Content for  TS 38.300  Word version:  18.4.0

The random access procedure is triggered by a number of events:

• Initial access from RRC_IDLE;
• RRC Connection Re-establishment procedure;
• DL or UL data arrival, during RRC_CONNECTED or during RRC_INACTIVE while SDT procedure (see clause 18.0) is ongoing, when UL synchronisation status is "non-synchronised";
• UL data arrival, during RRC_CONNECTED or during RRC_INACTIVE while SDT procedure is ongoing, when there are no PUCCH resources for SR available;
• Handover, except for when RACH-less HO is configured;
• SR failure;
• Explicit request by RRC upon synchronous reconfiguration;
• RRC Connection Resume procedure from RRC_INACTIVE;
• To establish time alignment for a primary or a secondary TAG;
• Request for Other SI (see clause 7.3);
• Beam failure recovery;
• Consistent UL LBT failure on SpCell;
• SDT in RRC_INACTIVE (see clause 18);
• Positioning purpose during RRC_CONNECTED requiring random access procedure, e.g., when timing advance is needed for UE positioning;
• Early UL synchronization with an LTM candidate cell;
• RACH-based LTM cell switch.

Two types of random access procedure are supported: 4-step RA type with MSG1 and 2-step RA type with MSGA. Both types of RA procedure support contention-based random access (CBRA) and contention-free random access (CFRA) as shown on Figure 9.2.6-1 below. The UE selects the type of random access at initiation of the random access procedure based on network configuration:

• when CFRA resources are not configured, an RSRP threshold is used by the UE to select between 2-step RA type and 4-step RA type;
• when CFRA resources for 4-step RA type are configured, UE performs random access with 4-step RA type;
• when CFRA resources for 2-step RA type are configured, UE performs random access with 2-step RA type.

The network does not configure CFRA resources for 4-step and 2-step RA types at the same time for a Bandwidth Part (BWP). CFRA with 2-step RA type is only supported for handover. The MSG1 of the 4-step RA type consists of a preamble on PRACH. After MSG1 transmission, the UE monitors for a response from the network within a configured window. For CFRA, dedicated preamble for MSG1 transmission is assigned by the network and upon receiving random access response from the network, the UE ends the random access procedure as shown in Figure 9.2.6-1 (c). For CBRA, upon reception of the random access response, the UE sends MSG3 using the UL grant scheduled in the response and monitors contention resolution as shown in Figure 9.2.6-1 (a). If contention resolution is not successful after MSG3 (re)transmission(s), the UE goes back to MSG1 transmission. The MSGA of the 2-step RA type includes a preamble on PRACH and a payload on PUSCH. After MSGA transmission, the UE monitors for a response from the network within a configured window. For CFRA, dedicated preamble and PUSCH resource are configured for MSGA transmission and upon receiving the network response, the UE ends the random access procedure as shown in Figure 9.2.6-1 (d). For CBRA, if contention resolution is successful upon receiving the network response, the UE ends the random access procedure as shown in Figure 9.2.6-1 (b); while if fallback indication is received in MSGB, the UE performs MSG3 transmission using the UL grant scheduled in the fallback indication and monitors contention resolution as shown in Figure 9.2.6-2. If contention resolution is not successful after MSG3 (re)transmission(s), the UE goes back to MSGA transmission. If the random access procedure with 2-step RA type is not completed after a number of MSGA transmissions, the UE can be configured to switch to CBRA with 4-step RA type. For the random access procedure towards an LTM candidate cell for early UL TA acquisition, CFRA triggered by a PDCCH order is used. The UE sends MSG1 towards the cell without monitoring for a response from it as shown in Figure 9.2.6-1 (e). To support UE power ramping, the UE may perform MSG1 retransmission as indicated by the network.

For random access in a cell configured with SUL, the network can explicitly signal which carrier to use (UL or SUL). Otherwise, the UE selects the SUL carrier if and only if the measured quality of the DL is lower than a broadcast threshold. UE performs carrier selection before selecting between 2-step and 4-step RA type. The RSRP threshold for selecting between 2-step and 4-step RA type can be configured separately for UL and SUL. Once started, all uplink transmissions of the random access procedure remain on the selected carrier. The network can associate a set of RACH resources with feature(s) applicable to a Random Access procedure: Network Slicing (see clause 16.3), (e)RedCap (see clause 16.13), SDT (see clause 18), and NR coverage enhancement (see clause 19). A set of RACH resources associated with a feature is only valid for random access procedures applicable to at least that feature; and a set of RACH resources associated with several features is only valid for random access procedures having at least all of these features. The UE selects the set(s) of applicable RACH resources, after uplink carrier (i.e. NUL or SUL) and BWP selection and before selecting the RA type. When CA is configured, random access procedure with 2-step RA type is only performed on PCell while contention resolution can be cross-scheduled by the PCell. When CA is configured, for random access procedure with 4-step RA type, the first three steps of CBRA always occur on the PCell while contention resolution (step 4) can be cross-scheduled by the PCell. The three steps of a CFRA started on the PCell remain on the PCell. CFRA on SCell can only be initiated by the gNB to establish timing advance for a secondary TAG: the procedure is initiated by the gNB with a PDCCH order (step 0) that is sent on an activated SCell of the secondary TAG, preamble transmission (step 1) takes place on the SCell, and Random Access Response (step 2) takes place on PCell. When two TAG IDs are configured for the serving cell, the TAG for which the TA command is applied is indicated in Random Access Response message or in MSGB. To establish timing advance for the other PTAG, CFRA is initiated by the gNB with a PDCCH order.

