Description
A Control Channel Element (CCE) is a logical grouping of physical resources used to transmit Downlink Control Information (DCI) on the Physical Downlink Control Channel (PDCCH). In LTE (Rel-8 onwards), a CCE is defined as a set of 36 Resource Elements (REs), which correspond to 9 Resource Element Groups (REGs) of 4 REs each, excluding those used for reference signals. This structure allows the PDCCH to be constructed by aggregating multiple CCEs (e.g., 1, 2, 4, or 8 CCEs) to support different DCI formats and coding rates, catering to various channel conditions and control information sizes. The mapping of CCEs to specific REGs within the control region of a subframe follows a predefined interleaving pattern to provide robustness against fading and interference.
In 5G NR (from Rel-15), the concept of CCE is retained and refined within the new control resource set (CORESET) framework. A CCE in NR consists of 6 Resource Blocks (RBs) in the frequency domain over the duration of one symbol (or multiple symbols if configured) within a CORESET. Each CCE is further subdivided into 6 Resource Element Groups (REGs), where a REG equals one RB in one symbol. This structure provides flexibility in control channel scheduling across the bandwidth part. The aggregation level (AL), defining the number of aggregated CCEs (1, 2, 4, 8, 16, or even 32 for extended coverage), is dynamically adapted based on the user's radio conditions, which is determined through channel state information and the chosen DCI format.
The operation of CCEs involves several key processes. First, the DCI message, containing scheduling assignments or grants, is channel coded and rate-matched. This coded bit sequence is then mapped to the allocated CCEs. The specific CCE indices for a user are derived from a hash function based on the user's Radio Network Temporary Identifier (RNTI), ensuring a pseudo-random distribution to minimize blocking collisions. The physical mapping of the CCE's REGs to actual REs within the CORESET follows a specified pattern, which can be interleaved or non-interleaved, offering trade-offs between diversity and localized transmission.
CCEs are fundamental to the operation of the control plane. They enable the transmission of critical signaling such as uplink and downlink resource grants, power control commands, slot format indications, and preemption indicators. The ability to scale the aggregation level allows the system to ensure reliable control channel reception for users at the cell edge (using high AL) while maintaining efficiency for users with good signal quality (using low AL). This dynamic adaptation is a cornerstone of the spectral efficiency and robust performance of LTE and NR networks. The entire process, from DCI generation to CCE mapping and transmission, is tightly integrated with the scheduling algorithms in the base station (gNB/eNB).
Purpose & Motivation
The CCE was introduced in LTE Rel-8 to provide a structured, scalable, and efficient method for transmitting downlink control information. Prior systems lacked such a granular and flexible unit for control channel construction, which limited the adaptability of control signaling to varying user conditions and control message sizes. The CCE architecture solves the problem of reliably delivering scheduling commands and other critical signaling in diverse radio environments by allowing the aggregation of multiple basic units to achieve different coding rates and robustness levels.
The primary motivation was to decouple the control channel design from the fixed payload sizes and to enable link adaptation specifically for the control channel. By defining a CCE as a fundamental building block, the system can dynamically decide how many CCEs (the aggregation level) to use for a particular user's DCI. This directly addresses the challenge of maintaining control channel coverage across the entire cell, from near the base station to the edge, without wasting excessive resources for users in good conditions. It provides a balance between reliability and efficiency.
Furthermore, the structured mapping of CCEs to physical REs, using interleaving patterns, was designed to exploit frequency and time diversity within the control region. This mitigates the impact of narrowband interference and channel fading, ensuring that control information remains decodable even under adverse conditions. The CCE concept thus underpins the dynamic scheduling and robust operation of the cellular network, forming a critical part of the radio interface's control plane architecture from LTE through to 5G NR.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (114 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, enhancements to the Control Channel Element (CCE) function were focused on the configuration and monitoring of PDCCH within CORESETs. Specific corrections and clarifications were introduced for PDCCH monitoring in scenarios involving overlapped CORESETs and for NR-DC (NR Dual Connectivity). Additionally, adjustments were made to the Quasi-Co-Location (QCL) assumptions for receiving PDCCH, particularly for Random Access Response (RAR) and for CORESETs other than CORESET#0.
- Correction on PDSCH resource allocation scheduled by PDCCH in Type 0 common search space TS 38.211CR0018
- CR on using CORESET#0 in dedicated DL BWP TS 38.213CR0017
- PDCCH monitoring for overlapped CORESETs TS 38.213CR0018
- Correction on physical downlink control channel TS 38.213CR0020
- Correction to align RAN1 and RAN4 specifications for EN-DC power control TS 38.213CR0021
- CR on QCL assumption for receiving PDCCH for RAR TS 38.213CR0026
+ 12 more changes
In Release 16, the enhancements for the Control Channel Element (CCE) function primarily involved corrections and refinements to PDCCH monitoring procedures. Specifically, this included a correction for PDCCH monitoring during DAPS handover and a separate correction for monitoring on cells configured with a Release 15 PDCCH monitoring capability. These updates ensured reliable control channel operation in these specific mobility and capability scenarios.
- Introduction of shared spectrum channel access TS 38.213CR0071
- Mapping of Uplink Traffic to Backhaul RLC Channels TS 38.300CR0255
- Correction on channel inference assumption for PUSCH repetition Type B TS 38.211CR0070
- Corrections on shared spectrum channel access TS 38.213CR0091
- Corrections on shared spectrum channel access TS 38.213CR0108
- CR on Power Control for NR-DC TS 38.213CR0128
+ 27 more changes
In Release 17, enhancements to the CCE function included corrections and clarifications for multi-slot PDCCH monitoring in scenarios involving NR-DC, CA, and mixed capability types, as well as specific adaptations for operation in the FR2-2 frequency range. These updates also addressed the BD/CCE budget for scheduling cells and introduced refinements for PDCCH monitoring when overlapping with rate matching patterns. Furthermore, the release provided corrections for PDCCH monitoring adaptation procedures in conjunction with BWP switching.
- CR on PDCCH repetition with SSSG switching TS 38.213CR0332
- Correction on the tables for determining Type0 PDCCH monitoring occasions TS 38.213CR0337
- Correction on multi-slot PDCCH monitoring in NR-DC and CA scenarios with mixed capability types TS 38.213CR0342
- CR on default PUSCH power control parameters for mTRP PUSCH TS 38.213CR0344
- CR for power control of mTRP PUSCH repetition TS 38.213CR0345
- Corrections on PDCCH monitoring enhancement for 52-71GHz spectrum TS 38.213CR0346
+ 27 more changes
In Release 18, specific enhancements were made to PDCCH monitoring behavior concerning CCEs, including adaptations for DRX groups and for RedCap UEs operating in HD-FDD mode on dedicated spectrum less than 5 MHz. Furthermore, clarifications and corrections were introduced regarding the TCI state application for CORESETs, including CORESET 0, during procedures like LTM cell switch. These updates refine the control channel's reliability and efficiency under new operational conditions and multi-TRP scenarios.
- Introduction of Network Controlled Repeaters TS 38.213CR0506
- Introduction of QCL-TypeD priorities for overlapping CORESETs in M-DCI/M-TRP operation [QCL-TypeD CORESET priority for M-TRP] TS 38.213CR0569
- Introducing support for Network-Controlled Repeaters to 38.300 TS 38.300CR0685
- Corrections to NR Network-controlled Repeaters TS 38.211CR0118
- Maintenance of Network Controlled Repeaters TS 38.213CR0577
- CR on PDCCH monitoring for dedicated spectrum less than 5 MHz TS 38.213CR0596
+ 13 more changes
In Release 19, the key new development for the Control Channel Element (CCE) function was the introduction of PDCCH repetitions for Type0-PDCCH CSS sets in Terrestrial Networks (TNs), including common PDCCH repetition for NTN in TNs. This enhancement also involved alignment and corrections on parameters for intra-slot PDCCH repetition to ensure proper operation. These changes specifically aimed to improve control channel reliability and coverage for common signaling.
- CR to TS 38.176-2: restriction of 7MHz channel bandwidth introduction TS 38.176CR0087
- Introduction of PDCCH repetitions for Type0-PDCCH CSS set in TNs [Common_PDCCH_Rep_TN] TS 38.213CR0748
- Introduction of control parameters for on-demand posSIB request [OdPosSIB_Req] TS 38.300CR1009
- Introduction of common PDCCH repetition (Rel-19 NTN) for TN [Common_PDCCH_rep_TN] TS 38.300CR1058
- (NR_FR1_7MHz_BW-Perf) CR to TS 38.176-2 with clarification for channel bandwidths below 10 MHz TS 38.176CR0094
- Corrections on R19 NES adaptation of common channel/signals TS 38.213CR0753
+ 5 more changes
Explore further
Broader topics and technologies where CCE plays a role.
Defining Specifications
3GPP specifications that define or reference CCE, 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 36.213 vj10 | LTE Physical Layer Procedures | 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 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.808 vh00 | Study on NR above 52.6 GHz to 71 GHz | Rel-17 |
| TS 38.824 vg00 | NR URLLC Physical Layer Enhancements Study | Rel-16 |
| TR 38.830 vh00 | NR Coverage Enhancements Study | Rel-17 |
| TR 38.912 vj00 | Study on New Radio Access Technology | Rel-19 |