5G-NR RACH (Random Access Procedure)
Random Access Procedure
RA Procedure is of two different types.
- Contention based RA Procedure
- Contention Free RA Procedure
Contention based RA Procedure
In CBRA, the UE randomly selects an RA preamble from a pool of preambles shared with other UEs in the cell. If multiple UEs select/transmit same preamble (Msg1), all those UEs decode same Msg2 content and transmit Msg3 on the same UL time/frequency resources. In the next step (Msg4), the network resolves the contention.
Contention Free RA Procedure
In CFRA, the UE uses a dedicated preamble provided by the network specifically to this UE via RRC signaling or PDCCH order.
The RA procedure is initiated by:
- A PDCCH order from the gNB.
- The UE’s MAC entity itself.
Event which trigger Random Access (RA) procedure:
- Initial access (CBRA).
- RRC Connection Re-establishment procedure (CBRA).
- SR failure (CBRA).
- DL and UL data arrival during RRC_CONNECTED when UL synchronization status is Out-of-Sync (CBRA or CFRA).
- During Handover (CBRA or CFRA).
- Transition from RRC_INACTIVE (CBRA)
- Request for On-demand System Information (CBRA or CFRA).
- Beam failure recovery (CBRA or CFRA).
Random Access Resource Selection
Random Access Resource Selection is almost similar to LTE.
- beam failure recovery
- On-demand System Information (SI reiquets)
- other CFRA cases that we will discuss.
RA resource selection for CBRA:
The UE first needs to select an SSB before selecting RA preamble. The UE then selects an RA Preamble randomly with equal probability from the RA Preambles associated with the selected SSB and the selected RA Preambles group.
RA resource selection for BFR:
For BFR, CFRA is used if RRC provides BeamFailureRecoveryConfig , otherwise, CBRA is used.
RA resource selection for SI request (On-Demand SI)
For UEs in RRC_IDLE and RRC_INACTIVE,Other SI request is triggered for RA procedure. If UE is configured by network with PRACH resources for SI request, CFRA is used, otherwise CBRA is used .
When CBRA is used for SI request via Msg3, the gNB acknowledges the request in Msg4
For reconfiguration with sync like Handover the RA configuration to be used is provided by the gNB within RACH-ConfigDedicated.
Random Access Preamble Transmission:
This is almost same as LTE.
RA_RNTI is used to address the UE on PDCCH which in turn used for decoding of PDSCH for Random Access Response (RAR).
Random Access Response Reception
Once the RA preamble is transmitted, the UE waits for the acknowledgement from the gNB in the form of Random-Access Response (RAR).
The UE monitors for RAR by attempting to detect a DCI format 1_0 with CRC scrambled by RA-RNTI/C-RNTI within a window configured by ra-ResponseWindow.
- For CFRA procedure, the UE after receiving RA response, considers the RA procedure as successfully completed.
- For CBRA procedure, the UE transmits Msg3 over PUSCH using UL grant received in MAC RAR and proceeds to the next step (Contention Resolution)
For CBRA procedure, the UE transmits Msg3 over PUSCH using UL grant received in the MAC RAR. The UE includes an identity in the Msg3 which is used later in the process of contention resolution.
Once Msg3 is transmitted, the UE starts or restarts (applicable for Msg3 retransmission) the timer ra-ContentionResolutionTimer and monitor for PDCCH
Case1: UE already had a C-RNTI
Case2: UE didn’t have a C-RNT
In both Case1 and Case2 above, if the contention resolution is not successful, the UE discards Temporary-RNTI and goes back to “Random Access Resource selection procedure” and retry RA procedure.