gNB Distributed Unit

Description

The gNB Distributed Unit (gNB-DU) is a key logical node in the 5G New Radio (NR) architecture, introduced in 3GPP Release 15. It hosts the Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers of the gNB or en-gNB. Its operation is partly controlled by the gNB Central Unit (gNB-CU), with which it communicates over the F1 interface. The gNB-DU is responsible for the real-time, low-latency processing of radio signals and is typically deployed at the cell site, close to the antenna. One gNB-DU can support one or multiple cells, but each cell is supported by only one gNB-DU, ensuring clear ownership and simplified management.

The primary functions of the gNB-DU include the RLC layer, which handles segmentation, retransmission, and flow control; the MAC layer, which manages scheduling, multiplexing, and HARQ; and the PHY layer, which performs modulation, coding, and antenna processing. These functions are highly time-sensitive and require proximity to the radio interface to meet the stringent latency requirements of 5G. By centralizing the higher layers in the gNB-CU, the gNB-DU can focus on these real-time tasks, leading to a more efficient and scalable architecture.

The gNB-DU connects to the gNB-CU via the F1 interface, which is split into F1-C (control plane) and F1-U (user plane). The F1-C uses SCTP over IP for signaling messages, such as those for cell configuration, UE context management, and paging. The F1-U uses GTP-U for user data transport, carrying the PDCP PDUs between the gNB-CU and gNB-DU. The gNB-DU also supports the F1 interface for dual connectivity, where it can act as an MgNB-DU (master node) or SgNB-DU (secondary node) in EN-DC or NR-DC scenarios. This flexibility allows the gNB-DU to be integrated into various deployment topologies.

In terms of deployment, the gNB-DU is often implemented as a physical network element at the cell site, but it can also be virtualized. The split architecture allows operators to deploy gNB-DUs in different locations, such as at the edge of the network or in a centralized location, depending on the use case. For example, in a C-RAN deployment, multiple gNB-DUs can be pooled in a central location, while the gNB-CU is further centralized. The gNB-DU also plays a role in network slicing, as it can be configured to support specific slices with dedicated resources.

The gNB-DU's operation is partly controlled by the gNB-CU, meaning that the gNB-CU makes decisions on radio resource management, mobility, and QoS, while the gNB-DU executes these decisions. This hierarchical control ensures coordinated operation across multiple cells and enables features like carrier aggregation and dual connectivity. The gNB-DU also supports the transmission of system information, paging, and random access procedures, which are essential for UE access. Overall, the gNB-DU is a critical component of the 5G RAN, providing the high-performance radio interface required for enhanced mobile broadband, ultra-reliable low-latency communications, and massive machine-type communications.

Purpose & Motivation

The gNB Distributed Unit (gNB-DU) was created to enable a flexible and efficient split of the 5G base station functions. In previous generations, base stations were monolithic, combining all protocol layers in a single unit. This made it difficult to centralize certain functions and scale the network cost-effectively. The gNB-DU/gNB-CU split allows the real-time, low-layer functions to be placed at the cell site, while the higher-layer, less time-critical functions are centralized. This separation addresses the need for lower latency, higher capacity, and more efficient use of network resources.

The gNB-DU solves the problem of deploying dense networks with many small cells. By centralizing the control and higher-layer processing in the gNB-CU, the gNB-DU can be made simpler and more cost-effective, reducing the total cost of ownership. It also enables advanced features like Coordinated Multi-Point (CoMP) and dual connectivity, which require tight coordination between cells. The split architecture also facilitates the introduction of new technologies, such as network slicing and edge computing, by allowing the gNB-DU to be deployed close to the user for low-latency services.

The gNB-DU also supports the evolution towards cloud RAN and virtualized RAN, where the gNB-DU can be implemented as a virtual function on commodity hardware. This provides scalability and flexibility, allowing operators to adapt to changing traffic demands. Overall, the gNB-DU is a key enabler of the 5G RAN's ability to meet diverse performance requirements and deployment scenarios.