RACH Decision-Making: How gNB Chooses CFRA or CBRA

In the 5G-NR RACH process, the gNB’s ability to configure Contention Free RACH to UE hinges on transmitting specific dedicated information. This includes the SSB index, PRACH Preamble, and RACH Occasion.

For CFRA RACH to be initiated, the network must configure the B1 Measurement Report to include SSB index reporting. Additionally, setting “reportRS-IndexResultsNR=True” in the B1 Measurement event is necessary for UE to transmit SSB beam level measurements in the Measurement Report (measResultRS-IndexList-r15). Conversely, if “reportRS-IndexResultsNR=False,” UE will not relay SSB/CSI-RS beam level measurements.

Upon receiving B1 Meas report with SSB indexes from UE, the gNB can select the optimal SSB index and configure CFRA RACH. This configuration can occur during NSA Addition or Handover procedures.

In instances where SSB beam information is unavailable, contention-based RACH is consistently performed.

It’s worth noting that the outlined procedure for RACH applies to both NSA and SA RACH procedures.

The Above procedure applies to the RACH procedure in SA as well

Other factors that influence this decision:

  • Network load: If the network is congested, CBRA might be preferred to manage access attempts and ensure fairness.
  • Service prioritization: Certain applications, like emergency calls, might require guaranteed access, making CFRA the better option.
  • UE capabilities: Newer UEs may support features that enable efficient CFRA communication, influencing the gNB’s choice.