RACH Decision-Making: How gNB Chooses CFRA or CBRA

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

For Further reading on RACH check out my article that I wrote a while back on 5G-NR RACH here