Description
Unused Transmission Occasion for Uplink Control Information (UTO-UCI) is a feature introduced in 3GPP Release 18 for 5G New Radio (NR). It addresses scenarios where a UE is scheduled for a Physical Uplink Shared Channel (PUSCH) transmission but, due to various reasons like lack of uplink data, the allocated resource would go unused. Instead of wasting this resource, the UTO-UCI mechanism allows the UE to utilize this 'unused transmission occasion' to send Uplink Control Information (UCI). UCI typically includes critical feedback such as Hybrid Automatic Repeat Request (HARQ) acknowledgments (ACK/NACK), Channel State Information (CSI) reports, and Scheduling Requests (SR).
The operation is governed by specific rules and configurations defined in the physical layer specifications (38.212, 38.213) and the MAC layer specification (38.321). The network can configure a UE with UTO-UCI parameters, potentially indicating which types of UCI are allowed to be transmitted in an unused PUSCH occasion. When a UE determines that a scheduled PUSCH occasion will not be used for its primary purpose of data transmission, it can, subject to timing and multiplexing rules, encode and map the pending UCI onto the physical resources of that PUSCH. This involves the standard UCI processing chain—including channel coding, scrambling, and modulation—but mapped onto the PUSCH resource grid instead of the Physical Uplink Control Channel (PUCCH).
Architecturally, UTO-UCI is a cross-layer optimization involving both the MAC and Physical layers. The MAC layer (specified in 38.321) handles the logical decision-making regarding resource availability and UCI prioritization, while the Physical layer (specified in 38.212 and 38.213) manages the precise encoding, multiplexing, and transmission procedures. This feature enhances the flexibility of the uplink frame structure. Its role is to improve spectral efficiency and reduce control signaling latency, which is crucial for advanced NR use cases requiring high reliability and low latency, such as ultra-reliable low-latency communication (URLLC). By making opportunistic use of already-allocated resources, it minimizes the need for separate, dedicated PUCCH resources for control signaling, leading to more efficient overall uplink resource management.
Purpose & Motivation
The primary purpose of UTO-UCI is to improve uplink resource utilization and reduce latency for control signaling in 5G NR networks. Prior to its introduction, if a UE had no uplink data to send in a scheduled PUSCH resource, that resource would be left empty, representing wasted spectral efficiency. Simultaneously, if the UE had pending UCI to report (e.g., a HARQ-ACK for a downlink transmission), it would typically need to wait for the next scheduled PUCCH resource or request a scheduling grant for a new PUSCH, introducing delay.
UTO-UCI solves this inefficiency by allowing the UE to 'piggyback' its UCI onto the otherwise-unused PUSCH resource. This is motivated by the increasing demand for low-latency and high-efficiency communications in 5G-Advanced (Release 18 and beyond). Features like enhanced mobile broadband (eMBB) with very dynamic traffic and URLLC with stringent delay requirements benefit from minimizing the time between an event (e.g., receiving a downlink packet) and the corresponding feedback (e.g., transmitting an ACK). By utilizing pre-allocated resources that are already in the UE's transmission timeline, UTO-UCI provides a mechanism for faster control feedback without additional scheduling overhead or resource contention.
Historically, similar concepts like piggybacking control information on data channels existed, but UTO-UCI formalizes and optimizes this specifically for the dynamic scheduling and flexible numerology of NR. It addresses the limitation of rigid separation between data and control channels, enabling a more fluid and efficient use of the radio interface. This is part of a broader 3GPP effort to introduce 'opportunistic' transmissions and enhance uplink efficiency for advanced network deployments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (301 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the UTO-UCI (Unused Transmission Occasion - Uplink Control Information) function was newly introduced as a specific type of control information that can be multiplexed with data on a PUSCH. The specification defines procedures for multiplexing UTO-UCI with HARQ-ACK on a PUSCH, analogous to the handling of CG-UCI, and states that a UE expects at most one of CG-UCI, UTO-UCI, or UEIRI to overlap with a PUSCH transmission.
