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Channel State Information Reference Signal (CSI-RS) enables UEs to report channel state information to the network. These signals are used for beamforming support and can be configured either beam specific or UE specific.

The resource elements carrying the CSI-RS can be configured to be either Zero Power CSI-RS (ZP-CSI-RS) or Non Zero Power CSI-RS (NZP-CSI-RS).

This post will only talk about ZP-CSI-RS and will skip discussions on CSI-RS or NZP-CSI-RS.

Configuration of Zero Power CSI-RS is same as NZP-CSI-RS, where as the use cases are different. In Case of ZP-CSI-RS, network doesn’t send any power/signal on the REs configured for the CSI-RS.

There are multiple reasons network configures ZP-CSI-RS:

Interference Measurement (CSI-IM)
There are two ways to obtain interference awareness: one by measuring the reference signal power from the interfernce signal directly and the second, by measuring RSSI during transmission gaps. In 5G, the physical resources used for interference measurement are defined as CSI-IM.
When the network assumes an interference situation, it configures ZP-CSI-RS as CSI-IM. UE uses these resources for interference measurement. In this case, the UE reports RSSI instead of CSI-RSRP.
When ZP-CSI-RS is configured for CSI-IM, network configures the CSI-IM resources such that they collide with PDSCH resources of the neighboring cell. For such configuration, resource mapping pattern of ZP-CSI-RS should align with resource mapping pattern of NZP-CSI-RS.
Following is an example of ZP-CSI-RS configured as CSI-IM:
Beam Mobility
In this case, network uses two beams with identical physical layer settings (like bandwidth part, CSI-RS resources etc.) Following is an example where gNB uses two beams with zero and non zero power CSI-RS. Here gNB has configured the CSI-RS resources in alternative mapping such that at one time instance only one of the two beams has non zero power.

In this example, the UE measures CSI-RSRP at two different instances and decides which beam is the best. The Best beam is the one where the UE receives heighest power level CSI-RSRP per beam.
Based on the CQI reports sent by the UE to the gNB, gNB decides which beam is the best and then it can configure accordingly.
PDSCH Rate matching
When a transport block size is not equal to the scheduled resources, rate has to be matched. This can be done either by puncturing/padding data bits or by inserting resource elements. In case of resource elements insertation, these resource elements are of zero power and not transmitted.
When a transport block size is smaller than the resources allocated for PDSCH, network may use ZP-CSI-RS resouce elements to match the rate accordingly. This configuration of ZP-CSI-RS is the same as CSI-IM.
ZP-CSI-RS rate matching allows RB level and RE level granularity.