CBG is layer 1 not MAC layer.
MAC delivers one CW to TB to lower layers.
After that MAC doesn’t know in how many CBGs that TB is split in.
MAC only knows if it gets an ACK or an NACK for that entire TB.
Wherever I read, CBG retransmissions seems to be based on Harq ACK/NACK.
Like here:
5G NR Physical Layer Procedure: Code block group (CBG) based transmission
I see that “UE will decode these code block groups (CBGs) and will send HARQ feedback (ACK/NACK) for each of the individual groups”.
ACK/NACK is at CBG level.
Splitting the TB in CBGs means it is MAC layer job?
I have to see a log where CBG is used to understand how CBG works.
It is not clear if CBG is used or not.
Yes, you need to check if each CBG has it’s own harq id.
I think it is not used (because entire TB is retransmiited) but then why do we see CB transmitted?
Like 24 CB total CB and 2 CB failed for example.
5G MAC UL Physical Channel Schedule Report there is a field called CBG bitmap.
I think there is the ACK/ NACk for each CBG.
CBG formation and its retransmission is performed by PHY layer only.
Then it cannot be done by MAC layer, right?
So ack nack for each CBG can not be transmitted at MAC layer.
It has to be at phy layer.
Mac give output of tbs.
Ack/nack from Mac harq algorithm if crc bit error there.
This is for the entire TB.
Our doubt is about CBGs.
In sharetechnote it is saying that UE is informed about CBG or not in DCI: CBG is indicated in DCI (Format 0_1, Format 1_1)
Yes. Does your log have the DCI info?
But first we need to see CBG configured at RRC level which I do nto see:
Please check in your logs, maybe you can find this RRC config for CBGs.
If that is not configured, nothing to send on DCI.
It’s not mandatory IE then.
I cannot find CBG configured in RRC, I checked few logs.
Yes, it says it is optional.
Yes CBG retransmission is transperent to MAC layer.
For MAC, TB is correctly received only when all CBGs are correctky decoded.
Not Mandatory for sure.
Sharetechnote says there is a CRC for each CB, for each CBG and for the entire TB:
I believe this CRC happens at phy layer for CB and CBGs and at MAC layer for entire TB.
Yes. So a CRC failure would.lead to Harq ACK/NACK, right?