In RRC_CONNECTED, the UE performs Radio Link Monitoring (RLM) in the active BWP based on reference signals (SSB/CSI-RS) and signal quality thresholds configured by the network. SSB-based RLM is based on the CD-SSB associated to the initial DL BWP and can be configured for the initial DL BWP, for DL BWPs containing the CD-SSB associated to the initial DL BWP, and, if supported, for DL BWPs not containing the CD-SSB associated to the initial DL BWP. Besides, SSB-based RLM can be also performed based on a non-cell defining SSB, if configured for the active DL BWP. RLM can be also performed based on CSI-RS, if configured for the active DL BWP. In case of DAPS handover, the UE continues the detection of radio link failure at the source cell until the successful completion of the random access procedure to the target cell. The UE declares Radio Link Failure (RLF) when one of the following criteria are met:

• Expiry of a radio problem timer started after indication of radio problems from the physical layer (if radio problems are recovered before the timer is expired, the UE stops the timer); or
• Expiry of a timer started upon triggering a measurement report for a measurement identity for which the timer has been configured while another radio problem timer is running; or
• Random access procedure failure; or
• RLC failure; or
• Detection of consistent uplink LBT failures for operation with shared spectrum channel access as described in clause 5.6.1; or
• For IAB-MT, the reception of a BH RLF indication received from its parent node.

After RLF is declared, the UE:

• stays in RRC_CONNECTED;
• in case of DAPS handover, for RLF in the source cell:
  • stops any data transmission or reception via the source link and releases the source link, but maintains the source RRC configuration;
  • if handover failure is then declared at the target cell, the UE:
    • selects a suitable cell and then initiates RRC re-establishment;
    • enters RRC_IDLE if a suitable cell was not found within a certain time after handover failure was declared.
• in case of CHO, for RLF in the source cell:
  • selects a suitable cell and if the selected cell is a CHO candidate and if network configured the UE to try CHO after RLF then the UE attempts CHO execution once, otherwise re-establishment is performed;
  • enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.
• in case of MCG LTM, for RLF in the source cell:
  • selects a suitable cell and if the selected cell is an LTM candidate cell and if network configured the UE to try LTM after RLF then the UE attempts RACH-based LTM execution once, otherwise re-establishment is performed;
  • enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.
• otherwise, for RLF in the serving cell or in case of DAPS handover, for RLF in the target cell before releasing the source cell:
  • selects a suitable cell and then initiates RRC re-establishment;
  • enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.

When RLF occurs at the IAB BH link, the same mechanisms and procedures are applied as for the access link. This includes BH RLF detection and RLF recovery. The IAB-DU can transmit a BH RLF detection indication to its child nodes in the following cases:

• The collocated IAB-MT initiates RRC re-establishment;
• The collocated IAB-MT is dual-connected, detects BH RLF on a BH link, and cannot perform UL re-routing for any traffic. This includes the scenario of an IAB-node operating in EN-DC or NR-DC, which uses only one link for backhauling and has BH RLF on this BH link;
• The collocated IAB-MT has received a BH RLF detection indication from a parent node, and there is no remaining backhaul link that is unaffected by the BH RLF condition indicated.

Upon reception of the BH RLF detection indication, the child node may perform local rerouting for upstream traffic, if possible, over an available BH link. If the IAB-DU has transmitted a BH RLF detection indication to a child node due to an RLF condition on the collocated IAB-MT's parent link, and the collocated IAB-MT's subsequent RLF recovery is successful, the IAB-DU may transmit a BH RLF recovery indication to this child node. If the IAB-DU has transmitted a BH RLF detection indication to a child node due to the reception of a BH RLF detection indication by the collocated IAB-MT, and the collocated IAB-MT receives a BH RLF recovery indication, the IAB-DU may also transmit a BH RLF recovery indication to this child node. Upon reception of the BH RLF recovery indication, the child node reverts the actions triggered by the reception of the previous BH RLF detection indication. In case the RRC re-establishment procedure fails, the IAB-node may transmit a BH RLF indication to its child nodes. The BH RLF detection indication, BH RLF recovery indication and BH RLF indication are transmitted as BAP Control PDUs.

For beam failure detection, the gNB configures the UE with beam failure detection reference signals (SSB or CSI-RS) and the UE declares beam failure when the number of beam failure instance indications from the physical layer reaches a configured threshold before a configured timer expires. For beam failure detection in multi-TRP operation, the gNB configures the UE with two sets of beam failure detection reference signals, and the UE declares beam failure for a TRP / BFD-RS set when the number of beam failure instance indications associated with the corresponding set of beam failure detection reference signals from the physical layer reaches a configured threshold before a configured timer expires. SSB-based Beam Failure Detection is based on the CD-SSB associated to the initial DL BWP and can be configured for the initial DL BWPs, for DL BWPs containing the CD-SSB associated to the initial DL BWP, and, if supported, for DL BWPs not containing the CD-SSB associated to the initial DL BWP. Besides, SSB-based Beam Failure Detection can be also performed based on the non-cell defining SSB, if configured for the active DL BWP. Beam Failure Detection can be also performed based on CSI-RS, if configured for the active DL BWP. After beam failure is detected on PCell, the UE:

• triggers beam failure recovery by initiating a Random Access procedure on the PCell;
• selects a suitable beam to perform beam failure recovery (if the gNB has provided dedicated Random Access resources for certain beams, those will be prioritized by the UE).
• includes an indication of a beam failure on PCell in a BFR MAC CE if the Random Access procedure involves contention-based random access.

Upon completion of the Random Access procedure, beam failure recovery for PCell is considered complete. After beam failure is detected on an SCell, the UE:

• triggers beam failure recovery by initiating a transmission of a BFR MAC CE for this SCell;
• selects a suitable beam for this SCell (if available) and indicates it along with the information about the beam failure in the BFR MAC CE.

Upon reception of a PDCCH indicating an uplink grant for a new transmission for the HARQ process used for the transmission of the BFR MAC CE, beam failure recovery for this SCell is considered complete. After beam failure is detected for a BFD-RS set of a Serving Cell, the UE:

• triggers beam failure recovery by initiating a transmission of a BFR MAC CE for this BFD-RS set;
• selects a suitable beam for this BFD-RS set (if available) and indicates whether the suitable (new) beam is found or not along with the information about the beam failure in the BFR MAC CE for this BFD-RS set.

Upon reception of a PDCCH indicating an uplink grant for a new transmission for the HARQ process used for the transmission of the BFR MAC CE for this BFD-RS set, beam failure recovery for this BFD-RS set is considered complete. After beam failure is detected for both BFD-RS sets of PCell concurrently, the UE:

• triggers beam failure recovery by initiating a Random Access procedure on the PCell;
• selects a suitable beam for each failed BFD-RS set (if available) and indicates whether the suitable (new) beam is found or not along with the information about the beam failure in the BFR MAC CE for each failed BFD-RS set;
• upon completion of the Random Access procedure, beam failure recovery for both BFD-RS sets of PCell is considered complete.

In RRC_CONNECTED, the gNB is responsible for maintaining the timing advance to keep the L1 synchronised. Serving cells having UL to which the same timing advance applies and using the same timing reference cell are grouped in a TAG. Each TAG contains at least one serving cell with configured uplink, and the mapping of each serving cell to a TAG is configured by RRC. For the primary TAG the UE uses the PCell as timing reference, except with shared spectrum channel access where an SCell can also be used in certain cases (see clause 7.1, TS 38.133). In a secondary TAG, the UE may use any of the activated SCells of this TAG as a timing reference cell, but should not change it unless necessary. Timing advance updates are signalled by the gNB to the UE via MAC CE commands. Such commands restart a TAG-specific timer which indicates whether the L1 can be synchronised or not: when the timer is running, the L1 is considered synchronised, otherwise, the L1 is considered non-synchronised (in which case uplink transmission can only take place through MSG1/MSGA). When two TAG IDs are configured for the PCell, both TAGs are regarded as primary TAG.

When extended DRX (eDRX) is used, the following applies:

• For RRC_INACTIVE, eDRX configuration for RAN paging is decided and configured by NG-RAN. In RRC_INACTIVE the UE monitors both RAN and CN paging;
• For RRC_IDLE, eDRX for CN paging is configured by upper layers. In RRC_IDLE the UE monitors only CN paging;
• Information on whether eDRX for CN paging and RAN paging is allowed on the cell is provided separately in system information;
• The maximum value of the eDRX cycle is 10485.76 seconds (2.91 hours) while the minimum value of the eDRX cycle is 2.56 seconds;
• The hyper SFN (H-SFN) is broadcast by the cell and increments by one when the SFN wraps around;
• Paging Hyperframe (PH) refers to the H-SFN in which the UE starts monitoring paging according to DRX during a Paging Time Window (PTW). The PH and PTW are determined based on a formula (see TS 38.304) that is known by the AMF, UE and NG-RAN;
• H-SFN, PH and PTW are used if the eDRX cycle is greater than 10.24 seconds;
• When the RRC_IDLE eDRX cycle is longer than the system information modification period, the UE verifies that stored system information remains valid before resuming/establishing an RRC connection.