5G NR · PDCCH building blocks

REG / CCE Structure

Every PDCCH is built from two LEGO bricks. A REG is a single resource block in a single symbol — 12 REs, three of them pilots. Six REGs stack into a CCE. Zoom into the RE layout, snap CCEs together, and compute exactly how many DCI bits each aggregation level can carry. TS 38.211 §7.3.2.

Inspect a REG Capacity calculator
12REs / REG
3DM-RS / REG
6REGs / CCE
54data REs / CCE
REG → CCE → PDCCH · pilots interleaved with control
1 The atom

Inside a single REG

A Resource Element Group = 1 RB × 1 OFDM symbol = 12 resource elements. Three carry PDCCH DM-RS (subcarriers k where k mod 4 = 1, i.e. 1, 5, 9); the other nine carry control payload. Hover the cells.

Total REs
12
DM-RS REs
3
Control REs
9
DM-RS spacing
every 4th
The DM-RS are frequency-multiplexed inside the REG (not separate symbols), so the receiver estimates the channel right where the control bits sit — vital because PDCCH must be decoded blind and fast.
2 The brick

Six REGs make a CCE

A Control Channel Element bundles 6 REGs = 72 REs → 54 control REs + 18 DM-RS. That is the smallest unit a PDCCH can occupy (aggregation level 1). Watch six REGs assemble.

REGs
6
Total REs
72
Control REs
54
QPSK coded bits
108
3 The maths

PDCCH capacity calculator

PDCCH is always QPSK (Qm = 2). Pick an aggregation level and a target code rate; the calculator turns control REs into coded bits and then into a maximum DCI payload — exactly how the scheduler decides which level a DCI needs.

Max DCI payload (incl. 24-bit CRC)
QPSK · AL × 108 coded bits
— bits
4 Precoding

REG bundles

Within a CCE, REGs are grouped into bundles (size L = 2/3/6) that share one precoder, so the UE can do wideband channel estimation across the bundle. The bundle is also the unit the interleaver shuffles.

L = 2

Duration-1 CORESETs. Finer frequency granularity, more precoder diversity.

L = 3

Available for duration 2–3; aligns bundles with the symbol span.

L = 6

Wideband precoding — best channel-estimation gain when the channel is flat across the bundle.

5 Knowledge check

Test yourself

CORESET Config CCE-REG Mapping