Code Block Group

Description

A Code Block Group (CBG) is a fundamental concept in the 5G New Radio (NR) physical layer, specifically within the Hybrid Automatic Repeat Request (HARQ) process for error control. In NR, a Transport Block (TB), which is the basic unit of data for transmission over the air interface, is segmented into smaller Code Blocks (CBs) before channel coding. A CBG is a collection of one or more of these Code Blocks. The primary function of CBG-based transmission is to enable more granular retransmissions. Instead of signaling HARQ feedback (ACK/NACK) and performing retransmissions at the granularity of an entire Transport Block, the network can operate at the CBG level. This means the receiver can identify and request retransmission of only those specific CBGs that contained errors, rather than the entire TB.

The operation involves several key components and procedures. During initial transmission, the gNB (base station) transmits a TB, which is segmented into CBs, and these CBs are logically grouped into CBGs based on configuration. The number of CBGs per TB is configurable by RRC signaling, with a maximum defined per specification (e.g., up to 8 CBGs for a TB in some configurations). Each CBG is independently channel coded. The receiver (UE) performs decoding and generates HARQ feedback in the form of a CBG-based ACK/NACK bitmap. This bitmap indicates the success (ACK) or failure (NACK) of each individual CBG. The gNB processes this feedback and schedules retransmissions specifically for the NACKed CBGs. The retransmitted CBGs may use the same or different redundancy versions (RVs) as defined in the HARQ process.

Architecturally, CBG processing is integrated into the physical downlink shared channel (PDSCH) and physical uplink shared channel (PUSCH) handling within the gNB and UE protocol stacks, primarily in the MAC and physical layers. The configuration, including whether CBG-based transmission is enabled and the number of CBGs, is determined by RRC and can be adapted based on channel conditions. The Downlink Control Information (DCI) formats include fields to indicate the CBG Transmission Information (CBGTI) and CBG Flushing Information (CBGFI), which manage which CBGs are being transmitted and whether previous soft buffer content should be flushed. This mechanism is crucial for managing the soft buffer memory in the receiver, as it stores log-likelihood ratios for incremental redundancy combining.

CBG's role in the network is to enhance the efficiency of the HARQ process, which is critical for achieving the high reliability and low latency targets of 5G, particularly for enhanced Mobile Broadband (eMBB) and Ultra-Reliable Low-Latency Communications (URLLC) services. By reducing the amount of data that needs to be retransmitted when only part of a TB is corrupted, CBG-based retransmission minimizes air interface resource consumption, reduces latency for error recovery, and improves overall throughput and spectral efficiency. It represents a significant evolution from the LTE approach, where retransmissions were primarily TB-based, offering finer control and better performance in challenging radio conditions.

Purpose & Motivation

CBG-based transmission was introduced in 5G NR to address limitations of the Transport Block (TB)-based HARQ used in LTE. In LTE, if any part of a TB failed decoding, the entire TB—potentially a large data unit—had to be retransmitted. This was inefficient, especially as TB sizes grew larger with higher data rates in 5G. Retransmitting an entire TB consumed excessive time and radio resources, increasing latency and reducing spectral efficiency, which was detrimental for latency-sensitive services like URLLC and for maintaining high throughput in poor signal conditions.

The primary problem CBG solves is the granularity of retransmissions. By grouping code blocks, it allows the system to identify and retransmit only the corrupted subsets of data. This partial retransmission capability significantly reduces the overhead and delay associated with error recovery. Historically, the motivation came from the need to support diverse 5G use cases with stringent requirements. For eMBB, large TBs are common, and CBG prevents small errors from triggering large retransmissions. For URLLC, minimizing latency is paramount, and faster error correction via targeted retransmissions is critical.

Furthermore, CBG enables more efficient use of the UE's soft buffer memory for HARQ combining. Instead of storing soft bits for an entire TB for potential retransmission, the UE can manage memory per CBG, allowing for better performance with limited buffer sizes. This addresses practical implementation constraints. The creation of CBG was driven by 3GPP's goal to design a more flexible and efficient air interface for NR, learning from LTE's experiences and anticipating the needs of future high-speed, low-latency applications.

Key Features

• Enables partial retransmissions at Code Block Group granularity instead of full Transport Block retransmission
• Configurable number of CBGs per Transport Block via RRC signaling for flexibility
• Uses CBG-based ACK/NACK feedback bitmap for precise error indication
• Includes DCI fields (CBGTI, CBGFI) for dynamic control of retransmissions and soft buffer management
• Reduces HARQ retransmission latency and improves spectral efficiency
• Supports both downlink (PDSCH) and uplink (PUSCH) channels

Evolution Across Releases

Defining Specifications