A.59 Monitoring of RACH usage

28.5523GPP5G performance measurementsManagement and orchestrationRelease 18TS

The RACH plays a vital role in the following procedures:

– Initial access from RRC_IDLE;

– Initial access after radio link failure;

– Handover requiring random access procedure;

– DL data arrival during RRC_CONNECTED requiring random access procedure;

– UL data arrival during RRC_CONNECTED requiring random access procedure;

Furthermore, the random access procedure takes two distinct forms:

– Contention based using a randomly selected preamble (applicable to all five events);

– Non-contention based using a dedicated preamble (applicable to only handover and DL data arrival).

In the use-case of RACH configuration optimization, received Random Access Preambles and a contention indicator are signalled across an OAM interface.

Monitoring of the preamble usage in a cell allows the operator to determine if the resources allocated to the RACH by the gNodeB are appropriate for the number of random access attempts. If the resources are underutilised, then the operator may reconfigure the gNodeB (via CM) to allocate less resource to RACH thereby freeing up resource for other uplink transmissions. Alternatively, if the resources are heavily utilised then this is indicative of RACH congestion leading to increased latency for the procedures listed above. To this effect, measurements directly reflecting RACH congestion experienced by the gNodeB and by the UEs are useful.

The gNodeB can partition the RACH resource between dedicated preambles, randomly selected preambles in group A and randomly selected preambles in group B. This partitioning can be evaluated when usage measurements are made on each set separately. In a cell configured with multiple SSBs, it is important to get the measurements per SSB.