Description
The Downlink Assignment Index (DAI) is a fundamental mechanism in 3GPP LTE and NR specifications that addresses the challenge of missed downlink control information detection in HARQ-ACK feedback procedures. In the physical layer control channel structure, DAI is embedded within Downlink Control Information (DCI) formats that schedule Physical Downlink Shared Channel (PDSCH) transmissions. This counter field specifically operates in scenarios requiring multi-subframe or multi-slot feedback, most notably in Time Division Duplex (TDD) configurations where multiple downlink assignments can be acknowledged in a single uplink subframe.
Architecturally, DAI functions within the UE's physical layer processing chain, specifically in the HARQ-ACK generation module. When a UE receives a DCI containing a DAI value, it interprets this as an indicator of how many downlink assignments have been transmitted up to that point within a specific bundling window. The UE maintains an internal counter that tracks received DAI values, allowing it to detect discrepancies that indicate missed DCIs. This detection capability is crucial because missed scheduling assignments would otherwise lead to incorrect HARQ-ACK feedback, potentially causing unnecessary retransmissions or protocol failures.
The technical implementation varies between LTE and NR, but follows similar principles. In LTE, DAI is typically 2 bits for TDD configurations, counting modulo-4 within the bundling window. In NR, the mechanism is more sophisticated with both counter-DAI and total-DAI fields in some DCI formats, providing enhanced reliability for multi-PDSCH scheduling scenarios. The UE's HARQ-ACK codebook construction directly depends on correctly interpreting DAI values to determine which HARQ-ACK bits to include and their proper ordering in the feedback payload.
DAI's role extends beyond simple error detection to enabling efficient spectrum utilization. By allowing accurate HARQ-ACK feedback even when some DCIs are missed, it prevents the radio link control (RLC) layer from triggering unnecessary retransmissions for correctly received data. This optimization is particularly valuable in TDD systems with asymmetric downlink/uplink ratios, where the feedback opportunity is limited relative to the number of potential downlink transmissions. The mechanism thus contributes directly to system throughput and latency performance in real network deployments.
Purpose & Motivation
DAI was introduced to solve a fundamental problem in HARQ-ACK feedback reliability: when a UE misses a downlink assignment (DCI), it cannot generate correct HARQ-ACK feedback for the corresponding PDSCH transmission. Before DAI implementation, missed DCIs would cause the UE to construct an incomplete HARQ-ACK codebook, leading to misalignment between the UE's feedback and the gNB's expectations. This misalignment could cause several issues: unnecessary retransmissions of correctly received data (wasting radio resources), failure to retransmit incorrectly received data (reducing reliability), and potential protocol timeouts or radio link failures.
The historical context for DAI's creation lies in the evolution from Frequency Division Duplex (FDD) to more complex Time Division Duplex (TDD) deployments in LTE. In FDD systems, HARQ-ACK timing is fixed and predictable, with each downlink transmission having a dedicated uplink feedback opportunity. However, TDD configurations with asymmetric downlink/uplink ratios (such as configuration 2 with more downlink than uplink subframes) required bundling multiple downlink HARQ-ACK responses into a single uplink subframe. This bundling created the possibility that a UE might miss some DCIs while receiving others, without any mechanism to detect these misses.
DAI addressed these limitations by providing explicit signaling that allows the UE to detect missing assignments and construct the HARQ-ACK codebook accordingly. The solution was particularly motivated by the need to support efficient TDD operation in LTE Release 8, where spectrum flexibility was a key requirement. By enabling reliable HARQ-ACK feedback in challenging radio conditions where control channel reception might be imperfect, DAI contributed to the overall robustness of the LTE system and paved the way for even more flexible scheduling mechanisms in subsequent NR specifications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (126 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, corrections and clarifications were introduced for the DAI function primarily within the procedures for the dynamic HARQ-ACK codebook and the Type-1 HARQ-ACK codebook determination. These included fixes to the associated pseudo-code in the specification and corrections to the timeline conditions for multiplexing HARQ-ACK information. The changes also addressed interactions with bandwidth part change and the time gap definition for HARQ-ACK transmission.
- Correction of wrong implementation on frequency domain resource assignment bitwidth TS 38.212CR0006
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
- Correction on physical downlink control channel TS 38.213CR0020
- Clarification of reference to PDSCH processing capability 1 in TS 38.213 TS 38.213CR0042
- 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
+ 4 more changes
In Release 16, enhancements to the DAI function focused on refining HARQ-ACK codebook procedures and addressing specific scheduling scenarios. Key corrections included the determination of the Type-3 HARQ-ACK codebook size to resolve ambiguities and the handling of codebook generation for multi-TRP transmissions with separate feedback. Furthermore, updates were made to HARQ-ACK processing timelines for cases like SCell dormancy indication without a scheduled PDSCH and for PDSCH repetitions with different subcarrier spacings in downlink and uplink.
- 38.212 CR on DAI size determination for DCI format 1_1/1-2 in CA TS 38.212CR0066
- Correction on HARQ-ACK codebook RRC parameter TS 38.212CR0069
- CR to 38.213 on HARQ-ACK processing timeline for DCI format 1_1 with Scell dormancy indication without scheduling PDSCH TS 38.213CR0135
- Corrections related to the sidelink slot index TS 38.213CR0154
- Correction of NRU HARQ procedure in the presence of SPS PDSCH TS 38.213CR0163
- 38.213 CR Correction on HARQ-ACK codebook for secondary PUCCH group TS 38.213CR0167
+ 34 more changes
In Release 17, the DAI (Downlink Assignment Index) function was updated to better support multicast transmissions and new configurations. Key enhancements included making the number of HARQ-ACK codebooks configurable specifically for multicast and aligning DCI sizes when configuring two such codebooks. Additionally, a correction was made to DAI counting for scenarios where a 'dci-enabler' in DCI indicates a value of 0.
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- Corrections on resource pool index TS 38.212CR0124
- 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
- CR on the description about HARQ-feedbackEnablingforSPSactive in 38.213 TS 38.213CR0340
+ 41 more changes
In Release 18, the DAI's operation was refined within the enhanced Type-2 HARQ-ACK codebook framework, particularly for scenarios involving multi-slot scheduling and HARQ-ACK multiplexing on a PUSCH with repetitions. Specific corrections addressed the proper generation of the codebook and the multiplexing of HARQ-ACK feedback associated with downlink assignments received after an uplink grant. These updates ensured robust DAI counting and HARQ-ACK reporting alignment for the new MIMO evolution features and complex scheduling scenarios introduced in the release.
- Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
- Introduction of MIMO Evolution for Downlink and Uplink TS 38.213CR0504
- 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 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0167
- 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
+ 21 more changes
In Release 19, the DAI function was updated to support the newly introduced capability of 32 HARQ process numbers. This enhancement required modifications to the DAI procedures to correctly manage the increased HARQ process space for downlink assignments.
Explore further
Broader topics and technologies where DAI plays a role.
Defining Specifications
3GPP specifications that define or reference DAI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 26.131 vj00 | Terminal Acoustic Performance Requirements | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |