PUCCH Formats & UCI in LTE 4G

Introduction

The PUCCH is the LTE uplink's dedicated control lane. It exists so a UE can always send the small pieces of feedback the network depends on — downlink acknowledgements, requests for uplink resources, and channel-quality reports — even when it has no user data scheduled. Its coding is specified in TS 36.212, and its physical structure and procedures in TS 36.211 and TS 36.213.

You meet the PUCCH the moment a connected UE has anything to say upstream without a data grant. Every downlink transport block the eNB sends must be answered with a HARQ-ACK; a UE with fresh data but no grant must raise a Scheduling Request; and the downlink link-adaptation loop is fed by periodic channel-state reports. All three ride the PUCCH whenever there is no PUSCH in the subframe to carry them instead.

It matters because without this back-channel the downlink is blind. HARQ cannot retransmit what it does not know failed, the scheduler cannot know a UE is waiting, and the eNB cannot pick a sensible modulation and coding scheme. The PUCCH is small in bandwidth but sits on the critical path of almost every downlink transaction.

Why the PUCCH is needed

Concretely, three loops depend on this feedback. HARQ needs an acknowledgement per downlink transport block or it cannot decide whether to retransmit. The scheduler needs a Scheduling Request or it never learns a UE has data waiting — and a UE with no grant has no other way to ask. Downlink link adaptation needs a channel-state report, or the eNB must guess the modulation and coding blindly. The PUCCH is the resource that guarantees all three can happen even when the UE has no data channel to speak on.

What UCI carries

UCI is the umbrella term for the control feedback the UE sends in the uplink. It has three ingredients, and any PUCCH transmission carries one of them or a defined combination.

The three components differ sharply in size and in how often they appear. HARQ-ACK is one or two bits per downlink transport block and must be sent with a fixed timing relationship to the downlink data it answers — in FDD, four subframes later (a downlink PDSCH in subframe n is acknowledged in subframe n+4). The SR is a single logical bit that a UE is granted a periodic on/off resource for; its period is RRC-configured (sr-ConfigIndex, giving periods from every 1 subframe up to every 80 ms), and it exists only to break the chicken-and-egg problem of needing an uplink grant to ask for an uplink grant. The channel-state report — CQI, PMI and RI together often abbreviated CSI — is the largest payload, a coded report of tens of bits that the eNB configures either periodically on PUCCH (via cqi-PUCCH-ResourceIndex) or aperiodically on PUSCH.

Where the PUCCH sits: band edges and hopping

The PUCCH is not placed in the middle of the band. It is deliberately pushed to the two outermost resource-block regions, one at the lower edge of the system bandwidth and one at the upper edge. Keeping control at the edges leaves a large contiguous block in the centre for PUSCH, which must stay contiguous to preserve the single-carrier property of SC-FDMA. A UE that were handed control resources in the middle would fragment the band and force every PUSCH allocation to route around it.

To fight fading, each PUCCH transmission uses slot-level frequency hopping: a UE that occupies the lower edge in the first slot of the subframe mirrors up to the upper edge in the second slot (and vice versa). If one edge is in a deep fade, the other edge usually is not, so the two halves of the acknowledgement or CQI report see independent channels. The mapping is symmetric about the band centre, so the amount of spectrum consumed at each edge is identical and the eNB can size the control region once for the whole cell.

The size of the PUCCH region is set by broadcast parameters. The number of RBs reserved for the mixed CQI/HARQ formats is set by NRB(2), the split between Format 1 and Format 2 regions by NCS(1), and the cyclic-shift spacing by deltaPUCCH-Shift (Δ_shift ∈ {1, 2, 3}) — all advertised in PUCCH-ConfigCommon (broadcast in SIB2) so every UE agrees on which RBs are control and which are data. As the number of active users grows, the eNB widens the region from the edges inward; when the cell is lightly loaded, more of the band is freed back to PUSCH.

The PUCCH formats

LTE defines a small family of PUCCH formats, each tuned to a different kind and size of UCI. Formats 1/1a/1b are built from a cyclic-shifted CAZAC base sequence with an orthogonal cover code, so many UEs share the same resource blocks. Format 2 carries a coded channel report. Format 3 was added in Release 10 to carry the larger HARQ-ACK payloads that carrier aggregation produces. Every format occupies exactly one resource block (12 subcarriers) per slot and hops to the opposite edge in the second slot.

Notice the split by job. The Format 1 family handles the tiny SR and HARQ-ACK payloads, riding on a length-12 base sequence whose cyclic shift and cover code separate users. In normal cyclic prefix each 0.5 ms slot has 7 SC-FDMA symbols: 3 carry DMRS (spread by a length-3 orthogonal sequence) and 4 carry the modulated ACK/SR symbol (spread by a length-4 orthogonal cover code), so up to 3×12 = 36 code combinations exist in principle before delay spread trims the usable count. Format 2 carries the moderate CQI report as 10 QPSK data symbols = 20 coded bits per subframe (2 DMRS symbols per slot in normal CP), the input being up to ~13 information bits protected by a (20, A) Reed–Muller block code. The Format 2a/2b variants let a CQI report and an acknowledgement share one transmission by modulating the second reference symbol with the ACK bits — which is why they are defined for normal cyclic prefix only, where two DMRS symbols per slot are available. Format 3 exists purely because aggregating several downlink carriers can require far more than two ACK bits at once; it block-codes the whole HARQ-ACK payload (up to ~22 information bits, i.e. 48 coded bits) and sends it as a DFT-spread-OFDM symbol stream, spread across the data symbols with a length-5 orthogonal cover in the time domain.

PUCCH resources: cyclic shift, cover code, and the resource index

A single pair of PUCCH resource blocks (one at each edge) is shared by many UEs at once. That sharing is possible because a PUCCH "resource" is not a raw slice of spectrum — it is a code-domain address. For the Format 1 family it has two ingredients, and a resource index nPUCCH that maps onto them:

• Cyclic shift of the length-12 base sequence. Twelve distinct shifts are possible in principle; the spacing between usable shifts is set by deltaPUCCH-Shift (Δ_shift = 1, 2 or 3), so a cell trades the number of shifts against robustness under delay spread — Δ_shift = 1 exposes all 12 shifts, Δ_shift = 3 leaves 4 well-separated ones.
• Orthogonal cover code (OCC) applied across the SC-FDMA symbols of a slot. Several UEs sharing the same cyclic shift are still separable if they use different cover sequences.

The product of usable cyclic shifts and cover codes is how many UEs can multiplex onto one PUCCH RB pair. The scalar resource index nPUCCH(1) (for Format 1/1a/1b) or nPUCCH(2) (for Format 2) is what higher layers and the derivation rules actually carry; the physical layer turns that index into the concrete (RB, cyclic shift, cover code) triplet with a fixed formula.

Deriving the ACK resource from the PDCCH

For a dynamically scheduled downlink, the UE is not told explicitly which PUCCH resource to use for its acknowledgement. Instead the resource is derived implicitly from the control channel that carried the grant. The PUCCH resource index for the HARQ-ACK is computed from the number of the first Control Channel Element (CCE) the scheduling PDCCH used:

Here nCCE is the index of the first CCE of the PDCCH, and NPUCCH(1) is a cell-wide offset broadcast in the system information (n1PUCCH-AN). Because each PDCCH occupies a distinct set of CCEs, this rule guarantees that two UEs scheduled by different PDCCHs land on different PUCCH resources — no extra signalling needed, and no collisions. For semi-persistent scheduling and other cases where there is no scheduling PDCCH, the resource is configured by RRC instead.

PUCCH, or piggyback on PUSCH

The default carrier for UCI is the PUCCH. But an uplink subframe can also contain a PUSCH — the data channel. In Rel-8/9 single-carrier operation the UE must not transmit PUCCH and PUSCH simultaneously, because doing so would break the single-carrier property and inflate the cubic metric of the transmitted waveform. So when the UE has a scheduled PUSCH, the UCI is multiplexed onto the PUSCH instead — commonly called UCI piggybacking or "UCI on PUSCH."

The HARQ-ACK and RI bits are placed on resource elements next to the PUSCH demodulation reference symbols (where the channel estimate is freshest), and the CQI/PMI bits are mapped across the data region. This keeps the transmission single-carrier and within one power budget, so the UE pays for one uplink transmission rather than two. The number of resource elements the UCI steals from the data is fixed by an offset (βoffset, configured per UCI type by RRC) so the eNB and UE agree on the rate matching; a larger offset gives the UCI more redundancy at the cost of data throughput.