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
- Correction to last PUCCH resource set configuration TS 38.213CR0019
- Correction on physical downlink control channel TS 38.213CR0020
- Correction to align RAN1 and RAN4 specifications for EN-DC power control TS 38.213CR0021
- Clarifying DL reception and UL transmission related restrictions TS 38.213CR0025
- CR on identifying transmission occasion after resetting a PC closed loop TS 38.213CR0027
+ 29 more changes
In Release 16, the UTO-UCI function was introduced as a new type of control information that can be multiplexed on a PUSCH, analogous to the existing CG-UCI. The specification defines a dedicated procedure for multiplexing UTO-UCI with data and other UCI types like HARQ-ACK on a PUSCH when the higher layer parameter `nrofBitsInUTO-UCI` is configured. This addition provides a mechanism for transmitting uplink control information specifically for unused transmission occasions.
- Introduction of UL transmission switching TS 38.213CR0114
- Correction on Transmission configuration indication in DCI format 1_2 TS 38.212CR0057
- Correction on HARQ-ACK codebook RRC parameter TS 38.212CR0069
- Correction for PUCCH repetition transmission TS 38.213CR0120
- CR on Power Control for NR-DC TS 38.213CR0128
- CR on correction on uplink power sharing for DAPS HO TS 38.213CR0131
+ 89 more changes
In Release 17, the UTO-UCI function was introduced as a new type of uplink control information for multiplexing on a PUSCH, analogous to the existing CG-UCI. The specification defines that when the higher layer parameter `nrofBitsInUTO-UCI` is configured, the existing multiplexing procedure for CG-UCI is applied by replacing CG-UCI with UTO-UCI in all notations. Furthermore, it is specified that a UE expects at most one of CG-UCI, UTO-UCI, or UEIRI to overlap with a PUSCH transmission.
- Introduction of small data transmission in RRC_INACTIVE state for NR TS 38.213CR0278
- Introduction of Small Data Transmission for MAC spec TS 38.321CR1198
- Start drx-HARQ-RTT-TimerUL after last repetition [ulHARQ_RTT_Timer] TS 38.321CR1407
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- CR on the description of the SRS resource set indication for PUSCH repetition TS 38.212CR0117
- Correction to support up to 32 HARQ process numbers for FR2-2 TS 38.212CR0126
+ 81 more changes
In Release 18, the UTO-UCI function was refined with corrections to its RRC parameter names and applicable configuration, as indicated in the Change Requests. The specification text clarifies that when the higher layer parameter `nrofBitsInUTO-UCI` is configured, the existing procedures for CG-UCI multiplexing are applied by replacing CG-UCI with UTO-UCI, specifically for when UTO-UCI and HARQ-ACK are transmitted together on a PUSCH.
- 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 RS for pathloss determination of Type 1 CG PUSCH [PL RS Type 1 CG] TS 38.213CR0567
- 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
+ 72 more changes
In Release 19, the UTO-UCI function was newly specified, defining procedures for multiplexing Unused Transmission Occasion Uplink Control Information with HARQ-ACK on a PUSCH. This was introduced analogously to the existing CG-UCI multiplexing procedure, where the higher layer parameter `nrofBitsInUTO-UCI` enables the function. The specification ensures that at most one of CG-UCI, UTO-UCI, or UEIRI can overlap with a PUSCH transmission.
- Introduction of 32 HARQ process numbers in Rel-19 [TN32HARQ] TS 38.212CR0222
- CR on UEIRI multiplexed into PUSCH in TS 38.212 TS 38.212CR0244
- Correction on WUS monitoring occasions for LP-WUS/WUR TS 38.213CR0750
- CR on UEIRI multiplexed into PUSCH in TS 38.213 TS 38.213CR0762
- Correction to UL Rate Control MAC CE TS 38.321CR2156
- Correction on UE transmissions during Cell DRX TS 38.321CR2129
Explore further
Broader topics and technologies where UTO-UCI plays a role.
Defining Specifications
3GPP specifications that define or reference UTO-UCI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |