5G NR CORESET: Control resource set

The parameters in CORESET include

• Frequency domain parameter
• Time-domain parameter

In frequency domain parameters, since the control region in LTE is always spread across the whole channel bandwidth, there are no parameters defining the frequency domain region for the LTE control region, but in the new radio, there is a need for a parameter defining the frequency domain width for CORESET. The frequency-domain width can be set in any value in the multiples of 6RBs. 

In time-domain parameters, Both NR CORESET and LTE control regions can vary in time domain length. So we need the parameter for time-domain length both in LTE and NR. 

It is made up of multiple levels of sub-structures. It can accommodate multiples of Aggregation levels. An Aggregation level is made up of N CCE. A CCE is made up of 6 REGs. A REG is made up of 1 RB and 1 OFDM symbol. 

• Resource element: It is the smallest unit of the resource grid made up of one subcarrier in the frequency domain and one OFDM symbol in the time domain. 
• Resource element group (REG): One REG is made up of one resource block (12 RE in frequency domain) and one OFDM symbol in the time domain. 
• REG bundles: One REG bundle is made up of multiple REG’s. The bundle size is specified by the parameter “L”. is determined by the RRC parameter.
• CCE (control channel element): A CCE is made up of multiple REG’s. The number of REG bundles within a CCE varies. 
• Aggregation level: It indicates how many CCE’s are allocated for a PDCCH. 

Control Resource Set (CORESET): It is made up of multiple resource blocks (i.e multiples of 12 REs) in the frequency domain and ‘1 or 2 or 3’ OFDM symbols in the time domain. It is equivalent to the control region in the LTE subframe. In NR, both the frequency region and the time domain region can be defined by the RRC signaling message. 

ControlResourceSetID: this corresponds to L1 parameter ‘CORESET-ID’

• Value 0: it identifies the common CORESET configured in MIB and in ServingCellConfigCommon
• Values 1: it is configured by dedicated signaling
• The controlResourceSetID is unique among the BWPs of the serving cell. 

A Special CORESET : CORESET 0 

These are configured by RRC. It is a special type of CORESET which is called CORESET 0. It is the one transmitting PDCCH for SIB1 scheduling. It cannot be specified by RRC since it should be used before any RRC is transmitted. It implies that CORESET0 should be configured by some predefined process and pre-defined parameters. 

The network can define common control regions and UE-specific control regions. The number of CORESET is limited to 3 per BWP (bandwidth part) including both common and UE specific CORESET. 

Search space: Each Search space is a set of control channel elements at a different aggregation level. It tells how many candidates are there to decode at different aggregation levels. There is a mapping between CORESET and search space. It is the space where UE attempts blind decoding. Different aggregation levels are there to mitigate the situation when UE is under bad coverage. 

Bandwidth part: 

• The motivation of BWP: In most cases, NR would operate in very wideband and there wouldn’t be any issues for the high-end UEs to handle the full operating band. There is also a need for the mechanism so that narrowband UE can operate. So BWP is no parameters are defining fraction of the spectrum that a UE only needs to care about. It is defined for both UL/DL. 
• Default BWP: DL BWP to be used upon expiry of the BWP inactivity time. When this field is absent then UE uses Initial BWP as the default BWP.

Important points: 

• The number of CORESET is limited to 3 for each BWP (Bandwidth Part) including both common and UE specific. 
• The number of BWP per serving cell is 4 and at a time there is only one active BWP. 
• The maximum number of  CORESET per serving cell is - 12. 
• The max number of search spaces per BWP is 10.
• Max search space per serving cell is the number of BWP per serving cell. 
• CORESET ID is unique among the BWPs of a serving cell. 
• Each search space is associated with some CORESET.