Release 10 added an optional mode, simultaneous PUCCH and PUSCH, enabled by the RRC flag simultaneousAckNackAndCQI and the more general capability that lets HARQ-ACK/SR stay on the PUCCH while data flows on PUSCH. This is only viable for UEs whose power amplifier can tolerate the higher cubic metric, and it trades waveform purity for the ability to always send control on its dedicated resource. When it is not configured, the piggyback rule applies.

HARQ-ACK bundling and multiplexing for TDD and CA

In FDD single-carrier operation a UE answers one downlink subframe with one HARQ-ACK four subframes later — simple. Two situations break that one-to-one relationship and force the UE to compress or combine multiple acknowledgements into a single PUCCH transmission: TDD, where several downlink subframes are acknowledged in one uplink subframe, and carrier aggregation, where several serving cells are acknowledged at once. LTE offers two mechanisms.

• ACK/NACK bundling — the UE logically ANDs the acknowledgements of multiple transport blocks (across subframes, or across codewords) into a single ACK/NACK. If everything decoded, it sends ACK; if any one failed, it sends NACK, and the eNB retransmits the group. Bundling keeps the payload tiny but is coarse: one failure taints the whole bundle. A Downlink Assignment Index (DAI) in the DCI tells the UE how many downlink subframes it should have received, so a missed assignment does not silently shrink the bundle.
• ACK/NACK multiplexing (channel selection) — the UE reports the individual acknowledgements by which PUCCH resource it transmits on and which QPSK constellation point it sends. With Format 1b with channel selection, the choice among a small set of configured resources plus the two modulated bits jointly encodes the ACK/NACK pattern of up to four transport blocks. This preserves per-block information at the cost of needing a set of resources.

For carrier aggregation with many cells, even channel selection runs out of room, which is exactly why Format 3 exists: it block-codes the full multi-bit HARQ-ACK payload and sends it on one DFT-s-OFDM resource selected from an RRC-configured set (up to four entries) by the ACK/NACK Resource Indicator (ARI) carried in the DCI's TPC field of the secondary cells.

SRS and PUCCH collisions

The Sounding Reference Signal (SRS) is a wideband uplink pilot the UE sends so the eNB can estimate the uplink channel for frequency-selective scheduling. SRS is transmitted in the last SC-FDMA symbol of a configured subframe, spanning much of the band — which is precisely where a PUCCH transmission also has energy. When an SRS occasion and a PUCCH transmission land in the same subframe, they collide, and LTE resolves it with deterministic rules rather than letting the UE choose.

• For the shortened PUCCH formats, the UE uses a shortened PUCCH format: it simply drops the last symbol of the PUCCH so the SRS can occupy it. This applies to cell-specific SRS subframes when the parameter ackNackSRS-SimultaneousTransmission is enabled, letting both signals coexist in the subframe.
• When simultaneous transmission is not configured, the UE drops the SRS in any subframe where it would collide with a PUCCH carrying HARQ-ACK or a positive SR — control feedback wins, because a lost acknowledgement is far more damaging than a skipped channel sounding.
• Periodic CSI on PUCCH that would clash with SRS is handled by dropping the lower-priority report according to configured rules, so the UE never has to transmit two overlapping signals it cannot power.

The theme throughout is priority: HARQ-ACK is the most protected UCI, SR next, and channel sounding or CSI yields when something has to give. The shortened-format trick is the preferred outcome because it preserves both signals; dropping is the fallback when the UE is not configured to shorten.

Summary

The PUCCH is the LTE uplink's dedicated control lane, carrying UCI — HARQ-ACK, SR and CQI/PMI/RI — whenever the UE has no PUSCH to piggyback on. It lives at the two band edges and hops across the slot boundary for frequency diversity, leaving the contiguous centre for SC-FDMA data. A small family of formats matches payload to job: Format 1/1a/1b for the 0–2-bit SR and HARQ-ACK, Format 2/2a/2b for the ~20-bit CQI report, and Rel-10 Format 3 for the large multi-bit HARQ-ACK that carrier aggregation demands.

The elegant core is that a Format 1 resource is a code-domain address — a cyclic shift plus an orthogonal cover code — so many UEs share one RB pair, and the dynamic ACK resource is derived for free from the scheduling PDCCH's first CCE. When data is scheduled, UCI multiplexes onto the PUSCH; when TDD or CA break the one-ACK-per-subframe relationship, bundling, channel selection or Format 3 compress the feedback; and when SRS collides, a strict priority order protects the acknowledgement above all else.

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