RACH in NR call flow

[new_moon]

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.

[pradeep]

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: 4G | ShareTechnote.

[new_moon]

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.

[pradeep]

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.

[new_moon]

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

[new_moon]

@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

[pradeep]

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.

[Shreyar]

@new_moon you can check here about RACH process.

RACH Procedure 5G/NR 4 Steps with IE

[Pinaki_Mohapatra]

Hello experts, I have some queries. Could you please share the detailed RACH procedure call flow in NSA?

It would be helpful if you could include the Information Elements (IEs) as well.

The RACH procedure is different in SA.

My specific concern is about how NR RACH is handled in NSA.

[Ankur]

@Pinaki_Mohapatra

Could you please share the detailed RACH procedure call flow in NSA?

You can try here:

5G-NR Non-standalone access call flows

[Pinaki_Mohapatra]

Thanks, but this is focused on the ENDC procedure, as seen on EventHelix and ShareTechnote.

My concern is specifically with RACH in NSA. In NSA, the UE can’t directly access the gNB, as it only uses Option 3x for the user plane. If RACH issues arise from the gNB side, it becomes a concern.

During the ENDC procedure, after the additional leg is established, the UE will perform RACH with the gNB. If RACH fails, the ENDC setup won’t be successful.

So, my question is: If your network has 100% healthy ENDC KPIs, but there’s a RACH issue in NSA, what are the possible scenarios?

I hope I’ve explained my query clearly.

[RFSpecialist]

@Pinaki_Mohapatra

If RACH fails, the ENDC setup won’t be successful.

Yes likely you will get SCG failure information cause randomaccessproblem.

[Manoj_deka]

@Pinaki_Mohapatra

So, my question is: If your network has 100% healthy ENDC KPIs, but there’s a RACH issue in NSA, what are the possible scenarios?

Please check Cell Radius i.e. if the UE attempts to send RACH preamble beyond cell radius, UL interference, b1 threshold (in case it is very low), RSI etc…

You can tune parameters like preamble receive target power / preambletransmax / power ramping step etc.

[Pinaki_Mohapatra]

@RFSpecialist

Yes likely you will get SCG failure information cause randomaccessproblem.

Yes, that’s at the end I know during ENDC Process part.

@Manoj_deka

Please check Cell Radius i.e. if the UE attempts to send RACH preamble beyond cell radius, UL interference, b1 threshold (in case it is very low), RSI etc…

You can tune parameters like preamble receive target power / preambletransmax / power ramping step etc.

Thanks! But here it’s optimisation part.

My concern is the flow in NSA RACH NR.

[Manoj_deka]

@Pinaki_Mohapatra

Thanks! But here it’s optimisation part.

My concern is the flow in NSA RACH NR.

RACH is the first message between the UE and gNB which is required for synchronisation, based on RACH preamble, the gNB calculate the Timing Advance.

[Pinaki_Mohapatra]

Great.

But do you have any sequence call flow message?

I don’t mean the 5G RACH SA part, only the stuff I need is NSA perspective.

[Manoj_deka]

In both SA and NSA, the RACH is the first message between UE and gNB.

In LTE as well.

RACH parameters are pretty common for both SA and NSA.

[Pinaki_Mohapatra]

In NSA option 3x how will UE send RACH message, control part here is Master eNodeB…

So how in NSA UE can directly communicate with gNodeB without ENDC?

[Firas_Attar]

All signaling goes through eNodeB.

[Manoj_deka]

@Pinaki_Mohapatra

In NSA option 3x how will UE send RACH message, control part here is Master eNodeB…

So how in NSA UE can directly communicate with gNodeB without ENDC?

The MeNB delivers the RACH configuration (related to gNB) to the UE within in RRC Reconfg message.