Hello experts,

I read in 5G NR book that during RACH procedure, a UE transmits prach preamble associated with RA-RNTI. The gnb then upon reception calculates the RA-RNTI associated with prach occasion in which the preamble is transmitted.

My question is does the UE need to calculate the RA-RNTI before it transmits the preamble? Does it send it to the UE That is during Msg1? Or does it calculate it when it awaits random-access response from the gnb?


UE needs to know RA-RNTI to decode msg2. Its value is based on freq-time resource used to send msg1 which is known to both UE and gNB

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Hi @O.B does the UE need to explicitly send the RA-RNTI to the gnb? Or is it inferred implicitly by the base station? @O.B

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PRACH occasions are broadcasted by gNB in SIB. Each occasion has an equivalent RA-RNTI based on a formula defined by 3GPP

For every occasion, gNB tries to detect a preamble. If no preamble is detected, no further action. If at least 1 preamble is detected in this PRACH, msg 2 is sent with all received preamble ID and the CRC of msg2 will include RA-RNTI (corresponding to PRACH occasion)

Any UE waiting for msg2 knows RA-RNTI for PRACH occasion it has used

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