CRC failure for CB, for CBG or for TB?
Retransmissions possible at CBG level only, not at CB level.
At CBG level.
Even if one CB fails, it would be CBG retransmission.
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.
I think it would be at PHY layer.
Crc is always as phy layer, right?
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.
CBs will always be there.
But whether retransmission to perform at CBG level or TB level is configured.
Isnât it?
I think you are right here.
This is a good article:
Transport Block - an overview | ScienceDirect Topics).
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.
Yes, right. 
I think the concept of code blocks is pretty clear now.
Yes, it seems usage of CB is mandatory while usage of CBG is optional.
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).
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In both LTE and NR, the size of the TB varies as per the no. of layers allocated by the eNB/gNB. As per 3GPP Relâ8, up to 2 layers, require 1 or 2 TBs; more than 2 layers requires 2 TBs. For e.g., if 2 layers are mapped to 1 TB (2X2 MIMO), the max size of the TB will be ~150000 bits. For 1 layer, the maz size of the TB is ~75000 bits.
YesâŚeach code block (CB) is associated with its individual CRC bits in both LTE and NR.
In LTE, the ACK/NACK is sent on per-TB level i.e. if the CRC fails in any of the CB of the TB, the entire TB has to be re-transmitted by the eNB.
But in NR, if CBG is configured which is a group of CBs, the UE will decode these CBGs and will send the ACK/NACK for each of the individual CBG and only the erroneously received CBGs are retransmitted by the gNB.