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5G CORESET (Control Resource Set): A Deep Dive

  • Writer: Venkateshu
    Venkateshu
  • Apr 11
  • 5 min read

Updated: Apr 14

1. Introduction to CORESET

In 5G NR, the Control Resource Set (CORESET) is a flexible and configurable area on the time-frequency grid that defines where the Physical Downlink Control Channel (PDCCH) can be transmitted. PDCCH is the channel through which the gNB (base station) sends control information to the UE (User Equipment). It carries the DCI (Downlink Control Information) that tells the UE how to decode the data, when to transmit or when to receive(scheduling), and more.

Unlike LTE, where control channels are static and span fixed regions, CORESET allows operators to define custom control regions suited for diverse deployment scenarios.


Why CORESET is needed?

In 5G, the carrier bandwidth (the width of the channel) can be very wide — up to 400 MHz. But most devices cannot listen to that entire bandwidth — it would be too complex and power-consuming.

So, instead of spreading control signals across the whole channel (like in 4G LTE), 5G allows the network to place control signals in a smaller area — and that smaller area is called a CORESET.

For example, consider CORESET as a TV Channel Guide

Imagine the 5G resource grid as your home television with hundreds of channels (PRBs and OFDM symbols).

  • The CORESET is like your TV channel guide screen that tells you what’s playing where and when (control information).

  • You don’t need the entire screen for the guide—just a small portion. Similarly, CORESET occupies a specific, configurable part of the 5G grid.

  • The guide may appear in different positions depending on your settings (morning vs night), just like CORESET can be placed at different time/frequency locations.


2. How is CORESET Different from LTE?

In 4G LTE:

  • Control messages are always at the start of each time slot, and they cover the whole frequency band (e.g., full 20 MHz).

  • That was okay when all devices supported the full bandwidth.

  • But newer LTE devices like eMTC (IoT) only support narrow bands, which made things complicated.

In 5G NR:

·       CORESET can be anywhere in time and frequency.

·       A CORESET has a fixed size, which makes it simpler and faster for the device to process control messages (PDCCH), without first having to decode something else.

·       It’s flexible — network can place it where it makes most sense. This is especially helpful for low-power devices or devices with limited bandwidth. 

Feature

LTE (PDCCH Region)

5G NR (CORESET)

Location

Always first symbols

Any symbol position

Bandwidth

Full BW only

Selective PRBs

Diversity

Limited

Interleaving, beamforming

Flexibility

None

Full control

Numerology

Fixed (15 kHz)

15–240 kHz

 

3. CORESET Timing and Location

  • A CORESET is made up of multiples resource blocks (i.e, multiples of 12 REs) in frequency domain and '1 or 2 or 3' OFDM symbols in time domain. 

  • It can be placed anywhere in a slot, but usually it’s placed at the beginning of the slot. This is similar to LTE, where control messages also come at the start.

  • A UE may be configured with up to three CORESETs on each of up to four BWPs on a serving cell. Therefore, a UE may be configured with up to 12 CORESETs on a serving cell in total, where each CORESET has an index of 0-11.

  • In general, CORESETs are configured in units of six PRBs on a frequency grid (starting from a reference point referred to as point A) and one, two, or three consecutive OFDM symbols in the time domain.

 

It’s important to know that a CORESET just tells the device where to look for control messages (PDCCH). It doesn’t guarantee that the network will actually send anything there.


Duration and Size Limits

The maximum length of a CORESET in time depends on where certain reference signals (DM-RS) are placed. Usually, it’s two or three OFDM symbols long to avoid overlapping with the start of actual data transmissions (PDSCH).

In terms of frequency:

  • A CORESET is made up of blocks of 6 resource blocks, and can go up to the full carrier width.


4. Mapping CORESET to 5G Resource Grid

A 5G NR resource grid consists of:

  • Time axis: Divided into OFDM symbols (14 per slot typically)

  • Frequency axis: Divided into subcarriers grouped into PRBs (12 subcarriers)



CORESET Mapping

  • A CORESET is mapped to a specific set of PRBs (frequency) and OFDM symbols (time).

  • Inside the CORESET, REGs (Resource Element Groups) are defined. Each REG = 12 subcarriers × 1 OFDM symbol.

  • CCE (Control Channel Elements) are composed of multiple REGs.

  • Mapping defines how CCEs are distributed—either sequentially or interleaved.

  • Aggregation Level (AL) defines the number of Control Channel Elements (CCEs) used to carry one DCI (Downlink Control Information) message in a CORESET. It’s a way to balance between coverage (robustness) and resource efficiency.

Aggregation Level

Number of CCEs

Number of REGs

Number of REs

1

1

6

72

2

2

12

144

4

4

24

288

8

8

48

576

16

16

96

1152


How it works:


5. Detailed Description of CORESET Parameters

Parameter

Description

Impact

CORESET-ID

Logical ID assigned to a CORESET

Helps UE identify which PDCCH belongs where

FrequencyDomainResources

Bitmask (45-bit) specifying PRBs used

Defines bandwidth of CORESET

Duration

1–3 OFDM symbols

Shorter = lower latency; Longer = more control data

CCE-to-REG Mapping Type

interleaved / nonInterleaved

Interleaved improves diversity; NonInterleaved helps with beamforming

REG Bundle Size

2, 3 or 6

Groups REGs into bundles for control decoding

Interleaver Size

Number of REG bundles interleaved together

Higher value increases frequency diversity

Precoding Granularity

How precoding is applied

Enables spatial beamforming for control

What is Search Space?

Search Space is an area within a CORESET that UE should monitor to detect a specific PDCCH/DCI. There are two large categories of Search Space(SS) called CSS (Common Search Space) and USS(UE specific Search Space). 

6. Call Setup Flow with CORESET

Initial Access Flow:

MIB Decoding

  1. UE reads CORESET#0 to decode PDCCH.

MIB ::5 SEQUENCE {

systemFrameNumber BIT STRING (SIZE (6)),

subCarrierSpacingCommon ENUMERATED {scs15or60, scs30or120},

ssb-SubcarrierOffset INTEGER (0..15),

dmrs-TypeA-Position ENUMERATED {pos2, pos3},

pdcch-ConfigSIB1 -> determines the bandwidth of PDCCH/SIB1 or the size of the CORESET

containing common search space

cellBarred ENUMERATED {barred, notBarred},

intraFreqReselection ENUMERATED {allowed, notAllowed},

spareBIT STRING (SIZE (1))

}

PDCCH-ConfigSIB1 ::5 SEQUENCE {

controlResourceSetZero ControlResourceSetZero,

searchSpaceZero SearchSpaceZero

}

PDCCH from CORESET#0 carries DCI for SIB1

  • CORESET#1 and beyond are explicitly defined in SIB1.

  • Used for UE-specific control signaling after initial access.

  • SIB1 includes:

o   CORESET-ID

o   BWP configuration

o   SearchSpace-to-CORESET mapping

SIB1 Example:

controlResourceSetId: 1

frequencyDomainResources: 0x3FC000 (PRBs 72-119)

duration: 3

mappingType: interleaved

searchSpaceId: 1

 

 

7. Conclusion

CORESET is one of the smartest innovations in 5G NR—designed to optimize how UEs receive control information across diverse deployment types. With its configurable nature and support for beamforming, it ensures more efficient use of control resources, enabling robust communication even in challenging RF environments.

Understanding how CORESET is defined, mapped, and used in real-world scenarios is critical for network testers, protocol engineers, and RF planners to ensure high-performance and low-latency 5G services.

 

8. References:

  • Understanding the Heart of the 5G Air Interface: An Overview of Physical Downlink Control Channel for 5G New Radiohttps://arxiv.org/ftp/arxiv/papers/1910/1910.01711.pdf

  • 5G NR Architecture, Technology, Implementation, and Operation of 3GPP New Radio Standards- Sassan Ahmadi

  • 5G NR: The Next Generation Wireless Access Technology, Erik Dahlman Stefan Parkvall Johan Sköld

 

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