RACH in NR call flow

Hello experts. Can someone please explain to me

The box where I have the arrow says the UE transmits PRACH preamble with RA-RNTI. What does that mean? How does the UE transmit the preamble with the RA-RNTI? Does the UE explicitly transmit preamble with RA-RNTI? I think I am confused whether the UE implicitly or explicitly transmits the RA-RNTI? The RA-RNTI is a 16 bit, does the UE transmit that? If it is implicit then what is reasoning behind the implicit rather than explicit transmit the RA-RNTI?
Thank you.

Just a suggestion. Please refer to sharetechnote and other free resources available online first. You will get some understanding. After that, if you still have some questions, you can narrow it down and ask here. The thing is, when the question is too basic/too broad, most folks just avoid answering it. For this question, please refer here: ShareTechnote.

Hi pradeep. Thank you. Actually I took a look at sharetechnote before here. I still couldn’t find the answer to my question. That is I am not sure how RA-RNTI is sent implicit or explicitly during transmission of msg1. Thank you.

MSG1 is just a PRACH sequence and is only to identify the preamble id. it doesn’t contain any info. RA_RNTI is calculated by UE based on the PRACH transmission timing. When network receives the MSG1, it also has the same timing info so, it knows the RA_RNTI to be used for PDSCH transmission for MSG2. The same info is given in the web page I shared earlier.

Thanks Pradeep. I am still confused. Does the UE explicitly sending the timing (subframe) or is it estimated by the gNB when it detects the sequence? In other words does UE send preamble + some more information (timing info) or does it just sent preamble

@pradeep Thanks Pradeep. I am still confused. Does the UE explicitly sending the timing (subframe) or is it estimated by the gNB when it detects the sequence? In other words does UE send preamble + some more information (timing info) or does it just sent preambl

PRACH sequence length, starting position etc are tied to the parameter called “PRACH configuration index” which is broadcasted in system info. UE sends RACH preamble with the assumption that initial timing advance is zero. At network, this preamble is received with some delay which is inferred by the network as timing advance needed for that UE. Network sending this TA command in MSG2 (RAR payload).

Please read the RACH section in 5G bullets book. The above is depicted in figures with which you can easily understand.

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