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

Hi All,
When there is 2x2 MIMO in the downlink, there are two streams of data being received in a slot.
Now when HARQ fail, we send the NACK per slot.
So does that mean, we discard data from both streams even if we fail to receive from any one stream?

NACK is given separately for each TB. (Transport Block)
So if only one TB is not received correctly, then NACK is given only for that TB and only that TB will be retransmitted.

But isn’t there only one TB till 4 layers in NR?

Yes. Up to 4 layer 1 TB, then 2 TB.
In LTE if layers more than 1 then 2 TB.
So basically ACK NACK, retransmissions are at TB level. (Precisely)

Yes. So now two layers in 2x2 MIMO are generating one TB.
And when a NACK is there, it effectively means retransmit from both layer, true?

Yes, true.
But fact is that, that TB will be retransmitted. But through 2 layers or 1 layer that would not be fixed I guess.
During retransmission, RI can be changed. Agree?

RI can change?
I thought it was RV. Not sure on RI part.
Anyway, how would a TB large enough of 2 layer be sent b 1 layer is what I am thinking.

TB size doesnt change with 1 layer or 2 layer.
For example , if there are 2 antenna port and number of layers are 2 then TB0 is send through Ant0 and TB1 is send through Ant1.
If number of layers is 1, then TB0 is send through both Ant0 and Ant1.
More than 1 layer means those many TBs can be transmitted simultaneously in simple words.

In 2x2 MIMO in NR, there is no TB0 and TB1.
There is only a single TB.
Isn’t that the case?

This understanding is not true.
In NR Till 4 layers you only have single TB. For more than 4 layers you have 2 TB/CW.
If you keep MCS and allocation same and increase your layers from 1 to 2 then your TB size will also be doubled.

Yes, dont get confused with TB0 , TB1 here.
Let’s name it 1st TB, 2nd TB.
If layer = 2 and antenna ports are 2, then 1st TB send throught Ant0 and 2nd TB in same que is send through ant1 (Both are TB0).
If layer = 1, then at a time onlt 1st TB will be transmitted.
Let me search image explaining this, then you will have no doubt

So with 2 layer operation it would end up increasing size of TB0, right?
And we cannot achieve the same size with 1 layer?

No, TB size should not changed with number of layers.

In the logs that I have, I can see TB size difference in terms of number of layers used.
But yes a document would be helpful to understand.

Check the calculation here (@Tech_Playon), it says TB size is directly proportional to number of layers:

TB size can be different based on RF condition, but it doesnt mean than it changes proportionally with number of layers.

Up to 4 layers in 5G there is only one TB.
Data from that TB is evenly distributed to all 4 layers like serial to parallel.

I would also imagine so.

Ok, so here my opinion now.

Your were right as in NR TB size is changing per layer. (In LTE it was not so as i remember). But in NR, TB is further divided in Code Block Groups (CBG) , and number of CBGs depend upon no of layers, but size of CBG remain fix.
And retransmission in NR is performed mainly based on CBG and not TB.

So i got confused between CBG and TB in case of NR.

I think we need to configure if we want CBG based retransmission. By default it is TB only, no?

Yes its configurable, but in NR no want to retransmit whole TB, just because 1 CBG got NACK.