HARQ and retransmission at MAC layer for VoLTE RLC UM

Hi Experts,
In VoLTE RLC is UM, no RLC retrans.
But it still has HARQ and retransmission at MAC layer, right?

And VoNR can be transmitted also with RLC AM.

In short, yes.

At RLC we have option to have UM or AM Mode for each QCI and the same can be configured in the system.
However, at MAC we don’t have any option. HARQ is mandatory as per specification for DL-SCH transport channel.

some logic channels using RLC TM mode also mapped to DL-SCH. HARQ is also used for these channels: CCCH, BCCH?

BCCH messages are System Information messages. Which will use HARQ for transmitting different RV versions of the message. However no feedback is expected from UE as they are broadcast messages and are not intended for specific users.

CCCH messages do use HARQ. EX: RRC connection request uses TM mode at RLC and HARQ is used at MAC. UE is expected to provide HARQ feedback as well.

Please refer to 3GPP 36.321, Sections on ‘DL SCH Data Transfer’ and ‘UL SCH Data Transfer’ for more details