Description
A Resource Element Group (REG) is a core building block in the LTE and NR physical layer resource grid. In the time-frequency domain, the resource grid is divided into resource blocks, each consisting of multiple resource elements (REs), which are the smallest units representing one subcarrier for one OFDM symbol. An REG groups a specific number of these REs together to form a logical unit for control channel mapping. In LTE, a REG typically consists of 4 consecutive REs in the frequency domain within one OFDM symbol, excluding any REs used for reference signals. This grouping is essential for the structured allocation of control information.
In LTE, REGs are primarily used for the Physical Downlink Control Channel (PDCCH) and other control channels like the Physical Control Format Indicator Channel (PCFICH) and Physical Hybrid ARQ Indicator Channel (PHICH). The PDCCH is mapped to Control Channel Elements (CCEs), where each CCE comprises a set of REGs (e.g., 9 REGs per CCE in LTE). This hierarchical structure—REs grouped into REGs, which are then aggregated into CCEs—allows for flexible and scalable control channel design. The mapping of REGs to physical resources follows specific patterns to ensure frequency diversity and interference randomization, enhancing reliability in varying radio conditions.
In NR, the concept of REGs is extended and refined to support more flexible numerologies and bandwidth parts. An NR REG is defined as one resource block in the frequency domain and one OFDM symbol in the time domain, containing 12 REs, but with some REs reserved for demodulation reference signals (DM-RS). NR introduces REG bundles, which group multiple REGs together for control resource set (CORESET) configuration. This enables efficient beamforming and support for massive MIMO. The use of REGs in both LTE and NR ensures that control signaling can be robustly transmitted, even in challenging environments, by leveraging diversity techniques and structured resource allocation.
Purpose & Motivation
The Resource Element Group (REG) was introduced to address the need for a standardized, granular unit for mapping control channels in the LTE physical layer, starting with 3GPP Release 8. Prior to LTE, 3G systems like UMTS used different mechanisms for control signaling, such as dedicated channels and code-based allocation, which were less efficient in OFDMA-based systems. REGs provide a structured way to allocate control information within the time-frequency grid, enabling efficient use of resources and supporting advanced features like frequency diversity and interference coordination.
REGs solve the problem of how to reliably transmit control channels, which carry critical information for scheduling, power control, and hybrid ARQ, in a dynamic and resource-constrained environment. By grouping REs into REGs, the system can apply channel coding and modulation schemes tailored for control data, ensuring robustness against fading and interference. This is particularly important in LTE and NR, where control channels must be decoded by all user equipment (UE) in the cell, often under varying channel conditions.
The evolution into NR further leveraged REGs to support flexible numerologies and beamforming, addressing the demands of 5G for higher data rates and lower latency. REGs enable the efficient mapping of control information in both LTE and NR, forming the foundation for reliable network operation and facilitating features like carrier aggregation, massive MIMO, and network slicing.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (22 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.
In Release 15, corrections were introduced for the mapping of physical resources, specifically addressing the PUSCH with configured grant and the PDSCH scheduled by PDCCH in Type 0 common search space. The release also included corrections to the mapping from virtual to physical resource blocks and to the frequency-domain starting position for SRS resource mapping. Furthermore, the specification defined CORESET#0, which is a control resource set used for initial access and system information.
- CR on PDSCH mapping to virtual resource blocks TS 38.211CR0006
- Correction on physical resource mapping for PUSCH with configured grant TS 38.211CR0008
- Correction to frequency-domain starting position for SRS resource mapping TS 38.211CR0009
- Correction on mapping from virtual to physical resource blocks TS 38.211CR0012
- Correction on PDSCH resource allocation scheduled by PDCCH in Type 0 common search space TS 38.211CR0018
- CORESET#0 TS 38.300CR0111
In Release 16, specific corrections were made to the REG function, primarily addressing inaccuracies in the CCE-to-REG mapping. The release also included a correction related to the mapping for RIM RS resources and set IDs. These updates provided more precise definitions for the mapping of control channel elements and reference signal resources within the resource grid.
In Release 17, the REG function saw no specific new enhancements; the provided CR titles and grounding context are focused on other areas such as slice group configuration, NAS procedures, and PDCCH operations, with no mention of changes to Resource Element Groups.
In Release 18, specific REG-related clarifications were made for sidelink channels, including corrections for mapping the PSFCH to physical resources and details on PSCCH DMRS sequence generation within a dedicated SL PRS resource pool. The release also provided clarifications on physical layer procedures such as PDCCH ordered CFRA for 2TA and the handling of RACH resources during an ongoing SDT procedure.
- Correction on mapping PSFCH to physical resources TS 38.211CR0141
- CR on PSCCH DMRS sequence generation in a dedicated SL PRS resource pool TS 38.211CR0148
- Resource handling for Alternative S-NSSAIs TS 38.300CR0816
- Clarification of PDCCH ordered CFRA for 2TA TS 38.300CR0868
- RACH resources while SDT procedure is ongoing TS 38.300CR0806
In Release 19, the REG function saw the introduction of Scheduling Request (SR) resources within the LTM cell switch MAC CE, enhancing MAC control element functionality. Additionally, common PDCCH repetition, initially developed for Non-Terrestrial Networks, was extended for Terrestrial Network use. No changes to the fundamental definition or mapping rules of Resource Element Groups themselves were specified in the provided materials.
- Introducing SR resources in LTM cell switch MAC CE [LTM_enh_SR] TS 38.300CR1054
- Introduction of common PDCCH repetition (Rel-19 NTN) for TN [Common_PDCCH_rep_TN] TS 38.300CR1058
- Correction on PDSCH resource mapping TS 38.211CR0178
- Removal of Request for CSI-RS resource configuration for Early CSI acquisition TS 38.300CR1079
Explore further
Broader topics and technologies where REG plays a role.
Defining Specifications
3GPP specifications that define or reference REG, 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 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.211 vj10 | LTE Physical Layer Specification | Rel-19 |
| TS 36.216 vj00 | LTE Relay Node Physical Layer | Rel-19 |
| TS 38.174 vj10 | NR Integrated Access and Backhaul Radio Spec | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.211 vj10 | NR Physical Channels and Modulation | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | 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 |