PDCCH — Physical downlink control channel — the assignment

5G Systems Notes · Concept map · PDCCH Route · Hierarchy · Index · All concepts · Hub

Channels — what carries what

Where it sits

Sits atLevel 8 of the hierarchy · Channels — what carries what · explained
Learn firstREG and CCE · CORESET · Polar coding — 16 concepts in the full chain, see the paths
UnlocksDCI · Search space
Primary clauseTS 38.211 §7.3.2
Used innr-pdsch 22 · ref-channels 6 · nr-frame-structure 4 · index 2 · ref-system 1
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.

Widget not found: sim_status

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

To understand PDCCH (level 8) you first need 16 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 Cyclic prefix · Resource element · Slot
4 Numerology · Physical layer · Resource block
5 Point A · REG and CCE · Transport block
6 Bandwidth part · CRC
7 CORESET · Polar coding
Immediately before PDCCH: CORESET, REG and CCE, Polar coding.
Keep going — where this sits on the route
◀PDCPstep 83 · level 8
Step 84 of 118level 8 · Channels
PBCHstep 85 · level 8▶
Level 8 → 9 · this unlocks
DCI · Search space
↳ all 8 concepts at level 9 · level 8 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.
5G Systems Notes · Concept map · PDCCH Top · Concept map · Hub