If HARQ fail, how is NACK sent for different MIMO streams of data?

RFSpecialist · March 28, 2021, 2:27am

CRC failure for CB, for CBG or for TB?
Retransmissions possible at CBG level only, not at CB level.

Nitin · March 28, 2021, 2:28am

At CBG level.
Even if one CB fails, it would be CBG retransmission.

RFSpecialist · March 28, 2021, 2:29am

I also understand the same.
Yet I am not clear if this CBG CRC is checked at MAC layer or at phy layer.
It has to be at phy layer in my opinion but I find no evidence for this.

sadanandk2 · March 28, 2021, 2:30am

I think it would be at PHY layer.

Nitin · March 28, 2021, 2:31am

Crc is always as phy layer, right?

RFSpecialist · March 28, 2021, 2:32am

I am confused about that excel file that I shared.
There is CB information inside yet the entire TB is retransmitted.
CRC for TB is at MAC layer.
CRC for CBG we believe is at PHY layer but need to double check this.

sadanandk2 · March 28, 2021, 2:32am

CBs will always be there.
But whether retransmission to perform at CBG level or TB level is configured.
Isn’t it?

Nitin · March 28, 2021, 2:33am

I think you are right here.

This is a good article:
Transport Block - an overview | ScienceDirect Topics).

Nitin · March 28, 2021, 2:34am

I think these segments are nothing but code blocks.
So CBs are always there.
Whether you want to base your retransmissions based on CBGs is the optional part.

sadanandk2 · March 28, 2021, 2:36am

Yes, right.

Nitin · March 28, 2021, 2:37am

I think the concept of code blocks is pretty clear now.

RFSpecialist · March 28, 2021, 2:37am

Yes, it seems usage of CB is mandatory while usage of CBG is optional.

Nitin · March 28, 2021, 2:38am

From NR logs no info for CBG. Seems not used as of now.

O.B · March 28, 2021, 2:39am

For HARQ, retransmission is performed through redundancy versions of the codeword. Since HARQ is adaptive, the MCS and number of layers can change for every retransmission.

To add, once TB is sent from MAC to PHY for initial transmission, it does not change. TB is used to generate a codeword with mother code rate (1/3 for LTE) the rate matching is used to generate the actual transmitted bits (taking into consideration mcs and layers in each (re)transmission).