Description
Zero power CSI-RS (ZP CSI-RS) is a specific configuration of the Channel State Information Reference Signal (CSI-RS) in the 5G New Radio (NR) physical layer. Unlike regular Non-Zero Power (NZP) CSI-RS, which transmits known reference symbols for channel estimation, a ZP CSI-RS resource is a set of time-frequency resources where the gNB deliberately mutes its transmission (transmits zero power). The primary function of these muted resources is to create measurement opportunities for the User Equipment (UE) to estimate interference. The UE is informed via Radio Resource Control (RRC) signaling about the configuration of ZP CSI-RS resources, including their periodicity, bandwidth, and location within the resource grid.
How it works is integral to advanced CSI acquisition frameworks. The gNB schedules ZP CSI-RS resources in coordination with neighboring cells, often as part of Coordinated Multi-Point (CoMP) or enhanced Inter-Cell Interference Coordination (eICIC) schemes. When the serving cell mutes its signal on these specific Resource Elements (REs), the UE receiver will primarily capture signals from other interfering transmitters (e.g., other gNBs or UEs) and noise on those resources. This measured power is used to calculate the Interference Measurement Resource (IMR) metric. The UE then uses this interference estimate, in conjunction with channel estimates derived from NZP CSI-RS, to compute CSI feedback such as the Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), and Rank Indicator (RI).
Key components involved include the CSI resource setting, which can contain both NZP and ZP CSI-RS resources, and the CSI reporting setting, which links measurements to a specific report. The ZP CSI-RS configuration is part of the CSI-MeasConfig IE signaled to the UE. Architecturally, this functionality is distributed between the gNB's scheduler, which determines when and where to mute, and the UE's physical layer measurement procedures. The gNB's decision to configure ZP CSI-RS is a network management task aimed at optimizing overall network performance, particularly in dense deployments where interference is a limiting factor.
Its role in the network is pivotal for enabling accurate link adaptation and multi-user MIMO scheduling. By providing a clean view of the interference environment, ZP CSI-RS allows the UE to recommend a modulation and coding scheme (MCS) that is robust to the actual inter-cell interference, rather than just the serving cell's signal. This leads to higher throughput and better reliability. Furthermore, in scenarios like dynamic point selection or joint transmission CoMP, ZP CSI-RS resources are used to measure interference from non-cooperating points, enabling the network to make intelligent decisions about which transmission points should serve a given UE.
Purpose & Motivation
ZP CSI-RS was introduced in 3GPP Release 15 with 5G NR to address the limitations of LTE's interference measurement techniques. In LTE, interference measurement was often based on the Cell-specific Reference Signal (CRS), which was always transmitted, making it difficult to isolate interference from specific cells or in specific resource blocks. This was inadequate for the advanced multi-antenna and coordination schemes envisioned for 5G, which require precise, resource-specific interference knowledge.
The technology exists to solve the problem of inaccurate CSI reporting in interference-limited environments. Without ZP CSI-RS, a UE's CQI report might be based on interference measured on resources that include the serving cell's own data or reference signals, leading to an overestimation of interference and overly conservative MCS selection. By providing dedicated, muted resources, ZP CSI-RS creates a "clean" interference snapshot, enabling more aggressive and accurate link adaptation. This directly improves spectral efficiency and user throughput, especially at the cell edge.
Historically, its creation was motivated by the need to support flexible and dynamic TDD, ultra-dense network deployments, and advanced CoMP techniques in 5G. These scenarios feature rapidly changing and potentially severe interference patterns. ZP CSI-RS provides the necessary tool for the network to manage this interference proactively. It allows for network-coordinated muting patterns, which can be used not just for measurement, but also as a primitive for interference avoidance, where strong interferers mute in certain resources for the benefit of victim UEs in neighboring cells.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (20 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Zero-Power CSI-RS (ZP CSI-RS) function was introduced as part of the new CSI framework, enabling more efficient channel state information reporting. This was integrated by allowing the configuration of a list of CSI-RS resource IDs within a `CSI-ReportSubConfig` for determining the CRI field bitwidth, where the parameter \(K_{s}^{CSI-RS}\) is defined as the number of CSI-RS resources in the corresponding set. This mechanism supports advanced features like multi-TRP operations and beam management by providing a resource for interference measurement without actual transmission.
In Release 16, the enhancements for Zero Power CSI-RS (ZP) were focused on enabling UE power saving, as indicated by the CR titles introducing and correcting power saving mechanisms. The technical groundwork involved defining specific reporting structures and bitwidth calculations for CSI, including CRI fields, which are essential for the UE to efficiently process and report on ZP CSI-RS resources configured for interference measurement. These changes integrated ZP CSI-RS functionality into the broader Rel-16 power saving framework across multiple protocol layers.
In Release 17, the Zero Power (ZP) CSI-RS function was enhanced as part of the broader UE power saving enhancements, with specific corrections and clarifications made to the related signaling procedures in the specifications. These included refinements to the determination of bitwidths for fields like CRI and PMI when ZP CSI-RS resources are configured within a `CSI-ReportSubConfig` or resource set. The updates also involved clarifying the handling of ZP CSI-RS resources in contexts like IAB restricted beam MAC CEs and ensuring consistent payload calculations for reports involving multiple CSI-RS resources.
- Introduction of Rel-17 UE power saving enhancements TS 38.212CR0091
- Corrections on UE power saving enhancements in 38.212 TS 38.212CR0103
- Corrections on UE power saving enhancements in 38.212 TS 38.212CR0111
- Corrections on UE Power Saving Enhancements for NR in TS 38.212 TS 38.212CR0120
- Correction to 38.321 on Integrated Access and Backhaul for NR Rel-17 concerning DL TX power adjustment range extension TS 38.321CR1540
- Clarification on which CSI-RS resources in IAB restricted beam MAC CEs TS 38.321CR1624
+ 1 more changes
In Release 18, the changes for the Zero Power CSI-RS function primarily involved clarifications and corrections to existing specifications. Specifically, this included a correction to CSI reporting procedures for scenarios involving a single CSI-RS port, ensuring accurate bitwidth determination for reported fields. Additionally, the release provided clarifications on power ramping procedures related to the function.
In Release 19, the primary new feature for the Zero Power CSI-RS (ZP) function was the introduction of a low-power Wake-up Signal for NR to enhance UE power saving. This was followed by subsequent corrections to this newly introduced low-power Wake-up Signal mechanism to ensure proper functionality. The specification updates included refinements to reporting field bitwidths and payload calculations, such as those for CRI and RI, which are integral to the CSI reporting framework that utilizes ZP CSI-RS resources.
Explore further
Broader topics and technologies where ZP plays a role.
Defining Specifications
3GPP specifications that define or reference ZP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |