Clarifying the Confusing RAN Types and Nomenclature

Radio access networks have many descriptors that confuse the non-specialists. This includes open, virtual, cloud, centralized, distributed and cooperative RAN. Confusion is understandable because there are many inter-related aspects. Here, I outline the fundamentals elements that define RAN types and clarify the nomenclature.

The Three Pillars of RAN

Architecture, implementation and deployment are the three elements that define the RAN type.

Architecture indicates the RAN functional blocks and the interfaces connecting these blocks. 5G formally introduced the concept of centralized and distributed units which host the air-interface protocol stack. There are different interfaces that define the connectivity between distributed unit and the radio. On the other side, 3GPP defines the the connectivity between the centralized unit and the core network; an interface that is interoperable between different vendors’ solutions.

Architecture impacts performance and economic aspects of network deployment such as requirements for fiber. Hence, it is important for service providers.

Implementation describes how the vendor implements the air interface in hardware and software. The hardware implementation includes the type of processors and accelerators. The software implementation of the air interface protocol functions include virtual network functions and containers in addition to classical embedded software development.

Implementation defines the performance – such as throughput and reliability. Implementation also impacts power consumption and even physical parameters such as size and weight. Vendors complete mostly on implementation aspects citing performance and cost advantages for operators.

Deployment describes how the operators choose to deploy the radio access network by centralizing some air interface protocol functions and distributing others. For example, centralized RAN denotes centralizing the baseband function whereas distributed RAN does the opposite. 5G Since 5G defines split distributed and centralized units, it may contribute to the confusion since the physical placement of these units could be at the cell site or remote [I recall using hybrid RAN to describe some deployments!].

Deployment is a preference of the service provider to match their network infrastructure capabilities and requirements. Service providers would weigh their options against their future service roadmap and technology strategy.

Nomenclature Map

I would map the RAN nomenclature as follows:

• Centralized and Distributed RAN: refers to deployment.
• Open RAN: refers to architecture; specifically to open interfaces. The definition could include HW/SW disaggregated implementation which is a point of disagreement in industry. New equipment vendors would include disaggregation because it differentiates their solutions, while incumbents would exclude it.
• vRAN: Virtual RAN refers to implementation.
• Cloud RAN: This would be vRAN where baseband is centralized. This is the original concept from CMCC and Intel.

O-RAN is an alliance of companies defining interface standards. It does not refer to any particular type of RAN.

A Historical Perspective

RAN architecture is not static and has been evolving over the decades to optimize many factors that include operational efficiency and cost, performance, security and deployment requirements.

The first attention to “Cloud RAN” came following the work China Mobile Research Institute conducted in the early 2010’s. Their October 2011 white-paper “C-RAN: The Road Towards Green RAN” provided some of the first quantitative and qualitative analysis on what they described as “Cloud infrastructure RAN”. It saw wide distribution – perhaps because earlier versions were not available in English or widely distributed. As for the term “Cloud RAN” itself, either Intel or China Mobile, or both, first coined it as both collaborated in evolving RAN virtualization.

Concluding Thought

In my first research report on Cloud RAN in 2013, it was evident that prospects would benefit greatly from defining interoperable interfaces. The O-RAN Alliance – founded in 2018 – now assumes this activity. I mention this for a perspective on the timeframe it took the industry to move on a critical element.