DMRS

DMRS is demodulation reference signal that is required to decode PDSCH/PUSCH/PBCH data. Following are the parameters related to DMRS configuration

1. DMRS is present in the same location where PDSCH is present/PUSCH/PBCH.
2. for MIB decoding, PBCH DMRS position is mentioned in the post  "Intial Cell Selection"
3. For SIB1, DMRS position is given in MIB message. 
4. For all other cases, DMRS configuration comes either in RRCRewconfiguration or SIB1 message.

Parameters related to DMRS :

Mapping Type :
Type A ==> Always start with Symbol 2 or 3 specified by parameter DMRS_TypeA_Pos in RRC configuration. In this case PDSCH starts from Symbol 0 in the slot.
Type B ==> Starts with the first symbol of PDSCH allocated. In this case PDSCH must be starting from any symbol in the slot.

Add_pos: 

Multiple DMRS symbols can be present in the PDSCH. This is defined by the parameter DMRS_Add_Pos. It can be 1, 2 or 3
Following is the possibility of having multiple DMRS symbols. Defined in 38.214
3 Additional DMRS symbols are possible only typically if no of symbols allocated for PDSCH is more than 12.
2 additional DMRS symbols are possible if no of PDSCH symbols is 10 or 11.
1 additional DMRS symbol is possible if No of PDSCH symbols is 8 or 9.
Otherwise only DMRS symbol at Symbol 2/3 will be present and no other DMRS can be configured.

ConfigType (Type 1 or Type 2):   

Indicates the frequency domain configuration of DMRS. It is different from Mapping Type (Type A or Type B) that defines the time domain start of DMRS symbol.
DMRS Type 1:

• Denser in frequency domain
• Every other RE in an RB is occupied by DMRS signal.
• In frequency domain 2 position for each DMRS signal is possible because alternate RE contains the DMRS signal.

DMRS Type 2:

•  Uses large number of orthogonal signals. Mainly used for MU-MIMO.
• In a group of 6 REs,  2 REs are used for DMRS. 
• Difference between 2 DMRS signals is 4 REs. 

Pictorial view of DMRS based on above parameters:

Based on above parameters, we can calculate max number of orthogonal DMRS signals possible in the system

No of Orthogonal signals = time * frequency * code

Time domain length can be either 1 or 2 symbol for DMRS hence possibility is 1 or 2

Code : 2 Orthogonal signals per RB is possible

Frequency :  For type 1 it can be 2 locations

For Type 2, it can be 3 locations

Hence No of Signals possible for Type 1 = 1 * 2* 2 = 4  or 2 *2 *2 = 8

No Of Orthogonal signals possible for type 2 =  1 *3 * 2 = 6 OR 2 *3 * 2 = 12

It shows the in type 2, we can have more orthogonal symbols which is why it is used for MU-MIMO where we need to serve more users.