Description
SFCI is a specific type of control information transmitted on the NR Sidelink interface, primarily used to convey feedback for transmissions between UEs without traversing the base station (gNB). It is carried on the Physical Sidelink Feedback Channel (PSFCH). The content of SFCI typically includes HARQ-ACK (Acknowledgement/Negative Acknowledgement) information for previously received Sidelink Shared Channel (SL-SCH) transport blocks, and can also include Channel State Information (CSI) reports to support link adaptation.
The generation and transmission of SFCI are governed by resource allocation modes and timing relationships defined in 3GPP specifications. A receiving UE, after decoding a Sidelink data transmission (PSSCH), will generate HARQ-ACK bits. These bits are then encoded, modulated, and mapped to specific resource elements within a PSFCH resource. The PSFCH resources are periodic and associated with specific PSSCH resource pools, allowing the transmitting UE to listen for feedback at a predetermined time and frequency location.
The structure of SFCI supports both broadcast/groupcast and unicast Sidelink communication modes. For groupcast, options like HARQ-ACK based on NACK-only feedback or all-ACK/NACK are defined. The inclusion of CSI in SFCI enables advanced features like Sidelink channel quality indication (CQI), rank indication (RI), and precoding matrix indicator (PMI), which are crucial for efficient MIMO operation and beam management in high-frequency NR Sidelink. This direct feedback loop is essential for achieving reliable, low-latency communication in V2X, public safety, and commercial D2D applications.
Purpose & Motivation
SFCI was introduced in NR Sidelink (Rel-16) to provide a reliable and low-latency feedback mechanism for direct device-to-device communications, which is a fundamental requirement for advanced V2X and proximity services. Unlike LTE Sidelink which had limited feedback capabilities, NR Sidelink demanded a more robust HARQ mechanism to support high-reliability use cases such as autonomous driving and industrial IoT, where timely acknowledgment of safety messages is critical.
The primary problem SFCI solves is enabling closed-loop link adaptation and retransmission for Sidelink, thereby improving transmission reliability and spectral efficiency. In the absence of such feedback, UEs would have to rely on open-loop transmissions with conservative modulation and coding schemes, wasting radio resources. SFCI allows the transmitting UE to adapt its MCS based on CSI and to retransmit packets only if a NACK is received, mimicking the efficiency of uplink/downlink HARQ in cellular networks.
Furthermore, SFCI supports the evolution towards more autonomous Sidelink operation, especially in partial and out-of-coverage scenarios. By establishing a direct control channel for feedback, it reduces dependency on network infrastructure for coordination, enabling reliable D2D communication even when a gNB is not available. This capability is vital for mission-critical communications and extends the operational range and resilience of 5G networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (174 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SFCI (Sidelink Feedback Control Information) function was newly introduced as a defined control information type for the NR sidelink. The specification established that SFCI is mapped for transmission on the PSFCH (Physical Sidelink Feedback Channel), as detailed in the new mapping table for sidelink control information. This provided the foundational framework for sidelink HARQ feedback procedures in New Radio.
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
- Correction on physical downlink control channel TS 38.213CR0020
- Correction to align RAN1 and RAN4 specifications for EN-DC power control TS 38.213CR0021
- Correction on the timeline condition of multiplexing two HARQ-ACK information in one slot TS 38.213CR0043
- CR on Type-1 HARQ-ACK codebook determination TS 38.213CR0044
- CR to 38.213 fix to HARQ-ACK Type-1 codebook pseudo-code TS 38.213CR0053
+ 6 more changes
In Release 16, the SFCI (Sidelink Feedback Control Information) function was formally introduced as a new control information type for the 5G V2X sidelink, with its mapping to the physical channel PSFCH explicitly defined. This addition was part of the broader introduction of 5G V2X sidelink features, which included establishing the complete sidelink physical layer framework encompassing transport channels, control information, and their corresponding physical channels. The release also involved subsequent corrections to related sidelink procedures, such as HARQ-ACK codebook configuration and prioritization between uplink and sidelink transmissions.
- Introduction of 5G V2X sidelink features into TS 38.212 TS 38.212CR0025
- Corrections on 5G V2X sidelink features after RAN1#100-e TS 38.212CR0036
- Corrections on 5G V2X sidelink features after RAN1#100bis-e and RAN1#101-e TS 38.212CR0040
- Corrections on 5G V2X sidelink features TS 38.212CR0052
- Correction on Sidelink Broadcast channel TS 38.212CR0062
- Correction on HARQ-ACK codebook RRC parameter TS 38.212CR0069
+ 54 more changes
In Release 17, the enhancements for Sidelink Feedback Control Information (SFCI) primarily involved corrections and refinements to the NR sidelink enhancement framework, as indicated by multiple CRs focused on corrections in the physical layer specifications. These corrections addressed the mapping and procedures for SFCI on the PSFCH, ensuring proper integration within the broader sidelink control channel structure defined in the specifications.
- Introduction of NR sidelink enhancement TS 38.212CR0094
- Introduction of sidelink enhancements in NR TS 38.213CR0279
- Corrections on NR sidelink enhancement in 38.212 TS 38.212CR0100
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- Correction on NR sidelink enhancement TS 38.212CR0122
- Correction to support up to 32 HARQ process numbers for FR2-2 TS 38.212CR0126
+ 57 more changes
In Release 18, the SFCI (Sidelink Feedback Control Information) function was enhanced as part of the NR sidelink evolution work item, with specific corrections and maintenance to its procedures. These included corrections on PSFCH prioritization for certain UEs, on determining PSFCH resources for a PSSCH, and on PSFCH power control. The updates aimed to refine the reliability and resource allocation for sidelink HARQ feedback transmitted on the PSFCH.
- Introduction of Rel-18 NR sidelink evolution TS 38.212CR0149
- Introduction of Rel-18 network controlled repeaters TS 38.212CR0150
- Introduction of Network Controlled Repeaters TS 38.213CR0506
- Introduction of NR sidelink evolution TS 38.213CR0509
- 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
- Corrections on Rel-18 NR sidelink evolution in 38.212 TS 38.212CR0164
+ 32 more changes
In Release 19, the key enhancement for the SFCI (Sidelink Feedback Control Information) function was the introduction of support for 32 HARQ process numbers, increasing the capacity for sidelink hybrid automatic repeat request processes. This change is reflected in the updated specifications governing physical layer and MAC procedures. The SFCI itself continues to be mapped to the PSFCH physical channel as per the established framework.
- Introduction of 32 HARQ process numbers in Rel-19 [TN32HARQ] TS 38.212CR0222
Explore further
Broader topics and technologies where SFCI plays a role.
Defining Specifications
3GPP specifications that define or reference SFCI, 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 |