UCI stands for Uplink Control Information. It is carried by PUCCH or PUSCH. It may remind you of DCI which is carried by PDCCH. Yes, UCI is the counter part of DCI, but the information/role of UCI is very small comparing to DCI ( I think).

Small does not mean unimportant, though. The eNB cannot schedule the downlink well without the channel report, and it cannot finish a HARQ process without the ACK or NACK. Let's look at UCI from three sides: what it carries, which channel carries it, and how the bits are placed on that channel.

• What does UCI carry ?
• When does PUSCH carry UCI instead of PUCCH ?
• Which PUCCH format carries which UCI ?
• How does UCI share a PUSCH with data ?
• Reference

What does UCI carry ?

The eNB needs three kinds of feedback from the UE. It needs to know whether the UE has data waiting, whether the last PDSCH was decoded, and what the downlink channel looks like. Each kind of feedback is one UCI type.

The information carried by UCI is mainly following three

• SR (Scheduling Request)
• HARQ ACK/NACK
• CQI

UE transmit a certain combination of these three information depending on situation. Sometimes it carries only SR, sometimes SR and HARQ ACK/NACK together etc.

Each type has its own size. SR is the simplest. The UE sends PUCCH format 1 on its SR resource for a positive SR, and sends nothing for a negative SR. So the SR information is in the presence of the signal, not in a bit value. HARQ-ACK is one bit per transport block. With one FDD serving cell, that is 1 bit for one transport block and 2 bits for two. With carrier aggregation or TDD, the UE reports for several cells or several DL subframes in one uplink subframe, so the HARQ-ACK payload grows.

The third item in the list is CQI, but 36.213 uses a wider name, CSI. CSI covers CQI, PMI and RI, and later releases added PTI and CRI. The size of the CQI and PMI report depends on the rank, so the eNB has to decode RI first. CSI is either periodic or aperiodic. Periodic CSI is sent with a period set by RRC, normally on PUCCH. Aperiodic CSI is sent only when a DCI requests it, and it always goes on PUSCH.

• SR is carried by presence : the UE sends PUCCH format 1 for a positive SR and nothing for a negative SR.
• HARQ-ACK size follows the number of transport blocks : it is 1 or 2 bits for one FDD cell, and more with carrier aggregation or TDD.
• CQI in the list stands for all CSI : CSI is CQI, PMI and RI, plus PTI and CRI in later releases.
• Aperiodic CSI always goes on PUSCH : periodic CSI normally uses PUCCH.

When does PUSCH carry UCI instead of PUCCH ?

A Release 8 UE does not send PUCCH and PUSCH in the same subframe. The two channels sit in different PRBs, and sending both at once would break the single carrier property of SC-FDMA. So in each subframe the UE needs a rule that picks one channel for the UCI.

There are two channels that can carry the UCI. Sometimes PUCCH carries UCI and sometimes PUSCH carries it.

Then when PUSCH carries UCI and when PUCCH carries it ?

36.213 section 10.1 UE procedure for determining physical uplink control channel assignment describe it as follows :

Uplink control information (UCI) in subframe n shall be transmitted

- on PUCCH using format 1/1a/1b or 2/2a/2b if the UE is not transmitting on PUSCH in subframe n

- on PUSCH if the UE is transmitting on PUSCH in subframe n unless the PUSCH transmission corresponds to a

Random Access Response Grant or a retransmission of the same transport block as part of the contention based

random access procedure, in which case UCI is not transmitted

Simply put, when UE transmit the user data and it has to use PUSCH. In this case PUCCH is not allowed to be transmitted, in this case PUSCH carries UCI. When there is no user data to be transmitted, PUCCH is transmitted carrying UCI in it.

The quote above is the Release 8 wording. The current 36.213, v19.4.0, keeps the same rule, but it states the scope first. The rule applies to a non-BL/CE UE with a single serving cell that is not configured for simultaneous PUSCH and PUCCH transmissions. The PUCCH list now reads format 1/1a/1b/3 or 2/2a/2b. The text also adds an SPS case. If the PUSCH belongs to an SPS configuration with totalNumberPUSCH-SPS-UL-Repetitions, the UE does not transmit periodic CSI.

Release 10 relaxed the one-channel rule. If the UE supports it, the network can configure simultaneousPUCCH-PUSCH-r10 in 36.331. Then the UE splits the UCI between the two channels. HARQ-ACK and SR stay on PUCCH, and the CSI goes on the PUSCH. If the UCI is only HARQ-ACK and SR, it stays on PUCCH even when the UE sends a PUSCH.

Carrier aggregation adds another question: which PUSCH carries the UCI. The UE transmits PUCCH only on the primary cell, or on the PUCCH-SCell for a secondary PUCCH group. For PUSCH, the primary cell comes first. If the UE sends PUSCH on the primary cell, the periodic CSI and HARQ-ACK go on that PUSCH. If it does not, they go on the PUSCH of the secondary cell with the smallest SCellIndex, excluding an LAA SCell. Aperiodic CSI goes on the PUSCH of the serving cell that the triggering DCI points to (36.213 clause 7.2.1).

SR is the one UCI type that never goes on PUSCH. The PUSCH multiplexing in 36.212 has no field for it. MAC signals an SR only when no UL-SCH resources are available in the TTI (36.321 clause 5.4.4). A UE that already has a grant reports its buffer with a BSR in the MAC PDU instead.

