PBCH DMRS(DeModulation Reference Signal)


PBCH DMRS is a special type of physical layer signal which functions as a reference signal for decoding PBCH. In LTE (at least in TM1, 2, 3, 4), we don't need this kind of special DMRS for PBCH because we can use CRS(Cell Specific Reference Signal) for PBCH decoding.  However, in 5G/NR there is no CRS. That's why we need the DMRS dedicated for PBCH decoding.




Sequence Generation


Following is the equation to generate PBCH DMRS. As in many other Physical Layer Signal, it is generated by Pseudo Random Sequence, but the part unique for PBCH DMRS is c_init(initialization value). As you see below, the initialization value is made up of various components like Physical Cell ID, SSB Index and Half Frame Number. That is, by decoding this DMRS UE can figure out SSB Index and Half Frame.





Physical Location (Resource Element Mapping)


The physical location (Resource Elements) of PBCH DMRS is determined as below. As you see here, the location shifts in frequency domain according to Physical Cell ID.






Location Shift by Physical Cell ID


As mentioned above, the array of PBCH DMRS shifts vertically (i.e, in frequency domain) with Physical Cell ID in a manner shown below.