What is the difference between TC-RNTI and C-RNTI in 5G?

Hi 5G Experts,
What is the difference between TC-RNTI and C-RNTI?
It’s known that TC-CRNTI allocated in msg 2 but for C-RNTI its mentioned its given to UE in rrc connection reconfiguration after sgnb addition.
So it’s like for NSA 2 C-RNTI given to UE. Is it correct?


Ideally yes, there should be unique UE identifier for each technology, isnt it?
For EN-DC UEs we are talking about.

It seems, but in LTE if we remember TC-RNTI allocated in msg 2 becomes permanent C-RNTI in msg 3 .
But for 5G NSA it’s not same.
C-RNTI transmitted in advanced.
So it seems contention resolution mechanism is different between 5G NSA and 4G.

Yes there is small difference.
If C-RNTI is given for NR through RRC Reconfiguration message during SgNB addition, then RACH performed toward NR is Non-contention based.
This is typical behaviour of EN-DC.
But in case of SA, UE is not given any C-RNTI in advance.
So RACH process here just like LTE. i.e. TC-RNTI in msg2 and convert it to C-RNTI.

Same like HO in LTE, C-RNTI for the taget cell is allocated via RRC

For both LTE HO and 5G NSA, random access does not need to be contention based however, it could be if the vendor did not implement contention-free RA

C-RNTI (Cell RNTI) is an ID used to identify the RRC Connection and scheduling that is dedicated to a particular UE - used for the transmission to a specific UE after RACH.
While TC-RNTI (or Temporary C-RNTI) is a temporary ID inside MAC RAR (Random Access Response), which is generated by eNodeB as a response to the Random Access Preamble transmitted by the UE, as part of the Random Access procedure.

I think it is still contention based, just that contention resolution happens on dci of msg4.

One point I got. There is some contention resolution id in 5G case tx in msg2 used for contention resolution. It’s some new IE maybe.

If preamble assignment and C-RNTI is given in advance during RRC reconf messgae, then NR RACH will be non-contention based.
Other wise its contention based.

Yes, if preamble assignment is the key.
C-RNTI allocation does not make it contention free.

Yes, but mostly either both of them will be given, or both not given.
That’s why I mentioned both.

It’s always contention based for RACH synchronisation.

Synchronization and contention are 2 different things, right?

Not really.
C-Rnti will be mandatorily given in NSA not dedicated RACH config.

Is it?
I didn’t checked that in detail.
Whatever I saw was both given together.

I mean in first stage after sgnb addition after reading mib, ssb beam finding UE will go for RACH sync.

Sync is two way, DL and UL.
For UL snyc RAC is the process, of which contention is a part.

RACH without contention is also used for UL sync.
Contention has nothing to do with synchronization.
This i was saying. :wink:

That by dedicated preamble given to UE used for HO or UL sync once it in connected mode.

Exactly. So synch and contention are different things.

In Principle, RACH-Contention resolution is used to ensure the resource assignment is given to single unique UE and avoid duplication.
And RACH-UL Synchronization is performed when UE have lost UL synch (Say TA etc…) and during HO.
So they both may or may not appear together.

But they are different. Right?