Hi Experts,
I’m trying to understand the UE attach of 5G procedures.
When UE camps on specific SSB, how does gNB knows what’s the SSB used?
Because gNB sends ssb1, ssb2, ssb3, etc.
And UE choose one of them.
So my question: How gNB knows what’s the chosen ssb?
It’s ssbrsrpthreshold parameter to select best SSB.
UE select SSB which having RSRP value less then configured threshold.
This I understand.
But I asked how gNB knows what’s the selected SSB by the UE.
By SSB index where UE will do RACH procedure.
Do you mean PRACH index?
Ssb index 0 to 7.
Do you know beam correspondence feature.
This I know, but in the msg1 there is no information about SSB index.
I mean UE gets msg1 and sib1 then it starts the RACH procedure, what I’m asking is how the UE at which message send SSB index?
You can see in L2, not in L3.
Do you mean in MAC?
I seen in logs UE always indicate.
So I can assume at each SSB burst there are maximum 7 indexed ssbs which UE can choose one of them and then inform gNB that it chooses this ssb by sending the index, right?
UE indicate SSB index ? I doubt… 
PRACH preambles are uniformly divided among the all SSB indexes in the cell.
gNB already know which preamble is assigned to which SSB index.
Hence when UE perform RACH, gNB comes to know which SSB beam UE is camp on from the Preamble UE has sent in msg1.
Here is my confusion also: I don’t see SSB index.
But let’s assume it’s right then we will have maximum at each ssb burst 8 ssbs because the indexes is from 0-7, right?
But I already know that ssb burst could be maximum 64 ssbs in 5g per burst.
Something don’t make sense here.
But assume two UEs sending the same RACH preamble?
Something dont make sense here also.
SSB indexes can be 4/8/64 based on FR range.
If 2 UEs use same preamble then Contention resolution will solve the issue.
Assume I have one UE and the gNB sends for instance 20 ssbs to it (maximum are 64 ssbs per burst) , so assume UE chose ssb2 and then it sends specific prach index (preamble).
You claimed each preamble is indexed by unique ssb index, so once the gNB sees this preamble it automatically knows the ssb index according to preamble that it gets.
Until here it makes sense.
But assume in the cell two UEs sending the same preamble. So…?
The logic isn’t work here.
It will send the same index to both UEs?
It will collide.
Do you mean the message 4 will solve the issue?
Will it send different RAPID?
Yes, based on your frequency carrier you will get a specific number of ssbs per burst, so the number of indexes per burst is determined by carrier frequency because by the number of ssbs per burst by default depends on your frequency carrier.
So number of ssbs per burst = number of indexes of ssbs per burst = related to carrier frequency.
Hope it’s clear. 
UE has to use only preamble, which are assign to the SSB index where UE camp on.
If 1 UE is in SSB index 0 and another UE in SSB index 1, then they both cannot use same preamble.
Appreciated, this I know.
But my confusion is that multiple UEs can send same RACH preamble.
Which means all UEs will get the same ssb index.
Which means that all the rach location is the same for all those UEs so It will collide.
Here is my doubt. 
Why multiple UEs will send same preamble if they are camped on different SSB beam?
Go to this link and go at the bottom. Read “Rach occasion” part:
So in case when NR is operating in Beamforming mode, UE need to detect and select a best beam for RACH process. This beam selection process would be the fundamental difference between LTE RACH and NR RACH process.
Preamble Sequence Generation - Like LTE Preamble Sequence, NR Preamble sequence is also based on Zadoff Chu based sequence.