Hi all,
In RAR (Random Access Response), how would an NR UE know whether it has only BI, or RAPID or RAPID and Mac RAR?
Because I don’t see any bit here telling this difference:
Basically the confusion here is only between Rapid only case and Rapid+Mac RAR case.
We got the TB block and we decoded those TB blocks, which we received from PDSCH in form of MAC PDU, and this MAC PDU consist one or more MAC subPDUs and optional padding & the presence and length of padding are based on the available TB size and the size of MacPDUs.
And each mac subPDU consist (BI,RAPID,RAR).
Your device may be received backoff indicator with MAC PDU, BI basically six bits which we use to set the value 5 ms to 1920 ms, it means you can enable the BI at gnB, when this happens, gnB will tell UE to back off for a period of time before attempting random access gain.
If no subMAC PDU, BI should be 0 ms.
RAPID we will use only in special cases.
Hello @Nitin,
Here you can find detailed information about 5G NR – Msg2 Random Access Response (RAR) – SA Mode:
But how are you differentiating a RAPID only and RAPID +MAC PDU subheader?
As there is no explicit bit to tell you this.
Hi @Tech_Playon. Sorry, could not find the answer to my question even though article is good. 
Subpdu signalling all the information BI rpaid and rar
But subpdus acknowledges SI request, where it consits of mac subheader for rapid.
If T= 0 then macsubheader carrying BI.
If T=1 then RAPID.
This is clear.
Now lets say my case is RAPID+ MAC RAR, how does the UE know it is not the case of RAPID only?
How many bits we have for RAPID and for RAR?
1 OCtet for RAPID, 7 Octets for RAR.
So total 8 Octets.
This is a little complicated:
Like we have MAC subpdus and one subpdus contain BI one contain RAPID and one contain RAPID plus RAR.
Now in RAPID and RAR we have both subheader.
When we have T=1 meand RAPID, plus we have RAPID subheader, and we have six but random access premable identifier .
If value of E = 1 it means more subpdus in one MAC pdu.
I hope it is somehow clear.
Not really…
After UE reads RAPID OCTET, what makes it not look for MAC RAR is the question here.
It can stop at RAPID or it can go ahead read MAC RAR.
BI and RAPID difference…but not RAPID+MAC RAR difference.
A UE that has sent SI request using dedicated RA resources for SI will not look further once RAPID matches, while a UE that needs RAR will look further after match. UE is already aware what it needs - therefore there is no need to explicitly inform UE.
