Open RAN

6G radio protocols

Radio protocols are sets of rules, configurations and procedures that govern the transmission and reception of data over wireless communication systems. They facilitate efficient and effective communication between devices within a network. For a radio network to operate successfully and support more advanced new use cases, it is essential to have seamless integration between modularly designed radio protocols. As the backbone of the network, each protocol plays a crucial role in managing and optimizing wireless communication.As illustrated in Figure, cellular radio protocols consist of the user plane and the control plane. The user plane handles the transmission of user data, including voice, pictures and videos. Meanwhile, the control plane manages the transmission process, overseeing aspects such as session setup, security and authentication, and mobility. The radio resource management (RRM) manages the broader network efficiency and performance. This includes scheduling, mobility, and spectrum utilization in the communication system to ensure seamless connectivity and QoS.

Open RANs (O-RAN) were introduced in 5G era with their programmability features resulted from the softwarization and cloudification of RAN functional splits and with the aim of realizing the optimum use of radio resources. The Open RAN concept separates the proprietary Base Band Unit (BBU) into distributed unit and centralized unit, allows their deployment as softwarized network functions over the general purpose server hardware.

O-RAN Alliance was established in 2018 and running as a global community from the industry and academia to specify the open architecture and interfaces to realize an Open RAN. According to the definitions given by Open Radio Access Network (O-RAN) Alliance, the reference O-RAN architecture has two logically separated segments for radio and management Network Function (NF)s. In particular, they consider the 7.2x RAN functional split defined by Third Generation Partnership Project (3GPP).

The radio segment includes Open Radio Unit (RU), Open Distributed Unit (DU), Open Centralized Unit (CU) (i.e., further divided as Control Plane (CP) and User Plane (UP)) and Near-Real-Time (Near-RT) RAN Intelligent Controller (RIC) functions, whereas the management segment includes the Service Management and Orchestration (SMO) framework with a Non-Real-Time (Non-RT) RIC function. O-Cloud is a collection of physical cloud infrastructure that supports to run O-RAN virtualized NFs mentioned above except RU, which is running only as a Physical Network Function (PNF) according to the current specifications. The interfaces include the existing 3GPP defined interfaces (F1, E1, Xn and X2) and O-RAN specified interfaces such as O1, O2, A1, Y1, E2, R1, and Open Fronthaul (O-FH). The O-RAN architecture supports at least three control loops involving different O-RAN functionalities: Non-RT control loops (in the order of 1 s or more); Near-RT control loops (in the order of 10 ms or more); Real Time (RT) control loops (below 10 ms). The SMO framework is responsible for handling the automation, control, management, and orchestration of RAN components including the Non-RT RIC which can expose all required functionalities via the rApps. The Non-RT RIC framework is responsible for exposing all required functionality to the rApps, whether from the Non-RT RIC framework or the SMO framework. The Near-RT RIC contains the platform and one or more xApps whereas their capabilities are exposed as services through Near-RT RIC Application Programming Interface (API)s.

As specified by 3GPP, the service-based architecture in the 5G core has been one of the key innovations compared to the so-called reference point architecture in Fourth-generation (4G). The evolution from 5G to 6G represents a paradigm shift not only in terms of technology but also in how networks are designed, standardized, and operated. The process of 6G network standardization is set to begin in 2024. The International Telecommunications Union (ITU) and 3GPP are the two major organizations committed to the development of standards for beyond 5G and 6G communication systems. To reinforce the commitment, a timeline for 6G standardization was established during the March 2024 3GPP meetings held in Maastricht, the Netherlands. In the meeting, the requirements, specifications, and objectives of the 6G standards were discussed, as well as how these standards can achieve the ITU 2023 goals for defining 6G networks.In addition to the ITU and 3GPP, other fora, such as the O-RAN Alliance, will also be involved in contributing to the development of standards for Open RAN in 6G. By reviewing the literature on 6G use cases and requirements, the O-RAN Alliance will primarily focus on identifying gaps within existing O-RAN standards and enhancing those standards to meet the requirements of the Open RAN architecture in the 6G era. While the integration of Open RAN into the 6G standardization process offers numerous benefits, it also presents several challenges. Ensuring interoperability among a diverse set of components, maintaining network security in an open environment, and managing the complexity of AI-driven operations are key areas that need to be addressed. The industry and academia must work collaboratively to overcome these challenges and fully realize the potential of Open RAN in the 6G landscape.

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