Demystifying CORESET and CCE-to-REG Mapping in 5G NR: A Deep Dive into Interleaved Resource Allocation
Understanding CORESET and CCE-to-REG Mapping in 5G NR
The 5G New Radio (NR) physical layer is designed for flexibility, efficiency, and reliability. A key component of 5G downlink control signaling is the Control Resource Set (CORESET), which lays out how control info—like scheduling grants, HARQ feedback, and configuration messages—is sent to user equipment (UE).
The diagram provided visually illustrates CORESET mapping for 54 Physical Resource Blocks (PRBs) and 2 OFDM symbols, highlighting interleaved CCE-to-REG mapping as well as the concept of aggregation levels (AL) in the control channel.
What Is CORESET in 5G NR?
The CORESET (Control Resource Set) defines the time-frequency resources for transmitting the Physical Downlink Control Channel (PDCCH). In simpler terms, it specifies where the control data is positioned within a slot.
CORESET Characteristics:
Defined by: Number of PRBs (in the frequency domain) and OFDM symbols (in the time domain)
Purpose: To transport the PDCCH that provides Downlink Control Information (DCI)
Configurable Parameters: * Number of PRBs (in multiples of 6) * Number of OFDM symbols (between 1 and 3) * Interleaving mode (either non-interleaved or interleaved) * REG bundle size and mapping pattern
In the image, we have a CORESET of 54 PRBs and 2 OFDM symbols, representing a moderately sized control area.
This CORESET is broken down into smaller units called REGs (Resource Element Groups).
Understanding REGs and REG Bundles
REG (Resource Element Group):
The smallest component within a CORESET.
Each REG usually has 12 subcarriers × 1 OFDM symbol (which is one resource block over one symbol).
REGs carry the QPSK-modulated control symbols used in the PDCCH.
To make control channel design simpler, REGs are combined together.
REG Bundles:
A REG Bundle is made up of 2 REGs (as shown in the image).
Grouping these helps cut down on control overhead and allows for flexible mapping patterns.
The image shows a 27 × 2 block interleaver, meaning there are 54 REG bundles organized in a two-dimensional interleaving setup—27 rows and 2 columns—to efficiently allocate control resources across frequency and time.
- From REGs to CCEs: Building the Control Channel
A CCE (Control Channel Element) is a higher-level structure that bundles multiple REG bundles together.
CCE Definition:
Each CCE = 6 REG Bundles = 12 REGs.
This structured grouping supports varying Aggregation Levels (AL).
CCE Indexing in the Image:
At the bottom of the image, the CCE indices are numbered #0 to #17, which show 18 CCEs within the CORESET. These indices map to logical control resource locations used by the PDCCH for different UEs.
- Interleaved CCE-to-REG Mapping Explained
A notable feature of the 5G NR CORESET design is the interleaved mapping, which randomizes how REG bundles are assigned to CCEs.
Why Interleaving Matters:
It distributes control information across both frequency and time resources.
Helps lower interference sensitivity, especially from narrowband interferers.
Enhances frequency diversity, boosting PDCCH decoding performance.
The diagram illustrates “Interleaved CCE-to-REG mapping using 27×2 block inter leaver for 54 REG bundles.” This indicates that each CCE is formed by selecting REG bundles according to an interleaving pattern rather than in a straightforward sequence.
As a result, CCEs draw REG bundles from various parts of the CORESET, making them more resilient to localized fading.
Aggregation Levels (AL) and Candidates
Users don’t always need the same reliability for decoding control messages. This is handled by Aggregation Levels (AL), which decide how many CCEs are bundled together for one PDCCH transmission.
Aggregation Levels and Meaning:
Aggregation Level (AL) Number of CCEs Used Typical Use Case
AL = 1 - 1 CCE - Good channel conditions, high SINR
AL = 2 - 2 CCEs - Moderate SINR, mid-tier reliability
AL = 4 - 4 CCEs - Poor channel conditions, enhanced robustness
The image nicely illustrates this concept with colored CCE candidates:
Red Blocks: 1 Candidate for AL = 4 (4 consecutive CCEs grouped together).
Blue Blocks: 2 Candidates for AL = 2 (each using 2 consecutive CCEs).
Teal Blocks: 4 Candidates for AL = 1 (each using 1 CCE).
This hierarchical setup enables the scheduler to adjust for coverage and capacity based on UE conditions.
Example Walkthrough: CORESET (54 PRBs, 2 OFDM Symbols)
Let’s break down the diagram step-by-step:
CORESET Configuration: * Frequency: 54 PRBs (648 subcarriers) * Time: 2 OFDM symbols * Total REGs = 54 × 2 = 108 REGs
REG Bundling: * 108 REGs / 2 = 54 REG Bundles
CCE Formation: * 1 CCE = 6 REG Bundles * Total CCEs = 54 / 3 = 18 CCEs (as shown at the bottom of the image)
Interleaving Process: * The 27×2 block interleaver rearranges the REG bundles before they’re assigned to CCEs. * This creates a scattered mapping across the frequency-time plane for improved reliability.
Candidate Generation: * The scheduler generates potential PDCCH allocations based on aggregation levels (AL = 1, 2, 4). * Each candidate utilizes a distinct combination of CCE indices as illustrated.
Non-Interleaved vs. Interleaved Mapping
In 5G NR, there are two types of CCE-to-REG mappings:
Mapping Type Description Use Case
Non-Interleaved - REG bundles are mapped sequentially to CCEs. Simple networks or low interference.
Interleaved - REG bundles are permuted using block interleaver. Dense deployments; improves diversity.
The image shows interleaved mapping, which increases reliability by ensuring that control data isn’t clustered in one frequency band or time symbol.
- Why CORESET Flexibility Matters
The design of CORESET is a key differentiator in 5G NR, allowing the network to optimize control signaling based on specific deployment scenarios.
Benefits of Flexible CORESET Configuration:
Customizable per UE: Different UEs can utilize distinct CORESETs for control signaling.
Bandwidth Efficiency: Smaller CORESETs mean lower control overhead; larger ones ensure coverage.
Robust Control Delivery: Adapts to shifting propagation and interference conditions.
Support for Beamforming: Works with spatially selective control channel mapping.
For example, high-mobility users in weak signal areas might get higher aggregation levels and interleaved mapping, while stationary users close to the base station could use smaller CORESETs with minimal overhead.
Real-World Application: Scheduler Perspective
From a scheduler’s perspective in the gNB (5G Base Station):
CORESET indicates where PDCCH resources are located.
Aggregation Level specifies how many CCEs are needed for a DCI.
Interleaving determines how REGs are spread across the grid.
The scheduler continuously tweaks these settings to balance latency, coverage, and throughput for UEs.
Conclusion
The CORESET and CCE-to-REG mapping process is fundamental to 5G NR control channel design. By utilizing interleaved mapping, REG bundling, and flexible aggregation levels, 5G enhances control reliability, interference resilience, and spectral efficiency.
The diagram illustrates how 5G NR smartly distributes control channel resources over time and frequency—ensuring that every DCI, no matter the user conditions, is delivered efficiently and reliably.
In short, CORESET sets the framework, REGs are the building blocks, and interleaved CCE mapping is the approach that makes 5G’s control signaling both resilient and scalable.