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5G functional split architecture

The 5G functional split is an architecture that involves dividing the base station functions across different physical nodes, namely the Centralized Unit (CU), Distributed Unit (DU), and Radio Unit (RU). The 5G split architecture offers mobile network operators the flexibility to optimize their networks for various performance requirements and deployment scenarios. By selecting the appropriate functional split, operators can balance the tradeoffs between cost, performance, and deployment flexibility to best serve their customers' needs.

The division of processing tasks between the CU, DU, and RU depends on the functional split chosen. The CU and DU play distinct roles in this architecture, with the CU focusing on higher-layer processing and the DU on lower-layer tasks, including real-time processing. The 3GPP has defined several split options, each with its set of responsibilities and performance characteristics:

Different functional split configurations in a 5G network can significantly affect network scalability, which is the ability to expand the network capacity to meet growing demands. The split architecture divides the base station functions across the Centralized Unit (CU), Distributed Unit (DU), and Radio Unit (RU), each taking on different roles in processing and managing network traffic.

abstract representation of the concepts of centralization versus distribution in 5G networks, viewed over a cityscape from a hilltop.webp

The choice of functional split configuration has a direct impact on the scalability of a 5G network. and involves tradeoffs between cost, performance, and deployment flexibility. Higher-layer splits tend to offer better scalability due to centralized resource management and reduced fronthaul demands. Split 7.2x, in particular, provides a good balance for 5G networks, enabling scalability while maintaining performance. Network operators must carefully consider their specific needs and the capabilities of their transport network when selecting a functional split to ensure that the network can scale effectively to meet future demands.

When evaluating 5G functional split options, the following should be considered:

Centralization vs. Distribution: Higher-layer splits, where more functions are centralized in the CU, can enhance scalability by allowing for resource pooling and more efficient management of network functions. Centralized functions can be scaled independently of the distributed elements, which is beneficial for managing numerous cells and handling frequent handovers.
Fronthaul Requirements: Lower-layer splits, which distribute more functions to the DU and RU, require high bandwidth and low latency in the fronthaul network. This can limit scalability if the transport network cannot meet these demands. Conversely, higher-layer splits reduce fronthaul bandwidth requirements, potentially allowing for easier scaling of the network.
Resource Management: The split architecture enables more efficient use of resources by distributing the workload based on the specific requirements of different network functions. This allows for independent scaling of control and user plane functions, which can adapt to changing demands in traffic.
Flexibility and Customization: Network operators can customize and optimize specific components based on the unique requirements of their service offerings and user demands. This flexibility is crucial for scaling the network to accommodate a wide range of services and applications.

Although there are numerous functional split options, only two have shown substantial adoption:

Split 7.2x: This split is considered optimal for 4G and 5G as it balances the demands on the fronthaul network with the benefits of centralization. It allows for a relatively simple RU, which is beneficial for network densification. This split is widely adopted for its balance between flexibility, cost, and performance. It allows for a relatively simple RU, which is beneficial for network densification and scalability. The DU handles more of the physical layer processing, which can be scaled to support increased traffic. Split 7.2x is the O-RAN Alliance fronthaul specification between O-DU to O-RRU. It has two variants: 7.2a and 7.2b based on where precoding occurs.
Split 8: Based on the CPRI interface, this split is best for 2G and 3G technologies. It places higher demands on the fronthaul network due to the centralization of more functions. This split is more suitable for 2G and 3G technologies and places higher demands on the fronthaul network. While it can be effective for these older technologies, it may not offer the same scalability benefits for 5G networks due to its higher fronthaul requirements.

5G functional split architecture

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