 23.0 dBm  (P_CMAX)
12:07:14.239  [MAC]    ra-ResponseWindow expired, no RAR
12:07:14.244  [MAC]    SSB reselected: idx 5 -> idx 6, RSRP -117 dBm
12:07:14.244  [MAC]    spatial filter changed -> counter NOT incremented
12:07:14.262  [MAC-UL] preamble 44  counter 2   P_PRACH 23.0 dBm
                       ... attempts 4..9 elided, all at 23.0 dBm ...
12:07:14.612  [MAC-UL] preamble 12  counter 10  P_PRACH 23.0 dBm
12:07:14.622  [MAC]    ra-ResponseWindow expired, no RAR
12:07:14.622  [MAC]    PREAMBLE_TRANSMISSION_COUNTER = preambleTransMax (n10)
12:07:14.622  [MAC]    -> Random Access problem indication to RRC
12:07:14.622  [RRC]    T300 still running; RA problem during establishment
12:07:14.622  [RRC]    -> connection establishment failure
12:07:14.622  [RRC]    -> store connEstFailReport (measured RSRP, numberOfPreamblesSent 10)
12:07:14.623  [RRC]    -> to RRC_IDLE, perform cell reselection

Listing 5. Ten attempts, eight of them at maximum power, ending in a connection establishment failure rather than an RLF -- because the trigger was initial access.

15. Release Deltas: Rel-15 to Rel-18

Table 9. RACH-relevant changes by release. Feature presence should always be confirmed against the UE capability exchange -- see the companion UE Capability document.

16. Reading RACH in Logs: A Checklist

1. Find the trigger first. Everything downstream is interpreted differently depending on it (§1). If the log does not state it, infer it from what RRC was doing: T300 running means initial access, T304 means handover, T319 means resume.

2. Establish CBRA or CFRA. A dedicated ra-PreambleIndex in a preceding reconfigurationWithSync or PDCCH order means CFRA, and contention resolution should be absent. Its presence in a supposed handover means the CFRA resource was unavailable -- worth investigating on its own.

3. Check the preamble index against the group boundary. An index below numberOfRA-PreamblesGroupA is group A; at or above it is group B and implies a large MSG3 with adequate headroom.

4. Recompute the RA-RNTI from the logged occasion (§7). A mismatch against the RNTI the UE actually monitored is a configuration or frame-timing problem, and it is silent otherwise.

5. Read the TA command as a distance (§9) and sanity-check it against the cell you believe answered.

6. Count attempts, then check the power on each. Many attempts at constant power means either powerRampingStep = dB0, or P_CMAX clipping, or repeated spatial filter changes suppressing the counter (§10). These three look identical in a naive read and have entirely different fixes.

7. Distinguish the four no-progress cases: no RAR, RAPID mismatch, no MSG4, MSG4 identity mismatch (§12). They are four different problems and only the first two can be helped by more power.

8. For 2-step, count fallbacks separately from failures. A fallbackRAR is a successful preamble. A high fallback rate with a low failure rate means msgA-RSRP-Threshold is too low, not that coverage is bad.

17. Glossary

18. References

• 3GPP TS 38.321 -- NR MAC protocol specification. Clause 5.1 (Random Access procedure), 5.1.1 (initialisation and triggers), 5.1.2 (resource selection), 5.1.3 / 5.1.3a (preamble / MSGA transmission), 5.1.4 / 5.1.4a (RAR / MSGB reception, RA-RNTI and MSGB-RNTI), 5.1.5 (contention resolution), 5.1.6 (completion), 6.1.5 / 6.1.5a (RAR and MSGB MAC PDU), 6.2.3 (MAC RAR), 6.1.3.3 (UE Contention Resolution Identity CE), Table 7.2-1 (backoff values).
• 3GPP TS 38.213 -- Physical layer procedures for control. Clause 4.2 (timing advance), clause 7.4 (PRACH power control), clause 8 (random access procedure, SSB-to-RO association, RAR window).
• 3GPP TS 38.211 -- Physical channels and modulation. Clause 6.3.3 (preamble generation), Tables 6.3.3.1-1 / 6.3.3.1-2 (formats), Tables 6.3.3.2-2 to 6.3.3.2-4 (PRACH configuration index).
• 3GPP TS 38.331 -- RRC protocol specification. RACH-ConfigCommon, RACH-ConfigGeneric, RACH-ConfigDedicated, MsgA-ConfigCommon, BeamFailureRecoveryConfig, UE-TimersAndConstants.
• 3GPP TS 38.300 -- NR overall description. Clause 9.2.6 (random access procedure overview and its place in the state model).
• 3GPP TS 38.104 / 38.133 -- RF and RRM requirements, for the measurement accuracy that underpins rsrp-ThresholdSSB and msgA-RSRP-Threshold decisions.

Companion documents in this set

• 01 Registration Process -- what MSG3's NAS payload goes on to do.
• 02 Radio Frame Structure -- SFN, slots and the symbol numbering that s_id and t_id index.
• 04 Timing Advance -- the TA maintenance loop after the initial command in §9.
• 06 Channel Mapping -- how CCCH/UL-SCH/PUSCH relate for MSG3.
• 07 MAC PDU and Control Elements -- the general subheader format that §8 specialises.
• 10 BSR, PHR, SR -- the SR mechanism that RACH substitutes for.
• 12 Beam Failure Recovery -- a CFRA user.
• 16 RLM and RLF -- where preambleTransMax exhaustion goes in RRC_CONNECTED.
• 17 System Information / 18 MIB and SIB1 IEs -- where rach-ConfigCommon is broadcast.
• 22-25 Handover documents -- the largest CFRA user.