RACH Decision-Making: How gNB Chooses CFRA or CBRA
A friend of mine asked me the above question so I thought it’s useful that to share the discussion
In 5G-NR RACH process is tied to the SSB index, For a gNB to configure Contention Free RACH to UE it needs to send the below-dedicated information to UE
- SSB index
- PRACH Preamble
- RACH Occasion
So for CFRA RACH to happen
- Network needs to configure the B1 Measurement Report that includes SSB index reporting.
- Network need to configure “reportRS-IndexResultsNR=True” in B1 Measurement event, only then the UE will send SSB beam level measurement in Measurement Report (measResultRS-IndexList-r15) , If “reportRS-IndexResultsNR=False” , then UE will not send SSB/CSI-RS beam level measurement.
- When UE reports B1 Meas report with SSB indexes, gNB can select the best SSB index and configure CFRA RACH
- gNB can configure CFRA RACH during NSA Addition or during Handover.
In the case there is no SSB beam info available then contention-based RACH is performed always.
The Above procedure applies to the RACH procedure in SA as well