PMI

Precoding Matrix Indicator

Physical Layer →
Introduced in Rel-8 Also in: Testing

PMI is a feedback index reported by a user device to the base station to indicate its preferred precoding matrix for optimizing downlink MIMO transmissions.

Category
Physical Layer
Introduced
Rel-8
Where
Radio Access Network › NG-RAN (5G)
Also touches
1 segments
Specifications
15 specs
PMI Description Purpose Related Classification Detected Changes Specifications

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

Part ofMIMO
Related approachesCQI

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (83 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Studied in Rel-8, normative work from Rel-15.

Rel-15 7 changes

In Release 15, clarifications and corrections were made to the bitwidth calculation and reporting for PMI-related fields in Downlink Control Information (DCI), including specific adjustments for the report type 1a used in FD-MIMO Class A. These changes also involved corrections to the UL/SUL indicator field within DCI formats 0_0 and 0_1, which interacts with scheduling assignments that can include PMI.

  • On bitwidth calculation for DCI fields using RRC parameter indicating maximum number of MIMO layers per serving cell TS 38.212CR0011
  • Clarification on UL_SUL indicator field and SRS request field TS 38.212CR0013
  • CR on correction to bitwidth of NNZC indicator TS 38.212CR0014
  • Correction on UL/SUL indicator in DCI format 0_0 TS 38.212CR0016
  • CR on UL/SUL indicator in DCI format 0_1 TS 38.212CR0021
  • MCC correction to remove duplication of DMRS position indicator statement for format 7-1F (clause 5.3.3.1.22) and format 7-1G (clause 5.3.3.1.23). TS 36.212

+ 1 more changes

Rel-16 13 changes

In Release 16, enhancements were introduced for NR MIMO and specifically for DL MIMO energy efficiency (EE) features, as detailed in the updated specification TS 36.212. These changes included corrections and clarifications following the RAN1#100-e meeting to improve the overall MIMO framework. The release also provided clarifications on UCI bitwidth and mapping order for non-PMI based CSI feedback mechanisms.

  • Introduction of Rel-16 DL MIMO EE features in 36.212 TS 36.212CR0330
  • Introduction of Enhancements on NR MIMO TS 38.212CR0027
  • Introduction of NR enhanced MIMO TS 38.214CR0055
  • Miscellaneous corrections for Rel-16 DL MIMO EE features in 36.212 TS 36.212CR0338
  • Corrections for NR MIMO after RAN1#100-e TS 38.212CR0033
  • Corrections in TS 38.212 for NR MIMO TS 38.212CR0042

+ 7 more changes

Rel-17 20 changes

In Release 17, enhancements to the PMI function included corrections to PMI indexing for Type II and enhanced Type II (eType II) CSI reporting to improve accuracy. The release also introduced new test cases and applicability for Type II PMI reporting and uplink MIMO operations, including scenarios with Supplemental Uplink (SUL). Furthermore, it provided corrections and clarifications to the overall "Further enhancements on MIMO for NR" work item, refining the associated signaling and procedures.

  • Introduction of Further enhancements on MIMO for NR TS 38.212CR0089
  • Introduction of further enhancements on MIMO for NR TS 38.214CR0228
  • Big CR for TS 38.307: release independent for UL MIMO bands (R17) TS 38.307CR0100
  • Big CR to TS 38.307: intra-band CA with MIMO requirements (R17) TS 38.307CR0101
  • Corrections on Further enhancements on MIMO for NR in TS 38.212 TS 38.212CR0106
  • Corrections on Further enhancements on MIMO for NR in TS 38.212 TS 38.212CR0113

+ 14 more changes

Rel-18 25 changes

In Release 18, the enhancements to the PMI function were part of the broader Rel-18 MIMO Evolution work, which introduced new specification support for MIMO enhancements on CSI. This included specific updates for procedures like uTCI_STxMP_DMRS_SRS_8Tx_2TA and corrections to the underlying technical specifications to refine these new CSI and PMI reporting mechanisms.

  • Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
  • Introduction of specification support for MIMO enhancements on CSI TS 38.214CR0437
  • Introduction of specification support for MIMO enhancements on uTCI_STxMP_DMRS_SRS_8Tx_2TA TS 38.214CR0438
  • Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0167
  • Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0185
  • Corrections on PRACH association indicator in PDCCH order in 38.212 TS 38.212CR0192

+ 19 more changes

Rel-19 18 changes

In Release 19, the PMI function was enhanced as part of NR MIMO Phase 5, introducing new capabilities such as 3Tx UL enhancements and asymmetric UL mTRP operation. Specific updates included refinements for 16Tx or 32Tx PMI reporting and the addition of applicability for test cases involving the 16TX Enhanced Type II codebook for predicted PMI. The release also focused on correcting and confirming the applicability of various PMI-related test cases and requirements.

  • Introduction of Rel-19 NR MIMO Phase 5 TS 38.212CR0220
  • Introduction of 3Tx UL enhancements and asymmetric UL mTRP operation for NR MIMO Phase 5 TS 38.214CR0676
  • Introduction of CSI enhancements for NR MIMO Phase 5 TS 38.214CR0677
  • Corrections on Rel-19 NR MIMO Phase 5 TS 38.212CR0231
  • Corrections on Rel-19 NR MIMO Phase 5 TS 38.212CR0237
  • Corrections on MIMO Phase 5 TS 38.214CR0693

+ 12 more changes

Explore further

Broader topics and technologies where PMI plays a role.

Topics
MEC (Edge Computing)MIMO & BeamformingLTE / LTE-Advanced5G NR (New Radio)Lawful InterceptSMS & Messaging
Technologies
LTE5G

Defining Specifications

3GPP specifications that define or reference PMI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.

SpecificationTitleRelease
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