5G RAN Control-User Plane Separation for Latency and Mobility
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Solution Overview
Problem
The transition to 5G wireless networks faces challenges such as network configuration, management, and interoperability due to the ultra-dense and heterogeneous nature of radio access networks (RANs) with a large number of small cells, leading to insufficient mobility management and high bandwidth requirements for fronthaul interfaces.
Innovation Solution
The proposed solution involves separating the functions of a base station into a Central Unit (CU) for control plane and user plane, allowing for flexible deployment and virtualization of RAN components, which includes centralizing control plane functions and partitioning user plane functions between a CU and a Distribution Unit (DU), thereby reducing latency and bandwidth pressures on interfaces and enabling efficient mobility management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If base station functions are centralized in a single unit, then control management is simplified, but latency and bandwidth pressures on interfaces increase
Solution Approach 1:
The base station is segmented into three functional units: Central Unit (CU) for control plane functions, User Plane Units (UPUs) for user plane data processing, and Radio Units (RUs) for radio frequency operations. This segmentation allows control functions to be centralized in the CU while user plane functions are distributed closer to the radio interface, reducing interface latency and bandwidth pressures while maintaining simplified control management.
2Productivity
If the number of small cells is increased to create ultra-dense RAN, then network capacity is improved, but mobility management becomes insufficient
Solution Approach 1:
The CU acts as an intermediary that coordinates mobility management across multiple UPUs and RUs in the ultra-dense RAN. The CU maintains centralized control plane functions including mobility management, while UPUs handle local user plane data processing. This intermediary architecture enables effective mobility management in ultra-dense deployments by coordinating handovers and resource allocation across multiple small cells without overwhelming any single node.
3Device complexity
If control plane and user plane functions are combined in a single base station unit, then device structure is simplified, but flexibility in deployment is reduced
Solution Approach 1:
The base station is divided into independently deployable functional units: CU, UPU, and RU, connected through standardized interfaces. The CU handles control plane functions and can be deployed centrally, while UPUs can be distributed at different locations closer to users. This segmentation provides deployment flexibility allowing operators to deploy units incrementally and in various configurations while maintaining a relatively simple overall structure through standardized interface protocols.
Solution Approach 2:
The CU is designed with universal control plane functions that can manage multiple UPUs and RUs, making it adaptable to different deployment scenarios. The standardized interfaces between CU, UPU, and RU allow these units to be combined in various configurations to meet different network requirements, providing versatility in deployment while maintaining a consistent architectural structure.
4Productivity
If fronthaul interface bandwidth is increased to support ultra-dense RAN, then network performance is improved, but infrastructure cost increases
Solution Approach 1:
User plane data processing functions are extracted from the centralized CU and placed in distributed UPUs closer to the radio interface. This extraction reduces the amount of user plane data that needs to be transmitted over the fronthaul interface between CU and RU, thereby reducing bandwidth requirements and infrastructure costs while maintaining network performance through localized data processing at the UPU.
Data Source
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AI summary
Functions of base station (BS) can be separated into a central unit control plane (CU-CP) for control plane functions, central unit user plane (CU-UP) for user plane functions and a distribution unit (DU) for user equipment communication functions. For example, the control plane (CP) of a base station is split, centralizing part of the CP in a Central Unit (CU). The user plane (UP) of the air interface is partitioned and part of the UP of the air interface and UP of the backhaul is centralized in a CU that can be co-located with the CU hosting the CP, or in a different location, while leaving the rest of the UP of the air interface distributed. Control plane/user plane (C/U) separation, user-plane partitioning and modular design can improve latency and/or bandwidth of interfaces, mobility among BS's, flexible deployment of RAN functions, and the virtualization of the CP and/or the UP.