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
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (336 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the UCI (Uplink Control Information) function was enhanced with new procedures for multiplexing multiple UCI types on PUSCH and the determination of Type-1 HARQ-ACK codebooks. Specific corrections were also introduced to align the dynamic HARQ codebook procedures and the timeline conditions for multiplexing two HARQ-ACK information bits within a single slot.
- Running 36.306 CR to introduce assistance information for local cache TS 36.306CR1535
- Control Plane latency reduction TS 36.306CR1614
- Introduction of support for MAC PDU containing UE contention resolution identity MAC control element without RRC response message in NB-IoT TS 36.306CR1570
- Introduction of Geofencing information in CMAS TS 36.306CR1637
- Correction on the interpretation of HARQ-ACK bitmap for FeLAA in 36.212 TS 36.212CR0297
- CR to TS 38.104 on Combined updates from RAN4 #90 This document combines the proposed changes in the following Draft CRs from RAN4 #90: - R4-1900284, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1900763, "Draft CR to TS 38.104: Update of performance requirement numbers for DFT-s-OFDM based PUSCH" - R4-1900876, "Draft CR to TS 38.104: On RX spurious emissions requirement" - R4-1900968, "Draft CR for 38.104: Performance requirements for NR PUCCH format 1" - R4-1901329, "Draft CR to 38.104: Annex C.6 correction" - R4-1901330, "Draft CR to 38.104: Abbreviations addition" - R4-1901387, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1901474, "Draft CR to TS 38.104: Corrections on transmitter co-existence and co-location requirements" - R4-1901483, "Draft CR to TS 38.104: Corrections on general intermodulation requirement" - R4-1902239, "Draft CR to TS 38.104: Addition of missing EIRP/EIS definitions in terminology in clause 3.1" - R4-1902241, "Draft CR to 38.104; clarification of BS power limits" - R4-1902245, "Draft CR to 38.104: Correction to FR2 OTA Interfering signal mean power units" - R4-1902246, "Draft CR to 38.104; Correction to definition of OTA reference sensitivity" - R4-1902260, "draft CR to TS 38.104 - update emissions scaling" - R4-1902338, "Draft CR: Update on FR1 range extension for TS38.104" - R4-1902389, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1902394, "Draft CR to TS 38.104 – PUSCH requirements with CP-OFDM for FR2" - R4-1902396, "CR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1902444, "Draft CR to TS 38.104: Editorial CR for BS demodulation requirements" - R4-1902561, "Draft CR for updating PRACH performance requirements in TS38.104" - R4-1902571, "Corrections to 38.104 Delay profile calculation" - R4-1902642, "Draft CR to TS 38.104: Correction on multi-band operation related requirements" TS 38.104CR0019
+ 48 more changes
In Release 16, UCI enhancements included corrections and updates to the HARQ-ACK codebook RRC parameter, UE capability for segmentation, and specific performance requirements for PUCCH formats 3 and 4. Furthermore, the release introduced corrections for UCI multiplexing procedures, including for CG-UCI on PUSCH and PUCCH repetition transmission, and clarified the UCI bit sequence generation. These refinements also extended to dual connectivity and handover scenarios, with changes to power control and prioritization between uplink and sidelink transmissions.
- Mapping of Uplink Traffic to Backhaul RLC Channels TS 38.300CR0255
- CR on UE capability of segmentation for UE capability information TS 36.306CR1783
- CR to TS 38.104 BS demodulation PUCCH format 0 requirements TS 38.104CR0100
- CR: Updates for PUCCH formats 3 and 4 performance requirements in TS 38.104 (Rel-16) TS 38.104CR0116
- UCI multiplexed on PUSCH requirement TS 38.104CR0172
- Correction on UCI bit sequence generation TS 38.212CR0047
+ 101 more changes
In Release 17, key enhancements for Uplink Control Information (UCI) included the clarification of PUCCH resource determination procedures and corrections to HARQ-ACK codebook generation for scenarios involving PUCCH cell switching, uplink BWP switching, and when both spatial and time bundling are configured. The release also introduced specific handling for the priority of CG-UCI and enabled the configuration of multiple HARQ-ACK codebooks for multicast services. Furthermore, corrections were made to ensure proper UCI assumptions when multiplexing Non-Coherent Joint Transmission (NCJT) CSI reports in PUCCH.
- On introducing height information reporting in MDT reports [LTE-Height-MDT] TS 36.306CR1838
- Start drx-HARQ-RTT-TimerUL after last repetition [ulHARQ_RTT_Timer] TS 38.321CR1407
- Introduction of uplink RRC Segmentation capability TS 36.306CR1853
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- Correction to support up to 32 HARQ process numbers for FR2-2 TS 38.212CR0126
- CR on priority of CG-UCI TS 38.212CR0128
+ 87 more changes
In Release 18, a key enhancement for Uplink Control Information (UCI) was the introduction of multiplexing HARQ-ACK on a PUSCH with repetitions, specifically for acknowledgments associated with downlink assignments received after the uplink grant for that PUSCH. This optimizes uplink resource usage by allowing late-arriving HARQ-ACK to be carried on an already scheduled PUSCH with repetitions. Additionally, corrections were made to the RRC parameter names for the Uplink Time Offset (UTO) UCI indication mechanism.
- Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
- Introduction of Rel-18 network controlled repeaters TS 38.212CR0150
- Introduction of MIMO Evolution for Downlink and Uplink TS 38.213CR0504
- Introduction of Network Controlled Repeaters TS 38.213CR0506
- Introduction of multiplexing in a PUSCH with repetitions HARQ-ACK associated with DL assignments received after an UL grant for the PUSCH [HARQ-ACK MUX on PUSCH] TS 38.213CR0568
- Introducing support for Network-Controlled Repeaters to 38.300 TS 38.300CR0685
+ 66 more changes
In Release 19, a key enhancement for Uplink Control Information (UCI) was the introduction of support for 32 HARQ process numbers, increasing the capacity for Hybrid Automatic Repeat Request acknowledgments. This required updates to the encoding and rate matching procedures for HARQ-ACK bits within the UCI framework. The release also included corrections and alignments for related control procedures, such as those for UL rate control MAC Control Elements.
- Introduction of 32 HARQ process numbers in Rel-19 [TN32HARQ] TS 38.212CR0222
- Introduction of PDCCH repetitions for Type0-PDCCH CSS set in TNs [Common_PDCCH_Rep_TN] TS 38.213CR0748
- Introduction of control parameters for on-demand posSIB request [OdPosSIB_Req] TS 38.300CR1009
- Support Aerial UE Flight Information Reporting TS 38.300CR1031
- Introduction of common PDCCH repetition (Rel-19 NTN) for TN [Common_PDCCH_rep_TN] TS 38.300CR1058
- Correction on PDCCH candidates skipping for receiving RAR when overlapping with candidate SSB TS 38.213CR0755
+ 4 more changes
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 |