PUCCH
Physical Uplink Control Channel: carries uplink control information including HARQ-ACK feedback, scheduling requests, and CSI reports from UE to gNB.
PUCCH carries the small but essential feedback that keeps the downlink running: HARQ-ACK/NACK (did I receive that block?), scheduling requests (I have uplink data, please grant me resources), and CSI reports (here's the channel quality I'm seeing). When a UE has no PUSCH scheduled, this control rides on PUCCH; when it does have PUSCH, the control can be piggybacked there instead.
NR defines five PUCCH formats, and the split that matters is short versus long. Formats 0 and 2 occupy just 1–2 OFDM symbols — quick, low-latency feedback for users near the cell with good signal. Formats 1, 3, and 4 span 4 to 14 symbols, using the extra time for repetition and coverage so a cell-edge UE's ACK still gets through. Format choice also depends on payload: format 0 and 1 carry only a couple of bits, while 2, 3 and 4 handle larger CSI reports. Getting HARQ-ACK timing and PUCCH resource allocation right is a common source of scheduler tuning.
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