What are the scenarios UE sends C-RNTI MAC Control Element?

Question

I had studied high level description in specification. But I need to have some clear explanation and all possible scenarios UE can send C-RNTI MAC Control element to eNodeB?

Answer 1

Suppose UE has valid c-rnti. Due to any reason UE wants to perform rach with eNB and does not have dedicated rach preamble index. In this case UE needs to send msg3 for contention resolution. So UE will include c-rnti in msg3 for contention resolution.

In the following rach scenario UE adds c-rnti Mac CE

1. HO without dedicated rach preamble index
2. When UL unsynchronized
3. Pdcch order without dedicated rach preamble index

Comments

Hi , Thanks for your explanation. can you please make clear, when UE does not have valid preamble index?

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During HO and Pdcch order, eNB can provide dedicated ra preamble index. UE uses this dedicated ra preamble for contention free rach and it gets successful after getting rar. In this case UE does not send msg3.

In case UE does not have dedicated ra preamble index then UE performs contention base rach and it will send msg3 for contention resolution.

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yes, i understand your explanation. i want to know why ue doent send msg3 , in case ue uses dedicated ra preamble. if not send msg3 how enodeb knows what type of connection request ue asking for?

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Dedicated ra preamble index is assigned by eNB and it is given to only one UE at a time. So that UE will perform rach using that dedicated ra preamble index and other UE can not use this same ra preamble index. So if UE get random access response using the dedicated ra preamble index will be considered successful rach procedure. Pls see 36.321 section 5.1.4 Random Access Response reception

if ra-PreambleIndex was explicitly signalled and it was not 000000 (i.e., not selected by MAC) -
consider the Random Access procedure successfully completed.

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thanks for explanation

Answer 2

When the UE has a valid C-RNTI, it sends to eNodeB in the MAC CE to resolve contention resolution.
One case that I know is handover. During the handover preparation, target eNodeB allocates a new-CRNTI to UE. The same new C-RNTI, UE uses during latch on to the target cell.