• Without simultaneous PUCCH-PUSCH, the UE sends one channel per subframe : UCI goes on PUSCH when a PUSCH exists, and on PUCCH otherwise.
• A PUSCH for an RAR grant carries no UCI : the same applies to its retransmission in contention based random access.
• simultaneousPUCCH-PUSCH-r10 splits the UCI : HARQ-ACK and SR stay on PUCCH, and CSI goes on PUSCH.
• With carrier aggregation, the primary cell PUSCH comes first : otherwise the UE uses the SCell with the smallest SCellIndex.
• SR never goes on PUSCH : a UE with a grant sends a BSR instead.

Which PUCCH format carries which UCI ?

Once the UE has chosen PUCCH, it still has to choose a format. The choice depends on which UCI types collide in the subframe and how many bits they need. Formats 1 and 2 date from Release 8 and carry only a few bits. Formats 3, 4 and 5 came later for carrier aggregation, when HARQ-ACK for many cells no longer fits in two bits.

The table below combines two sources. 36.211 Table 5.4-1 gives the modulation and the number of coded bits per subframe. 36.213 clause 10.1.1 lists the UCI that each format carries.

Read the table from the top. Format 1 has no bits at all, because it carries SR by presence alone. Formats 1a and 1b add one or two HARQ-ACK bits on the same structure. So a positive SR and an ACK can go together in one subframe. The UE sends the ACK bits on its SR resource instead of its HARQ-ACK resource. With channel selection, format 1b carries up to 4 HARQ-ACK bits. The UE picks one of several format 1b resources, and the chosen resource carries part of the information.

Formats 2a and 2b exist only with normal CP. They carry the 20 CSI bits and put the HARQ-ACK on the second reference symbol of each slot. With extended CP, format 2 carries CSI and HARQ-ACK together. A collision between periodic CSI and HARQ-ACK is a configuration choice, though. For a single serving cell without format 3, the UE multiplexes them only if simultaneousAckNackAndCQI is TRUE. Otherwise the UE drops the CSI and sends the HARQ-ACK alone.

• The format 1 family carries SR and HARQ-ACK : format 1 carries SR by presence, 1a carries 1 bit and 1b carries 2 bits.
• The format 2 family carries CSI : 2a and 2b add 1 or 2 HARQ-ACK bits, with normal CP only.
• Formats 3, 4 and 5 serve carrier aggregation : format 3 carries 48 coded bits, and formats 4 and 5 carry more than 22 UCI bits.
• simultaneousAckNackAndCQI decides a CSI collision : when it is not TRUE, the UE drops the periodic CSI.

How does UCI share a PUSCH with data ?

When UCI goes on PUSCH, it shares the resource with UL-SCH data. The UE does not simply append the UCI bits to the transport block. Each UCI type needs its own reliability, and HARQ-ACK needs the best of all. So 36.212 codes each type separately and places it in a fixed part of the subframe.

Let's start with the coding. 36.212 clause 5.2.2.6 codes HARQ-ACK, RI and CQI/PMI independently of each other and of the data. The number of coded symbols for each type comes from a formula with a beta offset. The network sets the offsets in 36.331 with betaOffset-ACK-Index, betaOffset-RI-Index and betaOffset-CQI-Index. A larger offset gives the UCI more resource elements and a lower code rate, and it leaves fewer resource elements for the data.

Next comes the placement, in the channel interleaver of 36.212 clause 5.2.2.8. Think of the PUSCH as a matrix with one column per SC-FDMA data symbol. With normal CP there are 12 columns, 0 to 11, because the DMRS takes the middle symbol of each slot. CQI/PMI is written first, row by row across all columns, and the data follows it. RI goes in columns 1, 4, 7 and 10. HARQ-ACK goes in columns 2, 3, 8 and 9, which sit right next to the DMRS. Both RI and HARQ-ACK are written from the last row upwards.

The order of writing matters. RI is written before the data, and the data skips the RI positions. HARQ-ACK is written after the data, so it overwrites the data in its positions. The data rate matching therefore does not depend on HARQ-ACK. This matters when the UE has missed a DL assignment, because the UE and the eNB then disagree on whether HARQ-ACK is present.

Two more rules apply when the PUSCH carries two transport blocks. CQI/PMI goes only on the transport block with the higher IMCS in the initial grant, and on the first one if the two IMCS values are equal. HARQ-ACK and RI go on both transport blocks.

• Each UCI type is coded separately : the beta offsets set how many resource elements HARQ-ACK, RI and CQI/PMI get.
• HARQ-ACK sits next to the DMRS : it uses columns 2, 3, 8 and 9 with normal CP.
• HARQ-ACK overwrites data, RI does not : the data is rate matched around RI and CQI/PMI only.
• CQI/PMI uses one transport block : the one with the higher IMCS, while HARQ-ACK and RI use both.

Reference

• 3GPP TS 36.211 v19.3.0 - clause 5.4, Table 5.4-1 supported PUCCH formats
• 3GPP TS 36.212 v19.3.0 - clause 5.2.2.6 channel coding of control information, 5.2.2.7 data and control multiplexing, 5.2.2.8 channel interleaver
• 3GPP TS 36.213 v19.4.0 - clause 10.1 and 10.1.1 PUCCH format information
• 3GPP TS 36.321 v19.3.0 - clause 5.4.4 Scheduling Request
• 3GPP TS 36.331 v19.3.0 - simultaneousPUCCH-PUSCH-r10, simultaneousAckNackAndCQI, betaOffset-ACK-Index, betaOffset-RI-Index, betaOffset-CQI-Index