6G radio protocols: Architecting for tomorrow’s diverse connectivity needs

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 1, 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.

As an analogy, think of vehicles on a road. The user plane corresponds to the vehicles transporting goods, while the control plane acts like traffic regulations, ensuring orderly and efficient movement. Radio resource management (RRM) plays a role similar to traffic management and urban planning, optimizing flow and resource allocation. In this analogy, the physical layer is the chassis connecting the vehicle with the road, while the spectrum serves as the road on which everything travels.

As we’ve seen, the UP protocols are responsible for handling the actual data transmission between devices. From the introduction of packet data services in 2G up to 5G-Advanced, a one-size-fits-all UP design has been specified to serve all possible use cases and device types. As we move to 6G, this singular UP design is proving less efficient. The network is increasing in complexity and diverse use cases will demand dynamic service provisioning, extreme data rates, and high reliability. Additionally, we are expecting the deployment of heterogenous networks, AI-native programmable networks, and support for network slicing and the distributed cloud, all the while reducing power consumption and computational overhead.

A singular UP design inevitably adds complexity to both standards specification and implementation, ultimately requiring compromises. For instance, cost- and power-efficient optimizations for low-bit-rate services may differ from those needed for high-bit-rate services like XR.

For 6G, we believe a modular UP design should be adopted to provide native support for various device types and user equipment (UE), optimized for different purposes. We see such a modular design consisting of the following two main components:

• An anchor user plane module for the lowest capability devices and control plane to anchor the existing services with known operation
• Flexible user plane modules for additional processing and implementation-friendly scalability targeted for higher capability devices such as immersive communication.
   

Figure 2. A modular user plane design to support various device types 

The proposed modular UP design can greatly simplify overall processing, support various UE types, and allow for hardware-based parallelization of processes, all of which can help to reduce power consumption for high-bit-rate services without compromising low-bit-rate performance.

In 6G, we anticipate that a large degree of application awareness will be essential across the entire end-to-end system to ensure a reliable, high-quality user experience. Unlike many of today’s use cases, future applications like XR and remote vehicle or drone control require guaranteed performance.

While discussions often center on new spectrum bands, new mechanisms, and user plane enhancements, CP procedures are frequently overlooked. However, as previously explained, they remain fundamental to the system. Without CP protocols, the UP cannot manage the data flow. Figure 3 illustrates this concept with a (virtual) cogged mechanism, where the CP dynamically adjusts gears to provide the necessary support based on the active UP modules, all of which acts as a scalable radio processing unit (RPU), as discussed in our vision for a modular UP design.

Figure 3. Control plane supports a modular user plane

The proposed modular UP design necessitates a more efficient and modular CP to function seamlessly. As the earlier analogy pointed out, vehicles transporting goods (UP) require traffic regulations (CP) for smooth and efficient movement. The current CP protocols are overly monolithic in their design, inflexible and complex. To support the agility, scalability and modularity required for efficient 6G UP operation, CP protocols should be redesigned with these principles in mind.

To achieve this, we need to focus on two key components: radio resource control (RRC) and radio resource management (RRM). While RRC serves as the mechanism for the network to communicate UP configuration to user equipment (UEs), RRM is the protocol that translates these intentions into actionable instructions for RRC.

RRC manages the communication between the network and the user equipment (UE). Some key functions include:

• Establishing and maintaining the radio link between the UE and the network
• Radio resource allocation by exact assignment and allocation of radio resources, such as bandwidth and power to the UE, to ensure optimized efficiency
• Configuration and control by sending configuration parameters to the UE, defining how it should operate on the network, and setting such things as data rates, strength of the UE's radio signal, and frequency bands among others.

A modular RRC design will enable more efficient signaling by allowing each component function to operate independently for specific aspects of UP operation. This modularity also allows network deployments to provide configurations efficiently, without requiring users to be aware of the underlying network architectural split. It enhances user experience and simplifies network management.

Radio resource management (RRM), which includes mobility, link management, QoS, UP configuration, failure recovery procedures, and performance monitoring, plays a critical role in optimizing the performance of cellular networks. RRM must be scalable to handle variable traffic loads, support diverse use cases and optimize spectrum efficiency. As networks expand and densify, the use of massive MIMO (multiple input, multiple output) and small cells requires more efficient coordination. With the growth of IoT and M2M communications, the RRM is additionally being challenged with more difficult scheduling, interference mitigation, and power control issues as the number of devices expands exponentially.

The RRM interacts with the CP to allocate and schedule radio resources according to network policies and real-time conditions, balancing fairness between users while optimizing radio resources for both transmissions and receptions.

In 6G, we believe an enhanced QoS framework will introduce greater flexibility and adaptability. Rather than relying on agreed-upon, hard QoS guarantees that the network is required to fulfill, regardless of the network conditions, soft QoS guarantees will allow services to specify acceptable ranges for QoS attributes such as data rate, latency and packet error rate, offering a more dynamic and efficient approach to managing network performance based on current conditions.

This new QoS framework will enable operators to meet minimum QoS requirements while aiming to provide QoS at or near the target values of specified attribute ranges, enhancing the overall quality of experience (QoE) for end users. The envisioned operation with soft QoS guarantees can be achieved by adding a new resource type such as adaptive QoS, along with the necessary operational enhancements within the existing 3GPP QoS framework.

Another important aspect in RRM is mobility management, which is essential to ensure seamless connection of a user while moving or stationary. 6G aims to enhance mobility by refining the handover procedures. 6G will build upon the various mobility solutions standardized by 5G-Advanced, integrating the most effective components into a unified framework. This will create a robust baseline for 6G mobility, leveraging the strengths of existing technologies to achieve an improved handover experience.

Being a fundamentally limited resource, spectrum is the first consideration for evolving wireless communications. Simply put, nothing happens without it. It is also well-known that not all MHz are born equal and that certain frequencies are more attractive for mobile operators due to their superior coverage and capacity properties. Not coincidentally, these prime spectrum assets are currently in use by 5G networks.

Consequently, provisions to ensure that 5G and 6G can coexist efficiently are essential for a fast and cost-effective rollout of new services. In practice, this means that the underlying UP and CP protocols must include native mechanisms to support Multi-RAT Spectrum Sharing (MRSS) and allow 6G to operate alongside 5G in various deployment scenarios.
 

Figure 4. MRSS and a powerful spectrum aggregation framework widen the 6G digital highway 

Unconstrained access to existing spectrum is key for 6G’s spectrum migration and commercial success. However, the gamut of emerging mobile applications will also require access to the new express traffic spectral lanes. This underscores the mission of the 6G protocol suite: the 6G-Spectrum Aggregation (SA) framework must adapt seamlessly to varying bandwidth combinations and traffic types, such as live streaming and gaming, with simplicity and flexibility at its core. Efficient SA relies on quick activation and deactivation of additional bandwidth resources while maintaining streamlined configuration and signaling. It can accomplish this, for example, by using a common configuration for multiple carriers and allowing dynamic enablement of SA for some carriers. Clearly defining device behavior in different modes can also help conserve power, especially during periods of low traffic. Knowing what device capability can be supported from the first release of 6G is critical to achieving efficient spectrum aggregation.

We’ve highlighted key areas where innovation is essential to support the emerging use cases, focusing on user plane and control plane protocols, as well as spectrum management technologies. Ultimately, the 6G system must meet the ambitious demands of the envisioned next-generation applications. We are excited to see how the telecom industry will shape 6G within 3GPP in the coming years. Meanwhile, stay tuned as we continue to explore these critical aspects of 6G networks and discover how they will redefine the way we connect and communicate.