Description
The Precoding Resource Block Group (PRG) is a fundamental concept in the 5G New Radio (NR) physical layer, specifically within the context of channel state information (CSI) reporting and precoding for multiple-input multiple-output (MIMO) transmissions. In the downlink, the gNB (base station) uses precoding to shape and direct transmission beams towards user equipment (UE), optimizing signal quality and throughput. The PRG defines the granularity at which the same precoding matrix can be applied across a set of contiguous physical resource blocks (PRBs) in the frequency domain. This grouping is crucial because wireless channel conditions can vary across frequency; applying a single precoding matrix over too wide a bandwidth might lead to suboptimal performance, while per-PRB precoding would incur excessive signaling overhead.
The size of a PRG is configurable by the network and can be adapted based on channel conditions, UE capability, and system bandwidth. Specifications such as 3GPP TS 38.213 and 38.214 define how PRG sizes are signaled to the UE, typically through higher-layer RRC configuration or dynamic DCI indications. The UE performs channel measurements and reports recommended precoding matrices (Precoding Matrix Indicators, PMI) for these PRG-sized chunks. The gNB then uses these reports to select and apply the appropriate precoding across the designated PRGs. This process enables efficient closed-loop MIMO operation, where the feedback overhead is managed by grouping PRBs expected to have similar channel characteristics.
Architecturally, PRG interacts with other physical layer components like the CSI-RS (Channel State Information Reference Signals) for channel estimation and the PDSCH (Physical Downlink Shared Channel) for data transmission. The choice of PRG size represents a trade-off: larger PRGs reduce CSI feedback overhead and precoding complexity but may not capture fine-grained frequency selectivity, potentially degrading performance in highly frequency-selective channels. Smaller PRGs offer more precise precoding at the cost of higher signaling. In massive MIMO (mMIMO) systems, efficient PRG configuration is vital for leveraging beamforming gains across wide bandwidths, such as in FR2 (mmWave) bands, where beam management is critical.
Purpose & Motivation
PRG was introduced in 5G NR (Release 15) to address the limitations of LTE's precoding granularity and to support the wider bandwidths and advanced MIMO schemes of 5G. In LTE, precoding was typically applied with a granularity tied to subbands or the entire system bandwidth, which could be inefficient for the larger bandwidths (up to 400 MHz in NR) and more diverse deployment scenarios of 5G. The motivation was to create a flexible mechanism that balances the trade-off between precoding accuracy and signaling overhead, enabling efficient beamforming and spatial multiplexing.
Without PRG, the network would face a dilemma: either use a single precoding matrix for the entire bandwidth, leading to performance loss in frequency-selective fading channels, or signal precoding information per PRB, resulting in prohibitive control channel overhead. PRG provides an intermediate, configurable granularity that allows the gNB to adapt to channel coherence bandwidth. This is particularly important for 5G's support of diverse use cases, from enhanced mobile broadband (eMBB) requiring high throughput to ultra-reliable low-latency communication (URLLC) needing robust link adaptation.
The creation of PRG was driven by the need to optimize massive MIMO operations, where precise beamforming is key to capacity and coverage. By grouping PRBs, the system reduces the payload size for CSI feedback (like PMI and CQI) and downlink control signaling, conserving uplink and downlink resources. This efficiency enables scalable MIMO deployments across FR1 (sub-6 GHz) and FR2 (mmWave) spectra, supporting features like multi-user MIMO (MU-MIMO) and coherent joint transmission, which are foundational to 5G's performance targets.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (123 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the new introduction for the PRG (Precoding Resource Block Group) function included specific correction and alignment procedures for frequency domain resource allocation. This involved a change request for aligning PUSCH resource allocation scheduled by a RAR UL grant with the procedures in 38.213 and corrections to the determination of the resource allocation table for PUSCH when using SP CSI. Additionally, corrections were made to the resource mapping of PDSCH as detailed in TS 38.214.
- Support maximum 8 SS/PBCH blocks for unpaired spectrum beyond 2.4GHz TS 38.213CR0006
- Correction to last PUCCH resource set configuration TS 38.213CR0019
- Correction on CRC assumption for multi-CSI resource selection and CSI report(s) selection TS 38.213CR0041
- Correction on uplink resource allocation type 1 TS 38.214CR0010
- Correction on determination of the resource allocation table for PUSCH with SP CSI TS 38.214CR0011
- Correction on PUSCH resource allocation TS 38.214CR0012
+ 4 more changes
In Release 16, the enhancements for the Precoding Resource Block Group (PRG) function were introduced as part of the broader NR enhanced MIMO work item. These changes included specific corrections and refinements to the MIMO enhancement specifications to improve performance and clarity. The updates aimed to optimize precoding granularity and resource allocation within the PRG framework for more efficient multi-antenna transmissions.
- Introduction of MIMO enhancements in NR TS 38.213CR0075
- Introduction of NR enhanced MIMO TS 38.214CR0055
- Corrections on MIMO enhancements TS 38.213CR0089
- Corrections on MIMO enhancements TS 38.213CR0109
- Corrections on MIMO enhancements TS 38.213CR0137
- 38.213 CR Correction on HARQ-ACK codebook for secondary PUCCH group TS 38.213CR0167
+ 26 more changes
In Release 17, the enhancements for the Precoding Resource Block Group (PRG) function were part of the broader "further enhancements on MIMO for NR" work item. These included corrections and clarifications to the PRG configuration and application, specifically addressing the availability of a soft RB set within an RB set group to ensure proper resource allocation. The updates ensured more precise and reliable PRG operation for multi-antenna transmissions in the NR system.
- Introduction of further enhancements on MIMO for NR TS 38.213CR0277
- Introduction of further enhancements on MIMO for NR TS 38.214CR0228
- Corrections on further enhancements on MIMO for NR TS 38.213CR0297
- Corrections on further enhancements on MIMO for NR TS 38.213CR0326
- CR on the clarification of PUCCH resource determination in 38.213 TS 38.213CR0339
- CR on PUCCH resource determination of SPS multicast HARQ-ACK TS 38.213CR0400
+ 26 more changes
In Release 18, the enhancements for the Precoding Resource Block Group (PRG) function were part of the broader "MIMO Evolution for Downlink and Uplink" work item. This included specification support for MIMO enhancements related to Channel State Information (CSI) and uplink transmission configuration indicators, such as those for dynamic multi-point transmission and demodulation reference signals. The release also involved maintenance and corrections to align RRC parameters and ensure proper resource mapping for these MIMO features.
- Introduction of MIMO Evolution for Downlink and Uplink TS 38.213CR0504
- 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
- Introduction of MIMO evolution for Downlink and Uplink TS 38.300CR0742
- Maintenance of MIMO Evolution for Downlink and Uplink TS 38.213CR0576
- Maintenance of MIMO Evolution for Downlink and Uplink TS 38.213CR0605
+ 26 more changes
In Release 19, the PRG function was enhanced as part of NR MIMO Phase 5, which introduced uplink enhancements for 3Tx and asymmetric uplink multi-TRP operation. These improvements were supported by corresponding Channel State Information (CSI) enhancements to optimize precoding across resource blocks. The release also included corrections and clarifications to the MIMO Phase 5 features to ensure proper implementation.
- Introduction of NR MIMO Phase 5 TS 38.213CR0710
- 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
- 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
- Introduction of Rel-19 MIMO Phase 5 TS 38.300CR1021
+ 11 more changes
Explore further
Broader topics and technologies where PRG plays a role.
Defining Specifications
3GPP specifications that define or reference PRG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TR 38.878 vi40 | Technical Report on Advanced Receiver for MU-MIMO | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |