Description
Hybrid Automatic Repeat Request (HARQ) is a fundamental error control mechanism employed in the physical layer of 3GPP radio access technologies, including UMTS (HSPA), LTE, and 5G NR. It operates by integrating two classical error control methods: Forward Error Correction (FEC) and Automatic Repeat Request (ARQ). The 'hybrid' nature stems from this combination. In operation, the transmitter sends a data packet encoded with FEC. The receiver attempts to decode it. If decoding fails, instead of discarding the corrupted packet, the receiver stores it and sends a Negative Acknowledgement (NACK) back to the transmitter. Upon receiving a NACK, the transmitter sends a retransmission. The receiver then combines the soft information (e.g., log-likelihood ratios) from the initial transmission and the retransmission before attempting decoding again. This process, known as soft combining, significantly improves the probability of successful decoding compared to treating each transmission independently.
HARQ is implemented using multiple parallel processes, known as HARQ processes, to maintain continuous data flow. Each process handles the transmission and potential retransmission of one transport block. While one process is waiting for an acknowledgement (ACK/NACK), another process can be transmitting new data, thus hiding the round-trip time latency. The protocol is managed by the Medium Access Control (MAC) layer, which handles the generation of ACK/NACK feedback, scheduling of retransmissions, and management of the HARQ buffers. The physical layer is responsible for the actual encoding, modulation, and the soft combining operation.
Key variants include Chase Combining, where identical copies of the packet are retransmitted, and Incremental Redundancy (IR), where each retransmission contains different parity bits, effectively increasing the code rate with each attempt. HARQ is tightly coupled with adaptive modulation and coding (AMC). The initial transmission uses a modulation and coding scheme (MCS) selected based on channel quality indicators (CQI). HARQ provides a second line of defense if the channel degrades unexpectedly after the MCS is selected. Its role is absolutely critical for achieving the high reliability and spectral efficiency targets of modern cellular systems, as it allows the system to operate closer to the capacity limit of the channel by efficiently recovering from errors.
Purpose & Motivation
HARQ was created to address the fundamental challenge of reliable data transmission over inherently unreliable and time-varying wireless channels. Traditional ARQ schemes, which simply discard erroneous packets and request retransmissions, are inefficient for wireless links due to high latency and wasted bandwidth. Pure FEC schemes, which add heavy redundancy to correct errors, become inefficient under good channel conditions. The purpose of HARQ is to synergistically combine the best of both: the proactive error correction capability of FEC to handle common channel variations, and the reactive error recovery of ARQ to handle deep fades or unexpected interference, but in a much more efficient manner than standalone ARQ.
Its introduction in 3GPP Release 5 with High-Speed Downlink Packet Access (HSDPA) was a pivotal moment for enabling high-speed mobile broadband. Prior 3G systems relied on RLC-layer ARQ, which had higher latency and was less efficient for real-time services. HARQ, operating at the physical/MAC layer with much shorter round-trip times, drastically reduced retransmission delay and improved throughput. This was essential for supporting latency-sensitive applications like voice over IP and interactive video. The evolution through LTE and 5G NR has further refined HARQ to support more complex scenarios like carrier aggregation, massive MIMO, and ultra-reliable low-latency communication (URLLC), where its fast and reliable error correction is a cornerstone technology.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (33 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, enhancements and clarifications were made to the HARQ function, including the introduction of an additional UE capability specifically for HARQ-ACK multiplexing on the PUSCH. The release also provided corrections to the configuration of MPDCCH uplink HARQ-ACK feedback and to the HARQ-ACK delay field for Rel-14 MTC, alongside a clarification for asynchronous HARQ procedures in the context of LTE mobility enhancements.
- Corrections to mpdcch-UL-HARQ-ACK-FeedbackConfig TS 36.331CR3840
- Introduction of additional UE capability on HARQ-ACK multiplexing on PUSCH TS 38.331CR1018
- Correction to field description for HARQ-ACK delay for Rel-14 MTC TS 36.331CR3222
- Clarification for the asynchronous HARQ with the LTE mobility enhancements TS 36.331CR3538
In Release 16, enhancements to HARQ focused on corrections and clarifications to improve reliability and configuration accuracy. Key updates included corrections to the HARQ-ACK codebook RRC parameter, refinements to HARQ-ACK spatial bundling configurations for the secondary PUCCH group, and clarifications on HARQ process sharing for Configured Grants (CGs). The release also addressed specific configurations for sidelink HARQ offsets and capabilities for Carrier Aggregation beyond five component carriers.
- Correction on HARQ-ACK codebook RRC parameter TS 38.212CR0069
- Correction on HARQ ACK spatial bundling configurations for secondary PUCCH group TS 38.331CR1993
- Clarification on HARQ process sharing for CGs TS 38.331CR2055
- Correction on HARQ ACK/NACK feedback configuration TS 38.331CR2181
- Correction on value range of sl-ConfigIndexCG and sl-HARQ-ProcID-offset TS 38.331CR2315
- HARQ-ACK codebook configuration for secondary PUCCH group TS 38.331CR2384
+ 6 more changes
In Release 17, enhancements to HARQ introduced support for configurable HARQ-ACK codebooks for multicast and aligned DCI sizes for these codebooks. The release also extended HARQ process number support up to 32 for the FR2-2 frequency range and included corrections for Enhanced Type 3 and Type1 HARQ-ACK codebook generation. Furthermore, it defined HARQ-ACK multiplexing on PUSCH in the absence of PUCCH and adjusted the drx-HARQ-RTT-TimerUL start for uplink repetitions.
- Start drx-HARQ-RTT-TimerUL after last repetition [ulHARQ_RTT_Timer] TS 38.331CR3479
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- Correction to support up to 32 HARQ process numbers for FR2-2 TS 38.212CR0126
- CR on number of HARQ-ACK codebooks configurable for multicast TS 38.212CR0129
- CR on aligning DCI sizes when configuring two HARQ-ACK codebooks for multicast TS 38.212CR0135
- Correction to RRC for 71GHz on scheduling and HARQ configuration for FR2-2 TS 38.331CR4144
+ 3 more changes
In Release 18, key HARQ enhancements introduced new RRC parameters to enable HARQ-ACK multiplexing on the PUSCH and introduced support for PTM (Point-to-Multipoint) retransmission reception for multicast DRX when HARQ feedback is disabled. The release also included specific UE capability corrections related to these new features, such as for HARQ-ACK multiplexing and for IoT NTN involving GNSS and HARQ enhancements. Furthermore, corrections were made to the rate matching behavior when HARQ-ACK is multiplexed with CG-PUSCH.
- PTM retransmission reception for multicast DRX with HARQ feedback disabled [PTM_ReTx_Mcast_HARQ_Disb] TS 38.331CR4504
- Introduction of RRC parameters for HARQ multiplexing [HARQ-ACK MUX on PUSCH] TS 38.331CR4597
- Corrections and Updates to UE capabilities for Rel-18 WIs, including TEI18 [HARQ-ACK MUX on PUSCH] TS 38.331CR4638
- IoT NTN UE capabilities correction for GNSS and HARQ enhancements TS 36.306CR1902
- Correction on the capabilities on PTM retransmission [PTM_ReTx_Mcast_HARQ_Disb] TS 38.331CR4867
- Correction on the rate matching when HARQ-ACK multiplexed with CG-PUSCH TS 38.212CR0158
In Release 19, the key new feature for HARQ was the introduction of support for 32 HARQ process numbers. This enhancement, specifically applied to the Transport Network (TN), increases the maximum number of concurrent HARQ processes from 16 to 32.
Explore further
Broader topics and technologies where HARQ plays a role.
