CSI-RS types and functions: There are 3 classifications.
1. Non-zero-power (NZP) CSI-RS
Non-zero-power (NZP) CSI-RS in NR serves multiple purposes:
- CSI-RS for Channel Measurement (CM): Supports channel state evaluation within the active BWP.
- CSI-RS for Beam Measurement (BM): Assists in beamforming and beam tracking within the active BWP.
- Tracking Reference Signal (TRS): Facilitates precise time-frequency synchronization, ensuring effective downlink reception and quasi-colocation (QCL) assumptions.
- CSI-RS for Mobility: Enables mobility-related measurements and handover processes within the active BWP.
This diverse functionality reflects NR’s flexible design for advanced wireless communication.
2. Zero-power (ZP) CSI-RS.
Zero-power (ZP) CSI-RS in NR is primarily designed to support PDSCH rate matching. It ensures that UEs skip specific REs configured with CSI-RS when receiving the PDSCH, particularly when the CSI-RS resources belong to other UEs. Without explicit notification, UEs cannot distinguish these resources, potentially leading to interference. ZP CSI-RS provides clear guidance for UEs to bypass the corresponding resources, maintaining efficient and interference-free data transmission. This mechanism is essential for optimizing resource allocation and maintaining high spectral efficiency in NR.
Although the term “zero power” (ZP) is used in NR protocols, it does not guarantee that no transmit power exists on the corresponding resources. A resource configured as ZP-CSI-RS for one UE may simultaneously be configured as an NZP-CSI-RS for another UE. UEs cannot assume any information is present on these resources; they only recognize that ZP-CSI-RS resources are not used for PDSCH transmission. This mechanism ensures clarity and proper resource handling, reducing interference and enhancing network efficiency.
3. CSI-RS for interference measurement (CSI-IM).
Channel State Information for Interference Measurement (CSI-IM) is defined in NR to estimate interference levels for resource optimization. It uses a four-RE-based pattern with time-frequency configurations of (2,2) or (4,1). While the structures of NZP CSI-RS, ZP CSI-RS, and CSI-IM are similar, UEs specifically measure NZP CSI-RS for channel information and regard ZP CSI-RS resources as unavailable for PDSCH. CSI-IM focuses solely on identifying interference, aiding in adaptive transmission strategies to enhance network performance. For detailed specifications, refer to 3GPP TS 38.214 V15.4.0, section 5.2.2.4.