Description
The CSI-RS Resource Indicator (CRI) is a critical feedback mechanism within the 5G New Radio (NR) and evolved LTE physical layer framework for channel state information (CSI) reporting. It operates within the context of beam management and multi-antenna (MIMO) operations. The network configures the User Equipment (UE) with a resource set containing multiple Channel State Information Reference Signal (CSI-RS) resources. Each resource corresponds to a specific transmission configuration, such as a particular beamformed signal from a gNB's antenna array or a specific port configuration. The UE measures the signal quality (e.g., based on Reference Signal Received Power (RSRP) or Signal to Interference plus Noise Ratio (SINR)) of each CSI-RS resource in this set.
Upon being triggered to report CSI, the UE selects the CSI-RS resource that yields the best measured quality according to the reporting configuration. The index of this selected resource within the configured set is encoded and reported back to the gNB as the CRI. This index is a compact representation, typically requiring only a few bits, which specifies which pre-configured beam or port configuration is currently optimal for the UE. The CRI is reported alongside other CSI parameters, such as the Rank Indicator (RI), Precoding Matrix Indicator (PMI), and Channel Quality Indicator (CQI), as part of a unified CSI report structure defined in specifications like 38.214.
The CRI's role is foundational for beam management procedures like beam selection and beam refinement. In initial access and mobility, the CRI helps identify the best Synchronization Signal Block (SSB) or CSI-RS beam for establishing a connection. In connected mode, it enables dynamic beam switching and tracking, allowing the network to adapt the transmission beam to the UE's changing position or radio conditions. For multi-TRP (Transmission Reception Point) or multi-beam operations, the CRI can indicate a preferred TRP or a combination of beams, facilitating advanced coordination schemes.
Architecturally, the CRI is generated by the UE's physical layer based on Layer 1 (L1) measurements. The reporting is configured via higher-layer RRC signaling, which defines the CSI-RS resource set, the reporting periodicity, and the event triggers. The CRI report is then transmitted via the Uplink Control Information (UCI) on the PUCCH or PUSCH. The gNB's scheduler uses the CRI feedback to select the appropriate downlink beamforming weights, thereby directing energy towards the UE and improving link reliability and data throughput. This closed-loop feedback is essential for harnessing the gains of massive MIMO and high-frequency bands (e.g., mmWave) where beamforming is mandatory for coverage.
Purpose & Motivation
The CRI was introduced to address the fundamental challenge of beam management in 5G NR, particularly as networks began utilizing higher frequency bands with massive MIMO antenna arrays. In sub-6 GHz LTE, wide-area coverage was achievable with relatively broad beams. However, in mmWave spectrum (e.g., FR2), signal propagation suffers from high path loss and blockage, necessitating the use of narrow, high-gain beams to maintain a viable link. The network must identify and track the best directional beam for each UE, a process that requires efficient feedback from the device. The CRI provides this mechanism by allowing the UE to indicate its preferred beam from a set of candidates configured by the network.
Prior to 5G, LTE's CSI feedback primarily focused on reporting wideband CQI and PMI for MIMO precoding across a relatively homogeneous cell coverage area. It lacked a standardized, efficient method for explicit beam indication. Early beamforming implementations often relied on cell-specific reference signals or required proprietary solutions. The CRI, standardized starting in 3GPP Release 13 for LTE enhancements and fully leveraged in NR from Release 15, created a unified, flexible framework for beam reporting. It solves the problem of overhead; instead of reporting full channel matrices for many beams, the UE simply sends a short indicator, conserving uplink resources.
Its creation was motivated by the need for dynamic and agile beam management to support high mobility, robust connectivity, and spatial multiplexing in dense deployments. The CRI enables key 5G use cases like enhanced Mobile Broadband (eMBB) in mmWave and ultra-reliable low-latency communications (URLLC) by ensuring the network can quickly switch to the strongest beam, minimizing interruption. It forms the basis for more advanced features like multi-beam CSI reporting, beam failure recovery, and multi-TRP operation, making it a cornerstone of 5G's physical layer design.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (88 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, the CRI (CSI-RS Resource Indicator) function was introduced as part of the new NR framework to explicitly indicate which CSI-RS resource is used for channel state information measurements and reporting. This is detailed within the CSI framework specifications, where corrections were made to the CSI-RS configuration in TS 38.214 and to aperiodic CSI-RS triggering to handle different numerologies between the PDCCH and the CSI-RS. These foundational specifications define the procedures for resource indication and management essential for NR operation.
