Description
The Transport Block Group (TBG) is a concept introduced in 5G New Radio (NR), particularly relevant for ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB) scenarios requiring high throughput. A Transport Block (TB) is the basic unit of data passed from the Medium Access Control (MAC) layer to the Physical (PHY) layer for transmission over the air interface. In earlier systems like LTE, typically one TB was transmitted per scheduled codeword per layer. NR enhances this by allowing the network to schedule a group of TBs—a TBG—within a single scheduling decision (via Downlink Control Information, DCI).
Operationally, when a TBG is configured, the gNodeB's scheduler allocates resources for multiple TBs simultaneously. These TBs within the same TBG are associated with a single Hybrid Automatic Repeat Request (HARQ) process identifier. This means that acknowledgment/negative acknowledgment (ACK/NACK) feedback is provided for the group as a whole or in a bundled manner, depending on the configuration, rather than for each individual TB. The TBs in a group are transmitted over the same set of time and frequency resources (e.g., within the same slot or mini-slot) but may be mapped to different spatial layers or use different modulation and coding schemes to optimize the link adaptation.
The specification of TBG parameters, including the maximum number of TBs per group and the associated HARQ feedback mechanisms, is defined in the physical layer procedures (specified in 38.213). The use of TBG allows for more efficient control signaling overhead, as a single DCI can schedule multiple data units. It also provides finer granularity for link adaptation and resource allocation, enabling the system to better match the transmission parameters to the channel conditions and data requirements for each TB within the group, which is crucial for meeting the stringent latency and reliability targets of NR.
Purpose & Motivation
The TBG concept was developed for 5G NR to address the limitations of single-TB scheduling in meeting the diverse and demanding Key Performance Indicators (KPIs) of new use cases. For eMBB, the ever-increasing demand for peak data rates requires more efficient ways to pack data into a scheduling interval. Transmitting multiple TBs in one grant reduces the relative control signaling overhead and allows better utilization of large contiguous bandwidths available in NR.
For URLLC and industrial IoT, low latency is paramount. The TBG mechanism supports transmission over very short durations (mini-slots). By scheduling multiple small TBs as a group, the system can achieve high reliability through diversity (e.g., different coding for each TB) without incurring the latency penalty of sequential single-TB transmissions and their associated HARQ feedback loops. It provides the scheduler with a flexible tool to trade off between latency, reliability, and throughput dynamically. This was a necessary evolution from LTE's approach to support the broader 5G service portfolio defined in IMT-2020.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (106 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the TBG (Transport Block Group) function was not newly introduced; the provided CR titles and grounding context exclusively discuss corrections and clarifications to existing HARQ-ACK procedures, including dynamic/Type-1 codebook determination, multiplexing timelines, and transmission conditions during BWP changes. The technical details focus on HARQ-ACK feedback mechanisms and SS/PBCH block configurations, with no mention of TBG. Therefore, Release 15 did not introduce TBG.
- Support maximum 8 SS/PBCH blocks for unpaired spectrum beyond 2.4GHz TS 38.213CR0006
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
- 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 TBG function was not a primary focus of the enhancements. The provided grounding context and Change Request titles indicate that Release 16 work centered on refining HARQ-ACK procedures, including corrections for Type-2 and Type-3 codebook construction, multi-TRP feedback, and sidelink HARQ-ACK reporting. The technical details from the context primarily describe SS/PBCH block transmission procedures and are not directly related to TBG.
- CR to 38.213 on HARQ-ACK processing timeline for DCI format 1_1 with Scell dormancy indication without scheduling PDSCH TS 38.213CR0135
- 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
- Correction on Type2 HARQ-ACK codebook construction TS 38.213CR0168
- Type-1 HARQ-ACK for PDSCH repetition with different SCSs in DL and UL TS 38.213CR0180
- Correction of Type-3 HARQ-ACK codebook generation for a PDSCH with one transport block for a configuration with a maximum number of two TBs TS 38.213CR0187
+ 28 more changes
In Release 17, the enhancements for the Transport Block Group (TBG) function were primarily focused on HARQ-ACK codebook generation and feedback procedures to support IIoT/URLLC and multicast services. Specific corrections and clarifications were introduced for Type-2 and Type-3 HARQ-ACK codebooks, including scenarios with spatial and time bundling configurations. Furthermore, new procedures were defined for multiplexing HARQ-ACK feedback from dynamic grant (DG) and semi-persistent scheduling (SPS) transmissions, particularly for both multicast and unicast traffic.
- Introduction of IIoT/URLLC enhancements in NR TS 38.213CR0272
- Corrections on IIoT/URLLC enhancements in NR TS 38.213CR0292
- Corrections on IIoT/URLLC enhancements in NR TS 38.213CR0321
- CR on the description about HARQ-feedbackEnablingforSPSactive in 38.213 TS 38.213CR0340
- Correction for HARQ-ACK codebook generation for PUCCH cell switching and UL BWP switching TS 38.213CR0347
- Correction on Type-2 HARQ CB generation when both of spatial bundling and time bundling are configured TS 38.213CR0349
+ 41 more changes
In Release 18, the new work for the Transport Block Group (TBG) function focused on enhancements and corrections for HARQ-ACK multiplexing, particularly on PUSCH with repetitions. Key introductions included enabling the multiplexing of HARQ-ACK associated with downlink assignments received after an uplink grant for such a PUSCH. The release also provided multiple corrections and clarifications for Type-1 and Type-2 HARQ-ACK codebooks, including aspects for multi-cell scheduling, multi-PDSCH scheduling, and operations on PUSCH and PUCCH.
- 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
- Correction on Type-2 HARQ-ACK codebook and DL BWP change TS 38.213CR0632
- Correction on multiplexing HARQ-ACK in a PUSCH transmission TS 38.213CR0636
- Correction on HARQ-ACK multiplexing on a PUSCH repetition [HARQ-ACK MUX on PUSCH] TS 38.213CR0646
- Correction on HARQ-ACK skipping for Rel-18 multi-cell scheduling TS 38.213CR0673
- Correction on CP Extension for S-SS/PSBCH Block TS 38.213CR0686
+ 11 more changes
In Release 19, the TBG (Transport Block Group) function was enhanced to support autonomous updates of the UE-specific Timing Advance (TA) or common TA within an OCC (Orthogonal Cover Code) group in NR-NTN. This allows a UE to maintain the adjusted TA values, \( N_{TA,adj}^{UE} \) or \( N_{TA,adj}^{common} \), without alteration during PUSCH transmissions that are part of the same OCC group, improving efficiency in non-terrestrial networks.
- CR for Autonomous updates of the UE-specific TA or common TA in an OCC group in NR NTN TS 38.213CR0763
Explore further
Broader topics and technologies where TBG plays a role.
Defining Specifications
3GPP specifications that define or reference TBG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |