Tech-invite3GPPspecsSIPRFCs
Overview21222324252627282931323334353637384‑5x

Content for  TS 38.300  Word version:  16.3.0

Top   Top   Up   Prev   Next
1…   4…   4.7…   5…   5.3…   5.4…   6…   6.2…   6.6…   7…   8…   9…   9.2.2…   9.2.3…   9.2.3.2   9.2.3.3…   9.2.4…   9.2.6…   9.3…   10…   11…   15…   15.5…   16…   16.3   16.4…   17…   A…   B…

 

9.2.6  Random Access ProcedureWord‑p. 85
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 when UL synchronisation status is "non-synchronised";
  • UL data arrival during RRC_CONNECTED when there are no PUCCH resources for SR available;
  • SR failure;
  • Request by RRC upon synchronous reconfiguration (e.g. handover);
  • Transition from RRC_INACTIVE;
  • To establish time alignment for a secondary TAG;
  • Request for Other SI (see clause 7.3);
  • Beam failure recovery;
  • Consistent UL LBT failure on SpCell.
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.
Reproduction of 3GPP TS 38.300, Figure 9.2.6-1: Random Access Procedures
Up
Reproduction of 3GPP TS 38.300, Figure 9.2.6-2: Fallback for CBRA with 2-step RA type
Up
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.
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 a scheduling cell of an activated SCell of the secondary TAG, preamble transmission (step 1) takes place on the indicated SCell, and Random Access Response (step 2) takes place on PCell.
Up

9.2.7  Radio Link FailureWord‑p. 87
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 SSB associated to the initial DL BWP and can only be configured for the initial DL BWP and for DL BWPs containing the SSB associated to the initial DL BWP. For other DL BWPs, RLM can only be performed based on CSI-RS. In case of DAPS handover, the UE continues the RLM 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 5.6.1; or
  • For IAB-MT, the reception of 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.
  • otherwise, for RLF in the serving 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.
In case the RRC reestablishment procedure fails, the IAB-node may transmit a BH RLF indication to its child nodes. The BH RLF indication is transmitted as BAP Control PDU.
Up

9.2.8  Beam failure detection and recoveryWord‑p. 88
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.
SSB-based Beam Failure Detection is based on the SSB associated to the initial DL BWP and can only be configured for the initial DL BWPs and for DL BWPs containing the SSB associated to the initial DL BWP. For other DL BWPs, Beam Failure Detection can only be performed based on CSI-RS.
After beam failure is detected, 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).
Upon completion of the Random Access procedure, beam failure recovery is considered complete.
Up

9.2.9  Timing Advance

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 on PRACH).
Up

Up   Top   ToC