CORESET (Control Resource Set)
Where 5G NR Control Information Lives
The CORESET is the answer 3GPP gave to a deceptively simple question: in a flexible, beam-swept, widely scalable air interface, where exactly does a device look for the instructions that tell it how to receive its data? Long Term Evolution fixed the control region to the first few OFDM symbols across the entire carrier. 5G NR replaced that rigid arrangement with a configurable rectangle of resources whose width, height, and position the network controls per bandwidth part. That flexibility lets an operator confine control overhead to a slice of spectrum, point it inside a specific analog beam at FR2, and trade control capacity against data capacity slot by slot.
A CORESET is described by three things: a frequency-domain allocation expressed as a bitmap of resource block groups (each group being six contiguous RBs), a duration of one to three OFDM symbols, and a CCE-to-REG mapping that is either interleaved or non-interleaved. Interleaved mapping spreads each control channel element across the allocated bandwidth to capture frequency diversity, which is important for cell-edge users who cannot rely on a single good subband. Non-interleaved mapping keeps a CCE contiguous, which suits beamformed or frequency-selective scheduling where the network already knows the best subband for a given user.
Because the CORESET is decoded before the UE knows its scheduling grant, the device performs blind decoding: it tries every PDCCH candidate at each configured aggregation level and checks the CRC, which is scrambled by the radio network temporary identifier. Keeping the number of candidates and CORESET size bounded is what makes that blind-decoding budget tractable, and it is the reason the standard caps the monitored candidates and non-overlapped CCEs per slot.
REG, CCE, and Aggregation Level Arithmetic
1 REG = 1 RB × 1 OFDM symbol = 12 subcarriers × 1 symbol = 12 REs
Control channel element (CCE):
1 CCE = 6 REGs = 72 REs (54 data REs + 18 DMRS REs)
Total CCEs in a CORESET:
NCCE = (NRB × Nsymb) ÷ 6
PDCCH aggregation level:
AL ∈ {1, 2, 4, 8, 16} CCEs → PDCCH occupies AL × 72 REs
Where NRB = allocated resource blocks, Nsymb = CORESET duration in symbols (1 to 3). Example: 48 RBs × 2 symbols = 96 REGs = 16 CCEs; an AL-8 PDCCH then consumes half of that CORESET.
CORESET#0 Frequency and Time Sizing
| Parameter | Allowed values | Typical FR1 | Typical FR2 | Notes |
|---|---|---|---|---|
| Frequency width | 24, 48, or 96 RBs | 48 RBs | 24 or 48 RBs | Contiguous for CORESET#0 |
| Duration | 1, 2, or 3 symbols | 1 symbol | 2 symbols | Up to 3 for general CORESETs |
| Subcarrier spacing | 15 / 30 / 60 / 120 kHz | 30 kHz | 120 kHz | Set by numerology μ |
| Occupied bandwidth | scales with SCS | ≈ 17.3 MHz | ≈ 69.1 MHz | 48 RBs example |
| CCE-to-REG mapping | interleaved / non-interleaved | interleaved | interleaved | Interleaved for diversity |
| Max CORESETs / BWP | 3 | 2 to 3 | 2 to 3 | IDs 0 to 11 cell-wide |
Frequently Asked Questions
What is the difference between a CORESET and a search space in 5G NR?
A CORESET defines the physical resources, namely the frequency-domain RBs and the 1 to 3 OFDM symbol duration, where control information can be placed. A search space set defines when and how often the UE monitors them, including periodicity, slot offset, and the number of PDCCH candidates per aggregation level. Each search space maps to exactly one CORESET, but one CORESET can serve several search space sets. Up to 3 CORESETs are configured per bandwidth part.
How large is CORESET#0 and why does it matter for initial access?
CORESET#0 is signaled by 4 bits in the MIB (pdcch-ConfigSIB1) and carries the PDCCH that schedules SIB1, so it must be decoded before any RRC connection. It spans 24, 48, or 96 contiguous RBs and 1, 2, or 3 OFDM symbols, with the exact pairing chosen from a table keyed by the SSB and PDCCH subcarrier spacings. A common FR1 setup is 48 RBs over 1 symbol at 30 kHz, equal to roughly 17.3 MHz, and it uses interleaved mapping so it remains receivable at the cell edge.
How many control channel elements fit in a CORESET and how is the PDCCH aggregated?
Each CCE is 6 REGs and each REG is one RB over one symbol, so a CCE holds 72 resource elements (54 data, 18 DMRS). Total CCEs equal (NRB × Nsymb) ÷ 6, so 48 RBs over 2 symbols give 16 CCEs. A PDCCH uses an aggregation level of 1, 2, 4, 8, or 16 CCEs; higher levels add coding redundancy for weak links but consume more of the limited CORESET, so link adaptation picks the lowest level meeting the target block error rate.