Contention based RACH procedure message 4

Hi All,
For contention based RACH procedure I have doubt for message 4.
Message 4 is rrc connection setup, but I find in few docs msg4 refer to contention resolution as well, Where UE compares tc crnti with crnti then it’s pass for contention?
What exactly msg 4 here?

Msg1 random acess request
Msg 2 random access response having tc crnti
Msg3 rrc connection request having crnti
Msg 4 is it rrc connection setup or contention resolution message where contention resolution done?

After msg3 there is time for harq which is contention resolution time.
If it fails we have msgmaxretx will go again rarequest but if you set 1 then rrcsetupreject.

In case of C-RNTI one, MSG4 is just a DCI scrambled with that CRNTI.
As soon as UE reads that DCI, contention resolution is done.

Setting of msg3maxretx and contention helps us to avoid msg4 failures.
A bit.
Apart from t300.

How to differentiate between RRC connection setup and contention resolution?
At L3 we can’t see contention resolution message.
In my view these messages are imposed on each other.
So basically UE compares tc-crnti with crnti if these are equal then contention resolution done. Is it correct?

Correct. Basically Msg4 is contention resolution msg only from MAC pespective, and RRC perspective it is RRC Connection setup.
So they are same.

In drive test logs, there are separate entries for Contention resolution and RRc connection setup, thats why this confusion arises.

Msg1 is sent by many UEs at the same resources.
So TC-RNTI sent by eNB in msg2 is assume to be for all UEs.
In msg 3 every UE transmit their own identity (s-tmsi or c-rnti if alrwady available with UE).
So msg4 is only for 1 UEfor which check is passed.
Other UE declare failure in RACH and start again with msg1.

1 Like

C rnti based.contention resolution never comes into picture for initial attach. It can only be done when UE has already connected before.
So RRC setup will never come into the picture here from what I can think of.
Mostly Cnrti based resolution is employed for scenarios where UE does not have a CCCH sdu available for msg3.

No it’s for initial attach as well after UE power on.