PUCCH — Physical uplink control channel — where HARQ-ACK and CSI go
Channels — what carries what
Where it sits
What it is
What it is. Where a device sends uplink control information: HARQ-ACK, scheduling requests, and CSI reports. TS 38.211 clause 6.3.2, with the selection rules in TS 38.213 clause 9.2.
Five formats, and the choice between them is mechanical. TS 38.213 clause 9.2.2 states it directly:
| Format | Clause | Symbols | Payload | How it carries it |
|---|---|---|---|---|
| 0 | 6.3.2.3 | 1–2 — short | 1–2 bits | Sequence selection — a cyclic shift of a base sequence is the message. No modulation at all. |
| 1 | 6.3.2.4 | 4–14 — long | 1–2 bits | A modulated sequence spread over many symbols — the coverage version of format 0. |
| 2 | 6.3.2.5 | 1–2 — short | > 2 bits | Scrambled, QPSK-modulated, with its own DM-RS. |
| 3 | 6.3.2.6 | 4–14 — long | > 2 bits | The large-payload long format — a full CSI report goes here. |
| 4 | 6.3.2.6 | 4–14 — long | > 2 bits | Like format 3, but spread so several devices share the same resource blocks. |
Formats 0 and 1 are the interesting ones, because they do not modulate anything. The acknowledgement is conveyed by which cyclic shift of a known sequence the device transmits. A receiver correlates against both possibilities and takes the larger — which works at signal levels where demodulating a QPSK symbol would not.
That is why a one-bit acknowledgement is so much cheaper than a one-bit data message, and it is what makes the HARQ loop in PDSCH §12 affordable enough to run on every transmission.
Before a device has a dedicated configuration, its PUCCH resource comes from Table 9.2.1-1 —
sixteen predefined rows, each a format, a first symbol, a duration and a set of cyclic shifts,
indexed by pucch-ResourceCommon. It is the PUCCH counterpart of CORESET 0.
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: