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
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (171 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the PSFCH was newly introduced as the Physical Sidelink Feedback Channel to support sidelink communication. Its procedures, including those for Hybrid Automatic Repeat Request (HARQ) feedback, are specified within the physical layer procedures for control. The channel enables feedback mechanisms essential for reliable sidelink data transmission.
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
- Correction on physical downlink control channel TS 38.213CR0020
- 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
- Correction on HARQ-ACK transmission with BWP change TS 38.213CR0064
+ 1 more changes
In Release 16, the PSFCH was enhanced as part of the introduction of 5G V2X sidelink features, which included new physical layer procedures for the sidelink shared channel. These enhancements were accompanied by corrections to sidelink physical layer procedures and the associated HARQ-ACK codebook. Furthermore, the release defined specific sidelink channel access procedures for operation in shared spectrum.
- Introduction of 5G V2X sidelink features into TS 38.212 TS 38.212CR0025
- Introduction of Physical Layer Enhancements for NR URLLC TS 38.212CR0026
- Introduction of shared spectrum channel access TS 38.213CR0071
- 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
+ 59 more changes
In Release 17, the PSFCH function was enhanced as part of the broader NR sidelink enhancement, which included corrections and refinements to procedures such as HARQ-ACK codebook generation for multicast and the enabling of HARQ feedback for SPS. These updates also involved corrections to multiplexing and channel access procedures to improve the reliability and efficiency of sidelink communications.
- 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
- CR on ChannelAccess-Cpext in Fallback DCI TS 38.212CR0118
- Correction on NR sidelink enhancement TS 38.212CR0122
+ 55 more changes
In Release 18, the PSFCH saw specific corrections and maintenance as part of the broader NR sidelink evolution work, including corrections for PSFCH prioritization, power control, resource determination, and resource mapping. These updates addressed detailed implementation aspects such as the OFDM symbol location for PSFCH transmission in SL-U and the CPE starting position for PSFCH. The release also introduced sidelink channel access procedures, which form the procedural framework within which the corrected PSFCH mechanisms operate.
- Introduction of sidelink channel access procedures for Rel-18 NR sidelink evolution TS 38.201CR0003
- Introduction of Rel-18 NR sidelink evolution TS 38.212CR0149
- 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
- Corrections on Rel-18 NR sidelink evolution in 38.212 TS 38.212CR0187
+ 29 more changes
In Release 19, the key new feature for the PSFCH was the introduction of support for 32 HARQ process numbers, increasing the capacity for sidelink feedback. This enhancement is detailed under the change request for the introduction of 32 HARQ process numbers in Rel-19. The release also included corrections related to the adaptation of common channels and signals for new energy savings (NES) features.
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 |