NR introduced a new concept of CORESETs (COntrol REsource SETs). This concept is related to Downlink Control signaling. To understand this, let’s first revisit “LTE CORESET”. You may argue that there is no such concept in LTE. So, Let’s first define what could be LTE CORESET (or concept closest to CORESETs).
As it can be seen from figure below, In LTE, up-to 4 OFDM symbols are used for Downlink control signaling and it uses the full carrier bandwidth. This region is known as “Control Region”. Control region is situated a the start of the subframe and length can change dynamically and indicated by PCFICH channel. This region can be known as LTE CORESET.
Below given are some of the drawbacks of LTE CORESET
- There do not exist mechanisms to perform neither frequency domain scheduling nor Intercell Interference Coordination (ICIC) over the PDCCH and hence, low Signal to Interference plus Noise Ratio (SINR) levels at cell edges.
- Release-12 eMTC devices does not use full system bandwidth. This leads to the complexities.
- UE first needs to decode PCFICH channel and then only it knows the length of control region. This dependency makes pipelined processing complex.
- Resources assigned for control region can not be used for data channels.
Although LTE-A provides some mechanisms like introduction of ePDCCH to overcome some of the issues related to LTE PDCCH yet those are not enough. Therefore, a more flexible structure was introduced in NR.
- In NR, CORESET is defined as time-domain and frequency-domain resources. In time domain, a CORESET is semi-statically configured with one or a set of contiguous OFDM symbols (up to 3 OFDM symbols) and it can be located anywhere in the slot. The configuration indicates the starting OFDM symbol and time duration. In frequency domain, a CORESET can be located anywhere in the frequency range of the carrier (However, not outside the active Bandwidth Part). And, is made up of multiple of resource bocks (i.e. multiples of 12 REs). This flexibility to blank-out certain CORESET or to configure overlapping CORESET provides greater flexibility and helps in avoiding intercell interference.
- There are two types of CORESETs : Common CORESET and UE-Specific CORESET. After UE has detected the serving cell and decoded PBCH in SS- block to acquire the essential system information such as bandwidth, SFN and etc. Then the UE needs to decode the remaining system information to get the initial access related parameters. The remaining system information is scheduled by the NR-PDCCH, however, UE cannot expect any configuration information for the CORESET except for the information carried in the NR-PBCH at this stage. In this sense, the configuration for CORESET 0 (common CORESET) is indicated by MIB (Master Information Block). In the connected mode, UE can be configured with UE-specific CORESETS. There can be up-to 12 CORESETs configured (up-to 3 CORESETs in each BWP).
- when the control resource set spans multiple OFDM symbols, NR support a control channel candidate to be mapped to multiple OFDM symbols or to a single OFDM symbol. (As shown in figure below). In this case, The gNB informs UE which control channel candidates are mapped to each subset of OFDM symbols in the control resource set.