R16新特性——two-step RACH

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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.

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.