- Clear SPS resources for Scell TS 36.321CR1428
- CR on PDCP duplication related operations in sidelink LCP procedure and resource reselection procedure TS 36.321CR1432
- Correction to DELTA_PREAMBLE for NPRACH resource format 2 TS 36.321CR1441
- Correction on resource selection TS 36.321CR1442
- Correction of wrong implementation on frequency domain resource assignment bitwidth TS 38.212CR0006
- Clarification on UL_SUL indicator field and SRS request field TS 38.212CR0013
+ 14 more changes
In Release 16, enhancements for the CRI function included corrections and support for aperiodic CSI-RS triggering with specific beam switching timing values of 224 and 336 symbols. These changes ensured proper configuration and reporting when using these extended timing values for beam management. Additionally, corrections were made to the handling of increased numbers of CSI-RS resources for mobility per measurement object.
- Aperiodic CSI-RS Triggering for UE reporting beamSwitchTiming values of 224 and 336 TS 38.214CR0060
- Introduction of carrier specific NRSRP thresholds for NPRACH resource selection TS 36.321CR1535
- RRC IE name fix to dynamic frequency domain resource allocation type selection (Rel-15 origin) TS 38.212CR0056
- Correction on SRS resource set configuration in TS 38.212 TS 38.212CR0070
- Correction on SRS resource set configuration for DCI format 0_2 in TS 38.212 TS 38.212CR0072
- Correction on aperiodic CSI-RS triggering with beam switching timing of 224 and 336 and on CSI reporting TS 38.214CR0107
+ 19 more changes
In Release 17, specific corrections were made to the CSI-RS Resource Indicator (CRI) framework to enhance multi-TRP (mTRP) operations, including a correction on CSI-RS port restriction for mTRP CSI and a correction on slot offsets of CSI-RS resource pairs for MTRP. Additionally, refinements were introduced for aperiodic CSI-RS timing in mixed numerologies configurations and for CSI-RS used for tracking during fast SCell activation.
- CR on the description of the SRS resource set indication for PUSCH repetition TS 38.212CR0117
- Corrections on resource pool index TS 38.212CR0124
- Correction on the short message indicator when TRS availability indication is present TS 38.212CR0132
- CR for CSI-RS power for inter-cell mTRP TS 38.214CR0313
- CR on default QCL for unified TCI state for PDSCH and A-CSI-RS TS 38.214CR0314
- Correction on CSI-RS port restriction for mTRP CSI TS 38.214CR0319
+ 18 more changes
In Release 18, specific corrections were made to the CRI function to address the threshold for A-CSI-RS reception within the Rel-18 TCI framework and to clarify the CSI-RS transmission occasion for Non-Coherent Joint Transmission (NCJT) CSI. These changes refined the conditions for receiving A-CSI-RS and the associated resource indication for advanced multi-transmission point operation. Additionally, a correction was made for CSI reporting scenarios involving a single CSI-RS port.
- Corrections on PRACH association indicator in PDCCH order in 38.212 TS 38.212CR0192
- CR on the PRACH retransmission indicator field included in the PDCCH order TS 38.212CR0213
- CR on PDSCH resource mapping for dedicated spectrum less than 5 MHz TS 38.214CR0518
- Correction on TRIV/FRIV resource indication of SL-U TS 38.214CR0548
- Correction on contiguous RB based resource allocation TS 38.214CR0550
- Correction to Sidelink resource allocation mode 2 TS 38.214CR0568
+ 7 more changes
In Release 19, the enhancements for the CRI function primarily involved clarifications and corrections to the counting rules for simultaneous NZP-CSI-RS resources, particularly in relation to multiple CSI reporting settings (N settings) and NES scenarios. These changes aimed to precisely define the maximum number of such resources a UE is required to handle concurrently. Additionally, corrections were made to the association mechanisms between NZP CSI-RS and CSI-IM resources for measurement accuracy.
- TEI19 Counting of CSI-RS resource referred by N CSI reporting settings [SimCSI_count] TS 38.214CR0681
- TEI19 Simultaneous NZP-CSI-RS resource counting with NES [SimCSI_countNES] TS 38.214CR0689
- Correction on Semi-persistent CSI/Semi-persistent CSI-RS for LTM TS 38.214CR0733
- Correction on association between NZP CSI-RS and CSI-IM TS 38.214CR0744
- Clarification on the number of simultaneous L1 CLI-RSSI and simultaneous L1 SRS-RSRP measurement resources TS 38.214CR0745
- Correction on the maximum number of SRS-RSRP measurement resource sets and CLI-RSSI measurement resource sets TS 38.214CR0746
Explore further
Broader topics and technologies where CRI plays a role.
Defining Specifications
3GPP specifications that define or reference CRI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |