Why is DMRS optional in LTE and mandatory in 5G NR?
Is it because crs is transmitted all the time in LTE and NR doesn’t have this leverage and also for mm as pusch coverage will be bad it will help a bit?
Because in LTE beamforming is optional, and you have CRS that can be used for channel estimation without beamforming. However , in 5G due to beamforming , channel estimation cannot be done without known reference that is DMRS. In LTE , DMRS become mandatory when beamforming comes in picture.
LTE got DM-RS and CSI-RS introduced along with TM9, which is not mandatory, while NR comes by default.
The way I see it, is that 3GPP understood DM-RS + CSI-RS is a better solution instead of CRS. Better in terms of overhead, interference, flexibility, etc…