REG and CCE — The units PDCCH is built from

5G Systems Notes · Concept map · REG and CCE Route · Hierarchy · Index · All concepts · Hub

The grid and its units

Where it sits

Sits atLevel 5 of the hierarchy · The grid and its units · explained
Also writtenresource-element group · control-channel element
The 2 of them
REG — one resource block in one OFDM symbol — 12 resource elements
CCE — six REGs — the unit a PDCCH candidate is built from
Learn firstOFDM symbol · Resource block — 5 concepts in the full chain, see the paths
UnlocksCORESET · PDCCH
Primary clauseTS 38.211 §7.3.2.1, §7.3.2.2
Used inref-channels 5
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The channel that carries the DCI, and therefore the channel that must be decoded before PDSCH can be. TS 38.211 clause 7.3.2. Its content is described in PDSCH §8; this section is about how it is physically built.

Two units, both defined in clause 7.3.2:

REG — resource element group. "a resource-element group equals one resource block during one OFDM symbol" — twelve resource elements, of which three carry the PDCCH's own DM-RS and nine carry data.

CCE — control-channel element. "A control-channel element consists of 6 resource-element groups" — so one CCE is 72 resource elements, 54 of them payload.

Aggregation level is the code rate knob. A PDCCH occupies 1, 2, 4, 8 or 16 CCEs (Table 7.3.2.1-1). The DCI payload is fixed by its format, so more CCEs means a lower code rate and a more robust message:

Aggregation levelCCEsREGsPayload REsBits at QPSKRate for a 68-bit DCI
116541080.63
22121082160.31
44242164320.16
88484328640.079
16169686417280.039
Table 1: PDCCH aggregation levels from TS 38.211 Table 7.3.2.1-1, with the resource elements each buys. The payload column is a 44-bit DCI format 1_0 plus its 24-bit CRC, polar-encoded - see the modulation reference note. The effective code rate is that payload over the available bits at QPSK, two bits per resource element. A device at the cell edge is given a higher aggregation level, which costs the cell sixteen times as much control resource for one message.

PDCCH is always QPSK (clause 7.3.2.4) — there is no MCS field for control, because there is no channel report to base one on at the moment the control message is being sent. Robustness is bought with resources instead of with modulation, which is the whole difference between control and data.

Read on

This concept was first written up in ref-channels, which reads the whole group as one argument.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand REG and CCE (level 5) you first need 5 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 Subcarrier
1 OFDM
2 OFDM symbol
3 Resource element
4 Resource block
Immediately before REG and CCE: Resource block, OFDM symbol.
Keep going — where this sits on the route
◀Point Astep 45 · level 5
Step 46 of 118level 5 · The grid and its units
Transport blockstep 47 · level 5▶
Level 5 → 6 · this unlocks
CORESET level 7 · PDCCH level 8
↳ all 11 concepts at level 6 · level 5 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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