Defining Specifications
3GPP specifications that define or reference HARQ, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 25.101 vj00 | UTRA FDD UE RF Requirements | Rel-19 |
| TS 25.212 vj00 | UTRA FDD Layer 1 Multiplexing & Channel Coding | Rel-19 |
| TS 25.221 vj00 | UTRA TDD Physical Layer Specification | Rel-19 |
| TS 25.222 vj00 | UTRA TDD Multiplexing & Channel Coding | Rel-19 |
| TS 25.301 vj00 | UE-UTRAN Radio Interface Protocol Architecture | Rel-19 |
| TS 25.302 vj00 | UTRA Physical Layer Services | Rel-19 |
| TS 25.308 vj00 | HSDPA Overall Description | Rel-19 |
| TS 25.309 v1600 | FDD Enhanced Uplink Support | Rel-6 |
| TS 25.319 vj00 | Enhanced Uplink for UTRA FDD/TDD | Rel-19 |
| TS 25.321 vj00 | MAC Protocol Specification for UTRAN | Rel-19 |
| TS 25.331 vj00 | UTRAN RRC Protocol Specification | Rel-19 |
| TS 25.420 vj00 | Iur Interface Introduction for UTRAN | Rel-19 |
| TS 25.427 vj00 | UTRAN Iub/Iur User Plane Protocols | Rel-19 |
| TS 25.430 vj00 | Introduction to Iub Interface Specifications | Rel-19 |
| TS 25.766 vd10 | Network-Assisted Interference Cancellation for UMTS | Rel-13 |
| TS 25.823 v800 | Synchronised E-DCH Study for UTRA FDD | Rel-8 |
| TR 25.912 vj00 | Evolved UTRA and UTRAN Technical Report | Rel-19 |
| TS 26.267 vj00 | eCall In-band Modem Specification | Rel-19 |
| TS 26.268 vj00 | eCall In-band Modem ANSI-C Code | Rel-19 |
| TR 26.926 vj00 | Traffic Models & Quality Evaluation for Media/XR in 5G | Rel-19 |
| TR 28.841 vi01 | Technical Report on IoT NTN Enhancements | Rel-18 |
| TS 36.104 vj10 | Base Station (BS) radio transmission and reception | Rel-19 |
| TS 36.116 vj00 | E-UTRA Relay RF Requirements | Rel-19 |
| TS 36.117 vj00 | E-UTRA Relay RF Test Methods & Requirements | Rel-19 |
| TS 36.133 vj20 | E-UTRA RRM Requirements | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.201 vj00 | LTE Physical Layer General Description | Rel-19 |
| TS 36.216 vj00 | LTE Relay Node Physical Layer | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.306 vj00 | E-UTRA UE Radio Access Capability Parameters | Rel-19 |
| TS 36.314 vj00 | E-UTRA Radio Measurements Specification | Rel-19 |
| TS 36.322 vj00 | E-UTRA Radio Link Control Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.747 ve00 | Enhanced CRS and SU-MIMO IM Performance Requirements | Rel-14 |
| TR 36.791 vg00 | E-UTRA 2.4 GHz TDD Band for US | Rel-16 |
| TS 36.825 vd00 | Study on Additional LTE TDD Configurations | Rel-13 |
| TS 36.863 vc00 | CRS Interference Mitigation for Homogeneous Networks | Rel-12 |
| TS 36.938 v900 | E-UTRAN to 3GPP2/Mobile WiMAX Mobility | Rel-9 |
| TS 37.105 vj10 | AAS Base Station Transmission & Reception Requirements | Rel-19 |
| TR 37.901 vf10 | UE Application Layer Data Throughput Performance | Rel-15 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.201 vj00 | NR Physical Layer General Description | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.521 vj20 | NR Physical Layer UE Conformance Testing | Rel-19 |
| TS 38.551 vi30 | User Equipment (UE) Multiple Input Multiple Output (MIMO) Over-the-Air (OTA) performance | Rel-18 |
| TS 38.762 vj00 | Dynamic MIMO OTA Test Methodology for NR FR1 | Rel-19 |
| TS 38.811 vf40 | Study on NR Support for Non-Terrestrial Networks | Rel-15 |
| TS 38.824 vg00 | NR URLLC Physical Layer Enhancements Study | Rel-16 |
| TR 38.830 vh00 | NR Coverage Enhancements Study | Rel-17 |
| TR 38.838 vh00 | Study on XR Evaluations for NR | Rel-17 |
| TR 38.878 vi40 | Technical Report on Advanced Receiver for MU-MIMO | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |
