A 5G Coreset, or Control Resource Set, is a set of time-frequency resources on the NR downlink resource grid where a Physical Downlink Control Channel (PDCCH) may be transmitted.

CORESETs are semi-statically configured by the network and communicated to the UE via RRC Reconfiguration messages.

CORESETs are used to transmit downlink control information, such as Downlink Control Information (DCI) messages, which instruct the UE on how to receive and process the data transmitted on the downlink. DCI messages contain information such as the time and frequency locations of the downlink data channels, the modulation and coding schemes used, and the transmit power.

CORESETs are designed to improve the performance of the PDCCH in a number of ways.

• By localizing the PDCCH to a specific region of the frequency domain, CORESETs can reduce inter-cell interference.
• By allowing the network to configure the size and location of the CORESET, the network can optimize the PDCCH for the specific channel conditions and traffic requirements.

PDCCH decoding is very important in 5G NR, as it allows the UE to receive downlink control information, such as the time and frequency locations of the downlink data channels, the modulation and coding schemes used, and the transmit power.

If the UE fails to decode the PDCCH, it will not be able to receive any downlink data.

• The Base Station transmits the PDCCH using Resource Elements (REs) that belong to a Control Resource Set (CORESET).
• A CORESET is a localized region in the frequency domain where the PDCCH may be transmitted. It is equivalent to the LTE PDCCH area, but in LTE the PDCCH is always spread across the whole channel bandwidth.
• The NR CORESET region can be configured to have a maximum of 3 symbols. This is in contrast to LTE, where the PCFICH channel is used to know the number of symbols allocated to the PDCCH at any point in time.
• NR does not have a PCFICH physical channel, which simplifies the decoding process because the UE does not need to decode the PCFICH before decoding the PDCCH.
• If the UE fails to decode the PDCCH in the CORESET, it will search for the PDCCH in a Search Space (SS).
• The SS is a larger area within the CORESET that the UE should monitor to detect the PDCCH. There are two types of Search Spaces:
• Common Search Space (CSS): The CSS is used to transmit PDCCHs for all UEs in the cell.
• UE-Specific Search Space (USS): while the USS is used to transmit PDCCHs for specific UEs.
• The size of a Search Space is determined by the aggregation level.
• The aggregation level is a parameter that specifies how many CCEs are grouped together to form a PDCCH candidate.
• The higher the aggregation level, the larger the Search Space will be.
• The number of CORESETs within a Channel Band (CBW) depends on how many CORESETs are configured in each Bandwidth Part (BWP) and how many BWPs are configured in the CBW.
• The larger the CORESET, the more CCEs are available to send DCI messages. This is because each CCE is made up of a group of PRBs and symbols.

PDCCH Building Blocks

The PDCCH is a physical control channel in 5G NR that is used to transmit downlink control information to UEs. It is built on top of the following building blocks:

• The PDCCH is made up of Control Channel Elements (CCEs).
• Each CCE consists of 6 Resource-Element Groups (REGs).
• A REG is equal to 1 resource block during 1 OFDM symbol (12 REs).
• An RE is equal to 1 subcarrier on 1 OFDM symbol.
• The Aggregation Level (AL) defines the number of CCEs which will be used for Downlink Control Information (DCI) transmission.

Aggregation Level vs REGs

Aggregation Level = 6 REG and REG = 12 RE, so one aggregation level means (72 RE)

Number of CCE/ Aggregation Level	Number of Resource-Element Groups (REGS)
1	6
2	12
4	24
8	48
16	96

Example:

The PDCCH is transmitted on a specific set of REs in the frequency domain. This set of REs is called the PDCCH carrier. The PDCCH carrier can be made up of 1 or more CCEs.

A CCE is a group of 6 REGs. A REG is equal to 1 resource block during 1 OFDM symbol. A resource block is a group of 12 REs.

The Aggregation Level (AL) defines the number of CCEs which will be used for DCI transmission. The AL can be 1, 2, 4, or 8.

For example, an AL of 2 means that the PDCCH carrier will be made up of 2 CCEs. Each CCE will consist of 6 REGs, which is equal to 72 REs. Therefore, the PDCCH carrier will be made up of 144 REs.

The AL is configured by the network and communicated to the UE via RRC signaling messages. The UE then monitors the PDCCH carrier for DCI transmissions.

A higher AL keeps the PDCCH transmission at a high robustness level, making it suitable for cells where coverage has the highest priority. However, a higher AL also consumes more CCEs, which could be used by other UEs in this slot, reducing the cell capacity.

CORESET vs. LTE Control Region

The Control Resource Set (CORESET) in 5G NR is equivalent to the Control Region in LTE. However, there are some key differences between the two:

Frequency Domain

In LTE, the Control Region spans the entire channel bandwidth (CBW). However, in NR, the CORESET is localized within each Bandwidth Part (BWP). This means that the CORESET can be configured to have any frequency domain width, in multiples of 6 Resource Blocks (RBs).

Time Domain

Both the CORESET in NR and the Control Region in LTE can vary in time domain length. In LTE, the time domain length of the Control Region is defined by the Physical Control Format Indicator Channel (PCFICH). However, in NR, the time domain length of the CORESET is defined by the RRC parameter ControlResourceSet.duration.

Benefits of CORESET

The CORESET offers a number of benefits over the LTE Control Region, including:

• Improved flexibility: The CORESET can be configured to have any frequency domain width and time domain length, which gives the network more flexibility to optimize the control channel for different traffic scenarios and channel conditions.
• Reduced interference: By localizing the CORESET to a specific region of the frequency domain, the network can reduce inter-cell interference.
• Improved power efficiency: By reducing the frequency domain width of the CORESET, the network can improve the power efficiency of the control channel.

Non-interleaved vs. interleaved CORESET

Non-interleaved

In a non-interleaved CORESET, the Control Channel Elements (CCEs) are mapped to Resource Element Groups (REGs) in a contiguous manner. This means that the CCEs are transmitted in the same frequency and time domain locations.

For example, a non-interleaved CORESET with an Aggregation Level (AL) of 4 would consist of 4 CCEs that are mapped to the first 4 REGs in a subframe. The 4 CCEs would be transmitted in the same frequency and time domain locations.

Interleaved CORESET

In an interleaved CORESET, the CCEs are mapped to REGs in a scattered manner. This means that the CCEs are transmitted in different frequency and time domain locations.

For example, an interleaved CORESET with an AL of 4 could consist of 4 CCEs that are mapped to the first, fourth, seventh, and tenth REGs in a subframe. The 4 CCEs would be transmitted in different frequency and time domain locations.

ControlResourceSet information element

Mapping between PDCCH and CORESET

• The figure shows an example of how PDCCH transmission is mapped to a CORESET in 5G NR. The CORESET is shown in blue and the PDCCH transmission is shown in red.
• The CORESET is a region of the frequency domain in which the PDCCH can be transmitted. The size of the CORESET is configurable and can be different for different subframes. The PDCCH transmission can be mapped to any location within the CORESET.
• In the figure, the CORESET is located in the lower half of the frequency domain and the PDCCH transmission is mapped to the first two OFDM symbols within the CORESET.
• The UE uses the CORESET configuration to search for PDCCH transmissions. The UE knows the frequency domain location of the CORESET and the time domain length of the CORESET. The UE then searches for PDCCH transmissions within the CORESET.

• If the UE finds a PDCCH transmission, it will decode the DCI information that is carried by the PDCCH. The DCI information contains instructions for the UE on how to receive and process the downlink data channels.

A Special CORESET: CORESET 0

CORESET0 is a special type of CORESET in 5G NR that is used to transmit the Physical Downlink Control Channel (PDCCH) for System Information Blocks (SIBs). SIBs contain important system information, such as the cell identity, the available frequency bands, and the supported features.

• CORESET0 is referred to as Type 0 CORESET because it is the first CORESET that is configured to the UE. It is a fundamental CORESET for both Non-Standalone (NSA) and Standalone (SA) 5G NR networks.
• CORESET0 is configured to the UE by using a pointer included in the Master Information Block (MIB) or in the RRC Reconfiguration message. The configuration index points to a row within a look-up table standardized by 3GPP. This look-up table contains the frequency domain location and time domain length of CORESET0.
• CORESET0 is used by Search Spaces belonging to both the Initial and Dedicated Bandwidth Parts (BWPs). The Initial BWP is the BWP that is used to transmit the control channels that are required for the UE to connect to the network. The Dedicated BWPs are the BWPs that are used to transmit user data.
• CORESET0 is a specific use case of Common Coreset, where SIB 1 message is broadcasted. SIB 1 contains essential system information that the UE needs to operate in the network, such as the cell identity, the available frequency bands, and the supported features.

Bullet Points of CORESET:

• Each Bandwidth Part (BWP) has the capacity for accommodating up to 3 Control Resource Sets (CORESETs), which can be either common or UE-specific.
• A single cell is capable of supporting up to 4 BWPs, although only one BWP can be active at any given time.
• Consequently, each cell has the potential to configure and manage a total of 12 CORESETs, considering the combination of BWPs.
• In addition, each BWP can define up to 10 search spaces, facilitating efficient resource allocation.
• Therefore, within a single cell, up to 40 search spaces can be configured, considering all BWPs and their respective search spaces.
• It’s important to note that each CORESET is identified by a unique CORESET ID within a cell.
• Furthermore, each search space is closely associated with a specific CORESET, ensuring a clear mapping of resources and control elements.

Example

A cell with 2 active BWPs can have up to 6 CORESETs and up to 20 search spaces. The UE would monitor all 20 search spaces for PDCCH transmissions.