The CSI-RS is a DL reference signal, transmitted by the gNB, towards the end device (UE). The UE measures this signal and computes the CQI, PMI, RI, and so on, and reports these measurements back to the gNB on a PUCCH. I have also read that the CSI-RS is used for facilitating coherent demodulation of data.
1.) What are the exact contents of the DL CSI-RS? How does the UE calculate CQI, PMI etc based on the received CSI-RS? Does the CSI-RS contain information of the radio conditions prevailing at the gNB, in terms of RSRP, RSPQ, SINR, etc?
I have made an analogy: The CSI-RS is like a previous year exam question paper, that the gNB sends to the UE. The UE reads it and gets an idea of the standard of the questions, and prepares itself for the actual exam by tuning its preparation. The UE then informs the gNB of its level of preparedness/ readiness for the actual exam.
Does my interpretation hold valid? Am I close to the understanding of CSI-RS?
CSI-RS signal it is an analogy of Cell-RS in 4G which is used for demodulation of different signals.
In 4G Cell RS is transmitted all time, whereas in 5G CSI-RS transmitted only with physical channels.
This is good in terms of DL interference and power savings.
I think there is no big difference in 4G Cell RS calculation and CSI-RS for RSRP/SINR calculations.
These are just reference signals.
P.S. didn’t get your analogy. Try to look at this video:
CSI-RS is just a symbol (or a sequence of symbols) known in advance (standardized) by the UE. Because UE knows exactly what has been transmitted it is able to calculate the instantaneous channel model from the received CSI-RS.
The channel model is then simplified and quantized by calculating the representative CQI, PMI, etc… (to reduce overhead) before signaling it back to gNB.
Your interpretation is not valid, CSI-RS contains no information about prior channel conditions. It is just a symbol to be used for calculating the channel conditions.