In RACH procedure, when UE does not receive RAR, then it will retransmit the PRACH.
Next PRACH will be trigger in two cases:
1> UE sent a PRACH but didn't get a RAR.
2> UE sent a PRACH and got RAR, but the RAPID in the RAR in missing.
Now backoff indicator is used here which is MAC subheader that carries the parameter indicating the time delay between a PRACH and the next PRACH.
It having 0-15 index which carrying backoff parameter value in ms.
Backoff indicator field ( BI) is 4 bits & values configured by network.
Also, in SIB2 parameter- preambleTransMax - for maximum number of preamble transmission . I think it is used to limit the re transmission.
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