Code Block Group Transmission Information

Description

Code Block Group Transmission Information (CBGTI) is a crucial component of the 5G New Radio (NR) physical layer, specifically within the downlink control information (DCI) framework. It operates as part of the Hybrid Automatic Repeat Request (HARQ) feedback mechanism, providing granular information about which code block groups (CBGs) within a transport block were successfully received and which failed. A transport block is divided into multiple code blocks for channel coding, and these code blocks are further grouped into CBGs for transmission and feedback purposes. The CBGTI field in the DCI indicates precisely which CBGs are being retransmitted, allowing the receiver to combine previously received CBGs with newly transmitted ones.

The architecture of CBGTI is tightly integrated with the 5G NR physical downlink shared channel (PDSCH) and physical uplink control channel (PUCCH) or physical uplink shared channel (PUSCH) for feedback. When a user equipment (UE) receives a transport block, it performs decoding on each CBG and generates HARQ-ACK feedback. If some CBGs fail, the gNodeB uses CBGTI in the scheduling DCI for the retransmission to specify which CBGs are included. The CBGTI field is a bitmap where each bit corresponds to a CBG, with '1' indicating that CBG is being retransmitted and '0' indicating it is not. This bitmap length is configurable based on the maximum number of CBGs per transport block, which is RRC-configured.

Key components include the CBG configuration parameters (maxCodeBlockGroupsPerTransportBlock), the CBGTI field in DCI formats 1_0, 1_1, and 1_2, and the HARQ-ACK codebook design that supports CBG-based feedback. The network configures whether CBG-based transmission is enabled via RRC signaling, and when enabled, both transmitter and receiver must maintain soft buffers per CBG rather than per transport block. This requires more sophisticated buffer management but enables significant efficiency gains. The receiver uses the CBGTI information to identify which CBGs to combine from previous transmissions and which to replace with new transmissions.

CBGTI's role in the network is to enhance the efficiency of retransmissions, particularly for large transport blocks where retransmitting the entire block would be wasteful. By enabling partial retransmissions, it reduces latency and improves throughput, especially in challenging radio conditions. It works in conjunction with other HARQ enhancements in 5G NR, such as enhanced dynamic codebook for HARQ-ACK feedback, to provide a robust and efficient link adaptation system. The implementation requires careful coordination between scheduling, HARQ processes, and buffer management to ensure correct operation across multiple transmissions.

Purpose & Motivation

CBGTI was introduced in 5G NR to address the inefficiencies of traditional transport block-based HARQ retransmissions, particularly for large packet sizes. In previous cellular systems (including LTE), when any part of a transport block failed decoding, the entire transport block had to be retransmitted, wasting bandwidth and increasing latency. This became increasingly problematic with 5G's support for very high data rates and large transport blocks, where retransmitting the entire block could consume significant resources and delay data delivery.

The primary motivation for CBGTI was to improve spectral efficiency and reduce latency by enabling more granular retransmissions. By allowing the network to retransmit only the failed code block groups rather than the entire transport block, CBGTI minimizes overhead and makes better use of available radio resources. This is especially important for applications requiring high reliability and low latency, such as ultra-reliable low-latency communications (URLLC), where efficient error correction is critical.

Historically, LTE introduced some concepts of partial retransmissions with limited success, but 5G NR's CBGTI represents a more systematic and flexible approach. It addresses the limitations of previous approaches by providing a standardized, configurable mechanism that works seamlessly with 5G's flexible numerology and wide range of use cases. The creation of CBGTI was driven by the need to support diverse 5G requirements, from enhanced mobile broadband (eMBB) with large transport blocks to URLLC with stringent reliability needs, making efficient retransmissions essential for overall system performance.