Description
The Precoding Matrix Indicator (PMI) is a key feedback mechanism in the Multiple-Input Multiple-Output (MIMO) physical layer of 3GPP LTE and NR systems. It is part of the Channel State Information (CSI) reported by the User Equipment (UE) to the gNodeB (gNB). The UE, after measuring downlink reference signals (e.g., CSI-RS), calculates the optimal or preferred precoding matrix from a predefined codebook. This matrix is a set of complex weights applied to the antenna ports to shape the transmitted signal, effectively performing beamforming. The UE then sends the index (the PMI) corresponding to this matrix in the codebook back to the gNB via uplink control channels (PUCCH) or shared channels (PUSCH).
The gNB uses the reported PMI, along with other CSI like Rank Indicator (RI) and Channel Quality Indicator (CQI), to select the precoding matrix for subsequent downlink transmissions to that UE. This process adapts the transmission to the current channel state, focusing energy towards the UE and minimizing interference, which is essential for spatial multiplexing gains. The codebook design is standardized (different for LTE and NR) and defines a set of possible precoding matrices for various antenna configurations (e.g., 2, 4, 8 antenna ports) and transmission ranks.
PMI reporting can be wideband (a single PMI for the entire system bandwidth) or subband (different PMIs for different portions of the bandwidth), offering a trade-off between feedback overhead and granularity of channel adaptation. In advanced MIMO modes like multi-user MIMO (MU-MIMO), the gNB may use PMI reports from multiple UEs to schedule simultaneous transmissions with minimal inter-user interference. The accuracy and timeliness of PMI feedback directly impact the spectral efficiency and reliability of the downlink.
Purpose & Motivation
PMI was introduced to enable efficient closed-loop spatial multiplexing in MIMO systems, starting with LTE Release 8. Before such feedback mechanisms, MIMO primarily used open-loop techniques like spatial diversity, which were robust but did not maximize throughput by adapting to channel conditions. The fundamental problem is that the gNB lacks perfect knowledge of the downlink channel to each UE, which is necessary for optimal precoding.
The PMI solves this by leveraging the UE's ability to measure the channel and recommend a precoding strategy. This allows the network to perform channel-dependent precoding (beamforming), which significantly increases signal strength at the intended UE and reduces interference to others. It was motivated by the need to boost cell capacity and user data rates to meet the growing demands of mobile broadband.
Over successive releases, PMI feedback has evolved to support increasingly complex antenna arrays (massive MIMO), higher frequency bands, and new use cases. Enhancements like enhanced CSI feedback (eCSI) and Type II PMI (with higher resolution) in NR were driven by the requirements for more precise beamforming in mmWave frequencies and for advanced multi-user MIMO schemes, pushing the limits of spectral efficiency in 5G and beyond.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (9 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, a clarification was introduced regarding the UCI (Uplink Control Information) procedures for non-PMI based CSI feedback. Specifically, the specification work focused on defining the UCI bitwidth and the UCI mapping order for these reporting modes. This provides clearer implementation guidelines for scenarios where CSI feedback does not utilize a Precoding Matrix Indicator.
- Clarify UCI bitwidth and UCI mapping order for non-PMI based CSI feedback TS 38.212CR0080
In Release 17, the specification introduced new Type II PMI reporting test cases to validate performance and ensure proper operation. It also included corrections to the PMI indexing for both Type II and enhanced Type II (eType II) Channel State Information to resolve potential ambiguities or errors in the reporting procedures.
In Release 18, specific clarifications were made to the PMI function regarding testing procedures. The focus was on correcting the applicability of PMI test cases for systems utilizing larger antenna configurations. Specifically, these corrections applied to test scenarios involving 16-transmit (16Tx) or 32-transmit (32Tx) CSI-RS antenna setups.
- Correction to test applicability of PMI test cases with 16Tx or 32Tx of CSI-RS TS 38.522CR0600
In Release 19, the updates to PMI focused on refining test applicability and correcting reporting rules. This included introducing and confirming test case applicability for eRedCap devices and for 2Rx FDD FR1 operations using a 16TX Enhanced Type II codebook with predicted PMI. The release also applied corrections to the applicable release for 16Tx or 32Tx PMI reporting and to release-independent rules for PMI test requirements.
- Correction to applicable release of 16Tx or 32Tx PMI reporting TS 38.522CR0648
- Introduction of Applicabilities for eRedCap PMI test cases TS 38.522CR0626
- Confirmation of Applicability for PMI test cases TS 38.522CR0627
- Addition of applicability for 2Rx FDD FR1 Multiple PMI with 16TX Enhanced Type II codebook for predicted PMI TS 38.522CR0663
- (NR_perf_enh-Perf) Correction on release independent rules for PMI and CA test requirements (R19 A) TS 38.307CR0191
Explore further
Broader topics and technologies where PMI plays a role.
Defining Specifications
3GPP specifications that define or reference PMI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 32.808 v1800 | Common User Profile Storage Framework | Rel-8 |
| TS 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 36.747 ve00 | Enhanced CRS and SU-MIMO IM Performance Requirements | Rel-14 |
| TS 36.863 vc00 | CRS Interference Mitigation for Homogeneous Networks | Rel-12 |
| TS 36.867 vd00 | LTE DL 4 Rx Antenna Port Study TR | Rel-13 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TS 38.307 vj20 | NR UE Release Independent Requirements | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TS 38.762 vj00 | Dynamic MIMO OTA Test Methodology for NR FR1 | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |
| TR 38.912 vj00 | Study on New Radio Access Technology | Rel-19 |
