Section: 9.2.6 Random Access Procedure
Content: 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; - 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. (a) CBRA with 4-step RA type (b) CBRA with 2-step RA type (c) CFRA with 4-step RA type (d) CFRA with 2-step RA type (e) CFRA without network response with 4-step RA type Figure 9.2.6-1: Random Access Procedures Figure 9.2.6-2: Fallback for CBRA with 2-step RA type 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.