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.
Key Features
- Enables scheduling of multiple Transport Blocks with one grant
- TBG shares a common HARQ process identifier
- Reduces control signaling overhead (DCI efficiency)
- Supports flexible link adaptation per TB within the group
- Facilitates low-latency transmission using mini-slots
- Enhances reliability through multi-TB diversity within a single transmission time
Evolution Across Releases
Introduced as a new physical layer feature in 5G NR to support enhanced URLLC, integrated access and backhaul (IAB), and other advanced use cases. It defined the fundamental framework for TBG, including its association with a single HARQ process, the DCI formats for TBG scheduling, and the initial rules for TBG size determination and resource mapping.
Defining Specifications
| Specification | Title |
|---|---|
| TS 38.213 | 3GPP TR 38.213 |