CORESET — The configured region PDCCH may occupy
The grid and its units
Where it sits
What it is
What it is. The configured region of the grid in which PDCCH may appear. TS 38.211 clause 7.3.2.2: a CORESET consists of resource blocks in frequency and $N^{\text{CORESET}}_{\text{symb}} \in {1, 2, 3}$ symbols in time.
This is the clause that deleted a channel. In LTE the control region's depth changed per subframe and had to be announced, on PCFICH, before anything else could be decoded — a whole physical channel whose entire content was one number (PDSCH §3.2).
In NR the depth is configured by RRC and simply known. PCFICH became unnecessary, and the control region gained something LTE's never had: it need not span the carrier. A CORESET is a rectangle, not a set of full-width symbols, so control for one device can sit in one part of the band while data flows in another.
CCE-to-REG mapping can be interleaved or non-interleaved (clause 7.3.2.2), via REG bundles. Interleaving scatters a PDCCH's REGs across the CORESET, which buys frequency diversity for exactly the same reason the PDSCH VRB interleaver does (PDSCH §6.5) — the scheduler cannot know a good place to put a message for a device it has not heard from.
CORESET 0 is the special one. It is the CORESET a device finds from the MIB before it has any configuration at all, and it is what the frequency-allocation exception in PDSCH §8.3 refers to when it says numbering starts from the lowest block of the CORESET rather than of the bandwidth part.
Read on
This concept was first written up in ref-channels, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: