Description
Uplink Control Information (UCI) is a fundamental component of the physical layer control signaling in 3GPP LTE and NR. It comprises the set of control data that a User Equipment (UE) transmits to the base station (eNodeB/gNB) to support the operation of the uplink and downlink data channels. UCI is primarily carried on the Physical Uplink Control Channel (PUCCH) and, in certain cases, can be multiplexed with uplink data on the Physical Uplink Shared Channel (PUSCH). The content, encoding, and resource mapping of UCI are meticulously defined in the physical layer specifications (e.g., 3GPP TS 36.212/38.212 for channel coding and TS 36.213/38.213 for physical layer procedures).
Architecturally, UCI consists of several distinct information types, each serving a specific purpose in the radio link control loop. The key components are: Hybrid Automatic Repeat Request (HARQ) Acknowledgment (ACK/NACK), which informs the gNB whether a downlink transport block was received correctly; Channel State Information (CSI), which includes Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), and Rank Indicator (RI) to report downlink channel conditions; and Scheduling Request (SR), which is a single-bit indicator used by the UE to request uplink resources for data transmission. The physical layer processes these bits through specific channel coding schemes (e.g., Reed-Muller codes, polar codes in NR) and modulates them for transmission.
How UCI works is tightly integrated with the scheduling timeline and frame structure. The gNB schedules downlink data and allocates specific resources for the UE to send the corresponding HARQ-ACK feedback after a fixed time offset. For periodic CSI reporting, the gNB configures the UE with a reporting configuration, dictating when and on which resources the UE should send CSI. The SR resources are semi-statically configured. The network must correctly receive and decode UCI to adapt its transmissions (via link adaptation based on CSI), initiate retransmissions (based on HARQ-ACK), and grant uplink resources (based on SR). Its role is therefore central to maintaining a robust, adaptive, and efficient radio link, directly impacting throughput and latency.
Purpose & Motivation
UCI was created to provide a reliable and efficient mechanism for the UE to send essential control feedback to the network. In the packet-switched, scheduled architecture of LTE and NR, the base station requires timely information from the UE to make optimal scheduling decisions. Previous systems had less dynamic control signaling. UCI addresses the need for low-latency, frequent feedback to enable advanced features like fast link adaptation, HARQ with soft combining, and dynamic scheduling.
The problems it solves are multifaceted. Without HARQ-ACK, the network would not know if a transmission failed, leading to higher-layer retransmissions with much greater latency. Without CSI, the network cannot adapt the modulation and coding scheme (MCS) to current radio conditions, resulting in either wasted capacity (if too conservative) or high error rates (if too aggressive). Without SR, the UE would have to rely on random access or pre-allocated periodic resources to request uplink grants, increasing latency and reducing efficiency for bursty traffic.
Its introduction and evolution were motivated by the increasing demands for higher data rates, lower latency, and more reliable connections. As technologies evolved from LTE to NR, UCI mechanisms were enhanced to support more antenna ports (for MIMO), wider bandwidths, ultra-reliable low-latency communication (URLLC) with very short feedback timelines, and operation in unlicensed spectrum. The design of UCI is a key factor in achieving the spectral efficiency and responsiveness that define 4G and 5G systems.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (20 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, a key new introduction for Uplink Control Information (UCI) was the definition of performance requirements for new PUCCH formats, specifically Formats 0, 1, 2, 3, and 4, as detailed in the TS 38.104 Change Requests. Furthermore, the release formalized the procedures and performance requirements for transmitting UCI on a PUSCH without UL-SCH data, including specific handling for AUL-UCI on Autonomous Uplink PUSCH.
- CR to TS 38.104 Combined updates from RAN4 #90bis and RAN4#91 From RAN4 #90bis: - R4-1903105, "Draft CR to TS 38.104: Corrections on terminologies and editorial errors" - R4-1903319, "Draft CR to TS 38.104: removal of unused definition: "minimum EIRP level under extreme condition"" - R4-1903320, "Draft CR to TS 38.104: OSDD information correction" - R4-1903457, "Draft CR to TS 38.104: Removal of FFS for FR2 TDD OFF power level requirement in clause 9.5" - R4-1903499, "Draft CR to 38.104: Correction to unwanted emissions mask for bands n7 and n38" - R4-1903836, "Draft CR to TS 38.104: Correction on description on multi-band operation in section 4.8" - R4-1904024, "Draft CR to TS 38.104 Applicability rules for BS demodulation" - R4-1904234, "draftCR: Correlation matrix for 8Rx in TS 38.104" - R4-1904723, "Draft CR to TS 38.104: Update of performance requirements for DFT-s-OFDM based PUSCH" - R4-1904726, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1904729, "Draft CR on PRACH performance requirements in TS38.104" - R4-1904734, "Draft CR on TS 38.104 Performance requirement for PUCCH format 1" - R4-1904735, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1904739, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1904745, "draftCR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1904799, "Draft CR to TS 38.104: FRC update for PUSCH FR1 mapping type B and FR2 DMRS 1+1" - R4-1904816, "Draft CR : Clarification on step 5 and step 6 for delay profiles calculation (38.104)" - R4-1904842, "Draft CR to TS 38.104 BS demodulation CP-OFDM PUSCH FR2 requirements" - R4-1905126, "draft CR to 38.104 for TAE requirements" - R4-1905139, "draft CR to TS 38.104 on EVM measurement (Annex B and C)" - R4-1905140, "Draft CR: editorial correction on FR1 spurious emission requirement in TS38.104" - R4-1905143, "Draft CR for TS 38.104: Addition of NOTE for transmitter intermodulation requirements in certain regions" - R4-1905144, "Draft CR to TS 38.104: FRC reference corrections for the Rx requirements" - R4-1905145, "Draft CR to TS 38.104: Clarification on application of interfering signal offsets for ACS, blocking and intermodulation requirements" - R4-1905148, "Draft CR to TS 38.104: Corrections on out-of-band blocking requirement" From RAN4 #91: - R4-1906002, "Draft CR to 38.104: Subclause 6.7 and 9.8 transmitter intermodulation – correction of interfering signal type" - R4-1906096, "Draft CR to 38.104: Correction of frequency range for OTA spurious emissions" - R4-1906311, "Draft CR to 38.104: Correction on FRC (Annex A)" - R4-1906346, "Removal of n65 in Rel-15 38.104" - R4-1906915, "Draft CR to TS 38.104: Clarification on application of interfering signal offsets for OTA ACS, blocking and intermodulation requirements" - R4-1906918, "Draft CR to TS 38.104: Clarification on type of interfering signal for ACS, in-band blocking and ICS requirements" - R4-1907110, "Draft CR to TS 38.104: correction of the fundamental frequency limit of 2.55GHz for the spurious emissions" - R4-1907246, "Draft CR to TS 38.104: Update of performance requirements for DFT-s-OFDM based PUSCH" - R4-1907249, "Draft CR to TS 38.104: Correction on the terminology in PUSCH FRC tables" - R4-1907252, "Draft CR to TS38.104: Updates of PRACH performance requirements" - R4-1907255, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1907258, "Draft CR on NR UCI on PUSCH performance requirements for TS 38.104" - R4-1907261, "draftCR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1907266, "Draft CR on TS 38.104 Performance requirement for PUCCH format 1" - R4-1907267, "Draft CR on TS 38.104 Performance requirement for multi-slot PUCCH format 1" - R4-1907272, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1907275, "Draft CR to TS 38.104 BS demodulation CP-OFDM PUSCH FR2 requirements" - R4-1907277, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1907629, "Draft CR to 38.104: Term “reference signal” replacing by term “ideal signal” in EVM context" - R4-1907634, "Draft CR to 38.104: corrections to the EVM annex" - R4-1907659, "Draft CR to TS 38.104 on Spurious emission Category B in FR2" - R4-1907661, "Draft CR to 38.104 Definition of contiguous transmission bandwidth" - R4-1907662, "Draft CR to 38.104: BS TAE requirements" - R4-1907664, "Draft CR to 38.104: Clarification of interferer RB frequency for narrowband blocking" - R4-1907672, "Draft CR for TS 38.104: Correction on EVM" - R4-1907689, "Correction to CA carrier spacing" TS 38.104CR0029
- Correction to UCI multiplexing TS 38.212CR0008
- CR on UE procedure for reporting multiple UCI types TS 38.213CR0033
- CR on UE procedure for reporting multiple UCI types TS 38.213CR0052
- CR on beta_offset values for UCI reporting in PUSCH TS 38.213CR0062
In Release 16, key enhancements for Uplink Control Information (UCI) included specific corrections and clarifications to its multiplexing procedures, particularly for scenarios involving PUSCH. These involved corrections for UCI multiplexing on PUSCH, including handling with PUCCH overriding, for partial PUSCH mode 1, and for configurations without LCH-based prioritization. Additionally, the release introduced performance requirements for UCI on PUSCH and provided clarifications on UCI bitwidth and mapping order for non-PMI based CSI feedback.
- UCI multiplexed on PUSCH requirement TS 38.104CR0172
- Correction on UCI bit sequence generation TS 38.212CR0047
- Corrections on CG-UCI multiplexing in TS38.212 TS 38.212CR0078
- Correction on UCI multiplexing with PUCCH overriding TS 38.213CR0183
- Clarification on UCI and SL HARQ-ACK TS 38.213CR0262
- Correction to PUSCH skipping with UCI without LCH-based prioritization TS 38.321CR1062
+ 4 more changes
In Release 17, key enhancements for Uplink Control Information (UCI) included the introduction of specific handling for Configured Grant UCI (CG-UCI), such as defining its transmission priority. Furthermore, the release specified mechanisms for transmitting Channel State Information (CSI) on low-priority PUSCH when it is concurrently carrying CG-UCI.
In Release 18, specific clarifications and corrections were introduced for Uplink Control Information (UCI). These included a clarification prohibiting the multiplexing of UCI on the MSG3 PUSCH, ensuring clear separation for initial access. Furthermore, corrections were made to RRC parameter names and their applicable configurations for the Uplink Tx Opportunity UCI (UTO-UCI) indication feature.
Explore further
Broader topics and technologies where UCI plays a role.
Defining Specifications
3GPP specifications that define or reference UCI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.306 vj00 | E-UTRA UE Radio Access Capability Parameters | Rel-19 |
| TS 38.104 vj20 | NR Base Station RF Requirements | Rel-19 |
| TS 38.141 vj20 | NR Base Station RF Conformance Testing Part 1 | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TR 38.802 ve20 | Study on New Radio Access Technology Physical Layer Aspects | Rel-14 |
| TS 38.824 vg00 | NR URLLC Physical Layer Enhancements Study | Rel-16 |
| TR 38.830 vh00 | NR Coverage Enhancements Study | Rel-17 |
| TR 38.912 vj00 | Study on New Radio Access Technology | Rel-19 |
