Description
Configured Grant Uplink Control Information (CG-UCI) is a critical mechanism within the 5G New Radio (NR) physical layer uplink control channel framework, specified in 3GPP TS 38.212. It refers to the Uplink Control Information (UCI) that a User Equipment (UE) transmits using pre-configured, semi-persistent uplink resources known as Configured Grants (CG), specifically Type 1 or Type 2 Configured Grants. Unlike dynamically scheduled UCI on the Physical Uplink Control Channel (PUCCH) or multiplexed on the Physical Uplink Shared Channel (PUSCH), CG-UCI is transmitted on a Configured Grant Physical Uplink Shared Channel (CG-PUSCH) resource. This means the UE does not require a dynamic Downlink Control Information (DCI) grant from the gNodeB for each transmission instance, enabling grant-free uplink access.
The operation of CG-UCI is intrinsically linked to the Configured Grant (CG) framework introduced for uplink data. When a CG resource is activated for a UE via Radio Resource Control (RRC) signaling (Type 1) or via a combination of RRC and an activating DCI (Type 2), the UE is provided with a periodic resource allocation on the PUSCH. The UE can use these resources to transmit uplink data without a scheduling request (SR) or grant, reducing latency. CG-UCI leverages these same pre-allocated resources to piggyback essential control information. The UCI payload for CG-UCI typically includes Hybrid Automatic Repeat Request Acknowledgement (HARQ-ACK) feedback for downlink transmissions, Channel State Information (CSI) reports, and Scheduling Request (SR) indications. This control information is multiplexed with the uplink data on the CG-PUSCH according to the physical layer multiplexing rules defined in TS 38.212.
Architecturally, CG-UCI resides within the UE's physical layer processing chain. The Medium Access Control (MAC) layer provides the HARQ-ACK and SR information, while the physical layer measurements generate CSI. This information is encoded, rate-matched, and multiplexed with the data transport blocks before being mapped to the allocated CG-PUSCH resource's time-frequency grid. The gNodeB, aware of the CG configuration, monitors these periodic resources to decode both the data and the embedded CG-UCI. Key components enabling CG-UCI include the CG configuration parameters (periodicity, time/frequency resources, modulation and coding scheme), the UCI encoding procedures, and the rules for resource sharing between data and control bits on the PUSCH.
CG-UCI's role in the network is pivotal for supporting Ultra-Reliable Low-Latency Communications (URLLC) and efficient uplink operation. By eliminating the need for a scheduling request and grant cycle for control feedback, it drastically reduces the control plane latency for uplink acknowledgements and reports. This is essential for applications like industrial automation, where timely HARQ-ACK for critical downlink commands is necessary. Furthermore, it improves spectral efficiency and UE power efficiency by allowing control and data to be transmitted together in a single, pre-arranged transmission burst, avoiding separate control channel transmissions.
Purpose & Motivation
CG-UCI was created to address the stringent latency and reliability requirements of 5G, particularly for URLLC use cases. Prior to 5G NR Release 16, uplink control information was primarily transmitted on dedicated PUCCH resources or multiplexed on dynamically scheduled PUSCH. Both approaches incur latency: PUCCH requires periodic dedicated resources which might not align with the need for immediate feedback, while dynamic scheduling on PUSCH requires a full scheduling request (SR) and grant procedure, adding several milliseconds of delay. For time-critical applications like closed-loop control in factory automation or tele-surgery, this latency was prohibitive.
The motivation for CG-UCI stems from the parallel development of the Configured Grant (grant-free) uplink data transmission. While CG allowed data to be sent without scheduling latency, there was a missing link: how to provide the necessary accompanying control feedback (like HARQ-ACK for downlink assignments) with equally low latency. Transmitting this feedback on a separate, dynamically scheduled resource would negate the latency benefits of grant-free data. CG-UCI solves this by enabling the UE to send this vital control information within the same grant-free transmission opportunity as its data, using the pre-configured resources. This creates a cohesive low-latency uplink path for both data and control.
Historically, LTE offered Semi-Persistent Scheduling (SPS) for uplink, but its control feedback mechanisms were not as tightly integrated or optimized for the microsecond-scale latencies targeted by 5G URLLC. CG-UCI, as part of the enhanced Configured Grant framework, represents a fundamental architectural shift to support deterministic, low-latency uplink communication, moving beyond the reactive, grant-based paradigm of previous generations.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (10 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the CG-UCI function was newly introduced, enabling uplink control information for configured grant transmissions to be multiplexed with data on a PUSCH. This is governed by the higher layer parameter `cg-UCI-Multiplexing`, which allows for the joint coding of HARQ-ACK and CG-UCI bits. The specification details the procedures for multiplexing these coded CG-UCI bits (`g_{0}^{CG-UCI}...`) with UL-SCH data and other UCI on the physical uplink shared channel.
In Release 16, the specification introduced explicit procedures for multiplexing CG-UCI with HARQ-ACK on a PUSCH when the higher layer parameter `cg-UCI-Multiplexing` is configured. This allowed the jointly coded bits for HARQ-ACK and CG-UCI to be mapped into the uplink data channel alongside other UCI types like CSI. Furthermore, the release included corrections to these CG-UCI multiplexing rules to ensure clear implementation.
- Corrections on CG-UCI multiplexing in TS38.212 TS 38.212CR0078
In Release 17, the specification introduced the handling of priority for CG-UCI, as indicated by the related Change Request. This is reflected in the procedures for multiplexing data and control information, where the network can configure the higher layer parameter `cg-UCI-Multiplexing` to allow CG-UCI bits to be jointly coded with HARQ-ACK on a PUSCH.
- CR on priority of CG-UCI TS 38.212CR0128
In Release 18, the enhancements for CG-UCI primarily involved its multiplexing procedures on the PUSCH. The specification now explicitly defines the handling when CG-UCI overlaps with other uplink control information types like UTO-UCI or UEIRI, stating the UE expects at most one of these to overlap with a PUSCH. Furthermore, the procedures detail the separate encoding and multiplexing of CG-UCI bits, both with and without HARQ-ACK, when the higher layer parameter `cg-UCI-Multiplexing` is configured.
- Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
- Introduction of Rel-18 network controlled repeaters TS 38.212CR0150
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0167
- Corrections on Rel-18 network controlled repeaters in 38.212 TS 38.212CR0174
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0185
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0200
Explore further
Broader topics and technologies where CG-UCI plays a role.
Defining Specifications
3GPP specifications that define or reference CG-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 |