In this chapter we shall learn about 5G NR CORESET.
CORESET stands for Control Resource Set.
It is used to decode PDCCH.
WKT UE needs to decode PDCCH and get the DCI info carried by PDCCH. Otherwise UE will never know the DL/UL grants for PDCCH and PUSCH.
In LTE PDCCH is located across the channel bandwidth. PCFICH gives the information of PDCCH OFDM symbol in time and frequency domain.
In 5G NR due to large bandwidth, it is not feasible to do the same.
Hence in 5G NR, CORESET has been introduced.
Below are few points related to CORESET:
The number of CORESET is limited to 3 per BWP including both common and UE specific CORESET.
> Frequency allocation in CORESET configuration can be contiguous or non-contiguous.
> In Time domain, CORESET configuration spans 1 to 3 consecutive OFDM symbols.
> REs in CORESET are organized in REGs (RE Groups).
> Each REG consists of 12 REs of one OFDM symbol in one RB.
Some of the CORESET terminologies:
> RE (Resource Element): It is the smallest unit of Resource Grid, 1 subcarrier x 1 OFDM symbol.
> REG (Resource Element Group): Made of 1 RB (Resource Block) i.e. 12 REs x 1 OFDM symbol
> REG Bundles: 1 REG bundle is made of multiple REGs, Bundle size is specified by parameter “L”.
> CCE (Control Channel Element): One CCE is made of multiple REGs.
> Aggregation level: It indicates the number of allocated CCEs for PDCCH. It can be 1/2/4/8/16.
Below image shows RRC parameter structure of CORESET:
> ControlResourceID: If 0 it is common CORSET, if 1, then it is dedicated.
> frequencyDomainResources: Each bit corresponds a group of 6 RBs, with grouping starting from the first RB group in the BWP.
> duration:Contiguous time duration of the CORESET in number of symbols.
> cce-REG-MappingType: Mapping of Control Channel Elements (CCE) to Resource Element Groups (REG).
Now we shall see 2 more important points:
1. Configuring CORESET Region.
2. Allocating PDCCH resources to a CORESET.
1. Configuring CORESET Region.
> CORESET is a region on the resource grid where PDCCH resources are located.
> Minimum unit of a CORESET is REG (Resource element group) , 1 REG = 12 SC’s on Freq Domain and Minimum 1 OFDM Symbol on Time Domain, i.e 1 REG = 1PRB
> A CORESET consist group’s of 6 PRB’s on frequency domain and minimum 1 OFDM symbols in time domain. (up to max 3 OFDM symbols can be allocated).
> Group of 6 PRB’s on the Resource grid is 6*12/1 = 72 Resource elements.
> Out of the 72 RE’s in a 6 PRB’s group, 54 RE’s are used for PDCCH resources & 18 are used for its associated DMRS.
> CORSET config parameters are communicated to UE by RRC Reconfiguration message.
2. Allocating PDCCH resources to a CORESET.
> PDCCH is composed of CCE’s (Control Channel elements).
> Number of CCE that a PDCCH is determined by CCE Aggregation Level.
> Each CORESET can only have one cce-REG mapping type.
> There are two types of cce-REG Mapping: Interleaved and Non-Interleaved
Time-domain and frequency-domain parameters of CORESET as defined in TS 38.21
3GPP 38.213 & 38.211