Description
The Physical Sidelink Feedback Channel (PSFCH) is a physical layer channel defined within the 5G New Radio (NR) sidelink (SL) protocol stack, specified primarily in 3GPP TS 38.212 and 38.213. It is the mechanism for conveying Hybrid Automatic Repeat Request (HARQ) acknowledgement feedback for transmissions received over the Physical Sidelink Shared Channel (PSSCH). The PSFCH carries the HARQ-ACK information, indicating whether a transport block was successfully decoded (ACK) or not (NACK), which is essential for enabling retransmissions and achieving reliable communication in sidelink Mode 1 (scheduled) and Mode 2 (autonomous) resource allocation.
Architecturally, the PSFCH is mapped to specific time-frequency resources within a sidelink resource pool configuration. Its transmission is tightly coupled with the PSSCH it provides feedback for. The channel structure involves a sequence-based transmission. The receiving UE generates a specific sequence (e.g., a Zadoff-Chu sequence) whose cyclic shift or resource index implicitly or explicitly represents the HARQ-ACK value (ACK or NACK) and may also identify the source of the feedback. This sequence is then transmitted in the designated PSFCH resources. The design is optimized for low overhead and robustness in potentially high-interference D2D environments.
How it works operationally involves a defined timing relationship. After a UE receives a PSSCH transmission in a slot, it prepares the HARQ-ACK based on the decoding outcome. The PSFCH transmission occurs in a subsequent slot, with the gap defined by the higher-layer parameter sl-PSFCH-Period. The transmitting UE monitors the PSFCH for feedback. Upon receiving a NACK, it can trigger a retransmission of the same transport block, following the configured HARQ process. This closed-loop feedback mechanism significantly improves the reliability of sidelink communication, which is vital for safety-critical V2X messages or data sessions.
The PSFCH is a key component of the NR sidelink's support for advanced QoS. It works in conjunction with other sidelink channels like PSCCH (control) and PSSCH (data). Its configuration, including periodicity, resource size, and power control, is managed by the network (in Mode 1) or pre-configured (in Mode 2). The channel's role extends beyond basic HARQ; in later releases, enhancements have supported multi-cast feedback (e.g., NACK-only feedback for groupcast) and coordination with sensing procedures for autonomous resource selection, making the sidelink interface robust and efficient for dynamic direct communication scenarios.
Purpose & Motivation
The PSFCH was introduced to fulfill the need for reliable, low-latency direct communication in 5G NR-based sidelink, a successor to the LTE sidelink (PC5). While LTE sidelink provided basic broadcast capabilities for public safety and V2X, it had limited support for reliable unicast and groupcast with HARQ feedback. The creation of the PSFCH was motivated by the more stringent requirements of advanced V2X use cases (defined by 3GPP), industrial IoT, and public safety applications, which demand high reliability and guaranteed packet delivery for safety-critical information exchange.
Previous approaches in LTE sidelink primarily relied on blind retransmissions (repeating a transmission a fixed number of times) to achieve reliability, which is inefficient and wasteful of spectral resources. The absence of a dedicated, low-latency feedback channel limited the ability to implement adaptive retransmissions based on actual channel conditions. The PSFCH solves this by introducing a standardized, efficient method for HARQ-ACK reporting, enabling closed-loop link adaptation and retransmission on demand. This directly addresses the limitations of the open-loop reliability mechanism, leading to more spectrum-efficient and reliable communications.
The historical context is the evolution from LTE Device-to-Device (D2D) to the 5G NR sidelink, which was designed from the ground up to support a wider range of services, including enhanced V2X (eV2X). The PSFCH, as part of the NR physical layer design from Release 16 onwards, was a fundamental enabler for supporting NR QoS levels over the sidelink interface. It allows the sidelink to support services with packet error rate requirements as low as 10^-5, meeting the needs of autonomous driving coordination, remote driving, and sensor sharing, where timely and confirmed delivery of messages is non-negotiable.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (15 CRs across 3 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, the specification for the PSFCH was refined through corrections. These corrections addressed the mapping between the PSFCH and PSCCH, HARQ reporting procedures for scenarios involving multiple resource pools, and the priority order used in its power control mechanism.
In Release 17, the PSFCH function was enhanced with several specific corrections and clarifications. These updates addressed the determination of transmission priority in conflict scenarios and provided corrections for PSFCH power control procedures. Furthermore, the specification clarified the transmission and reception timeline for PSFCH carrying control information and corrected the description of valid PSFCH occasions for a specific scheduling scheme (Scheme 2).
- Correction on the determination of the priority value of PSFCH transmission with conflict information due to Condition 2-A-2 of Scheme 2 TS 38.213CR0404
- Clarification on the timeline of transmitting/receiving PSFCH with control information TS 38.213CR0449
- Correction on description of valid PSFCH occasion for scheme 2 in TS 38.213 TS 38.213CR0484
- Corrections on PSFCH power determination in TS 38.213 TS 38.213CR0460
In Release 18, the enhancements to the PSFCH primarily involved corrections and refinements to existing procedures. Key updates included corrections for PSFCH prioritization for UEs incapable of transmission in multi-RB sets for Sidelink Unlicensed (SL-U) operation, adjustments to the OFDM symbol location for PSFCH transmission in SL-U, and refinements to PSFCH power control mechanisms. Furthermore, corrections were made to the determination of PSFCH resources for a PSSCH, the CPE starting position for PSFCH, and the PSFCH resource mapping for contiguous RB resource pools.
- Correction on PSFCH prioritization for UE incapable of TX in multi-RB sets for SL-U TS 38.213CR0613
- Correction on OFDM symbol location for PSFCH transmission in SL-U TS 38.213CR0617
- Correction on PSFCH Power Control TS 38.213CR0618
- Corrections on determination of PSFCH resources for a PSSCH TS 38.213CR0628
- Correction on CPE starting position for PSFCH TS 38.213CR0670
- Correction on PSFCH Power Control TS 38.213CR0671
+ 2 more changes
Explore further
Broader topics and technologies where PSFCH plays a role.
Defining Specifications
3GPP specifications that define or reference PSFCH, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.201 vj00 | NR Physical Layer General Description | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TR 38.786 vi20 | Technical Report for NR Sidelink Evolution | Rel-18 |
