TBS calculation in Msg4

Hello Experts,
For TBS calculation in Msg4, I see number of symbols being taken for DMRS is three.
In which case can this be? I thought DMRS configuration type 1 only can be used for Msg4 and the length can only be 1 symbol.
In NR.

Type 1 and 2 in freq domain resource.
While in time domain you differentiate by type A and B.

Usually Type A can be 1 or 2 symbols. Type B 1 symbol.
If I remember correctly.
As Type B is used for URLLC APPs.

I think type 1 can be actually 4 symbol long! With one mandatory and 3 additional positions.
But for Msg4, only the mandatory position should come into the picture, as the additional ones are sent in RRC setup.

DMRS has 2 types configuration type 0 and 1. The main difference is their allocation.
Config type 0 has dmrs in every alternative Subcarrier.
So occupying 6 SCs out of 12 in single PRB.
Where type 1 has 4 SCs out of 12 in single PRB.

Type A and B are mapping types of PDSCH.
Mapping type B has front loaded DMRS always and it can start anywhere in the slot.

DMRS can have 1 to 4 symbols in a slot out of 14 OFDM symbols.
Where 1 symbol is mandatory for channel estimation.

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Agreed. Only that it is called config type 1 and 2 I think
Also, how many maximum RE for DMRS in msg4 can be reserved?

I guess you are asking about the number of additions symbol for DMRS, which can be 1 to 3.
I believe for DCI_1_0 reception this totally depends on the number of PDSCH symbols used. There are some set of rules defined for UE assumption in 214 for DCI_1_0 reception.

But would you agree that additional DMRS positions can only be defined in dedicated signalling?
And hence cannot come into the picture for Msg4?

Following are the parameters of DCI_1_0 scrambled with TC_RNTI. Where time domain position points to some table in the spec, those tables (Table 5.1.2.1.1-2, 5.1.2.1.1-3, 5.1.2.1.1-4, 5.1.2.1.1-) only provide Mapping type, K0, S and L.

Index Parameter Size
1.Frequency domain resource assignment Variable
2.Time-domain resource assignment 4
3.VRB-to-PRB mapping 1
4.Modulation and coding scheme 5
5.New data indicator (NDI) 1
6.Redundancy version (RV) 2
7.HARQ process number 4
8.Downlink assignment index 2
9.TPC commands for scheduled PUCCH 2
10. PUCCH resource indicator 3
11.PDSCH to HARQ feedback timing indicator 3

Correct me if I am wrong.

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Correct. There are default and customized table for time domain resources allocation on PDSCH configuration.
PDSCH config common & PDSCH config this will be sent on SIB1 and RRC setup msg.
msg4 max RE can go upto 16 CCE and that DCI will allocate PDSCH resource on time and frequency domain.

But does this have anything do to with how many symbols can be used for DMRS in Msg4?

DMRS type is linked with TDRA allocation DMRS type A and B and implementation of DMRS vendor specific on PDSCH & PUSCH, correct?

I think for Msg2 and Msg4 it is always single symbol Type1 configuration with position depending on Type A or type B being sent in SIB1.

I don’t think UE need SIB1 to decode Msg2 and Msg4.
Both the messages have a DCI_1_0 scrambled with RA_RNTI and TC_RNTI.
Mapping types are sent by DCI.
The number of DMRS symbols depends on number of PDSCH symbols and DMRSAdditionPosition.
Where:
Number of PDSCH symbols are carried by DCI.
DmrsAdditionPosition is assumed by the UE based on many factors.

I think yes DCI are scrambled with RNTI in different search space in corset but PDSCH time domain allocation for different search space will be sent on SIB1 & RRC setup msg.
Mapping row index of PDSCH in SIB1 that will calculate k0 ,S,& L based on 3GPP default table RIV decoder in case of customized PDSCH configuration.

DMRS additional position being assumed is something new for me.
Anywhere I could read more on this?

I think here lies my answer… Thanks!

5.1.6.2 DM-RS reception procedure 
When receiving PDSCH scheduled by DCI format 1_0 or receiving PDSCH before dedicated higher layer configuration 
of any of the parameters dmrs-AdditionalPosition, maxLength and dmrs-Type, the UE shall assume that the PDSCH is 
not present in any symbol carrying DM-RS except for PDSCH with allocation duration of 2 symbols with PDSCH 
mapping type B (described in subclause 7.4.1.1.2 of [4, TS 38.211]), and a single symbol front-loaded DM-RS of 
configuration type 1 on DM-RS port 1000 is transmitted, and that all the remaining orthogonal antenna ports are not 
associated with transmission of PDSCH to another UE and in addition 
- For PDSCH with mapping type A, the UE shall assume dmrs-AdditionalPosition='pos2' and up to two 
additional single-symbol DM-RS present in a slot according to the PDSCH duration indicated in the DCI as 
defined in Subclause 7.4.1.1 of [4, TS 38.211]