What is CORESET in 5G(NR)? (1)

I. A CORESET is a Control Resource Set used in 5G (NR). It is a set of physical resources within a specific area of ​​the Downlink Resource Grid used to carry the PDCCH (DCI). In 5G (NR), the PDCCH is specifically designed to be transmitted within a configurable Control Resource Set (CORESET).

II. PDCCH Location The CORESET in 5G is similar to a Control Region in LTE because its Resource Set (RB) and OFDM symbol set are configurable, and it has a corresponding PDCCH search space. The flexibility of NR Control Region configuration, including time, frequency, parameter set, and operating point, allows it to meet a wide range of application scenarios. While PDCCHs in LTE Control Regions are allocated across the entire system bandwidth, NR PDCCHs are transmitted within a specially designed CORESET area, located in a specific region of the frequency domain, as shown in the diagram below.

III. 4G PDCCH and 5G PDCCH CORESET Frequency allocation in a CORESET configuration can be continuous or discontinuous. A CORESET configuration spans 1-3 consecutive OFDM symbols in time. REs in a CORESET are organized into REGs (RE groups). Each REG consists of 12 REs from one OFDM symbol in an RB. The PDCCH is confined to a CORESET and transmitted using its own demodulation reference signal (DMRS) to achieve control channel beamforming for the UE. To accommodate different DCI payload sizes or different coding rates, the PDCCH is carried by 1, 2, 4, 8, or 16 Control Channel Elements (CCEs). Each CCE contains 6 REGs. The CCE-to-REG mapping of a CORESET can be interleaved (for frequency diversity) or non-interleaved (for local beamforming).

IV. CORESET Mapping Each 5G terminal (UE) is configured to blindly test multiple PDCCH candidate signals with different DCI formats and aggregation levels. Blind decoding increases the complexity of the UE, but is necessary for flexibly scheduling and processing different DCI formats with low overhead.

V. CORESET Characteristics

• The CORESET control resource set in 5G (NR) is similar to the LTE PDCCH control area;
• 5G (NR) CORESETs are divided into two types: general CORESETs and UE-specific CORESETs;
• Each active downlink BWP can configure up to 3 core sets, including general CORESETs and UE-specific CORESETs;
• A serving cell can have up to 4 BWPs, and each BWP can have up to 3 CORESETs, for a total of 12 CORESETs;
• Each CORESET can be identified by an index ranging from 0 to 11, named Control Resource Set Id;
• The Control Resource Set Id is unique within the same serving cell;
• When a specific CORESET is defined, its index is CORESET0; this CORESET is configured using a 4-bit information element in the MIB (Master Information Block), which is associated with the cell-defined synchronization signal and Physical Broadcast Channel (PBCH) block (SSB);
• CORESETs are only configured within their associated Bandwidth Weighted (BWP) Activation occurs only upon activation, except for CORESET0, which is associated with the initial bandwidth-weighted packet (the bandwidth-weighted packet with index 0);
• In the frequency domain, CORESETs are configured on 6 PRB frequency grids in units of 6 PRBs;
• In the time domain, CORESETs are configured as 1, 2, or 3 consecutive OFDM symbols.