What exactly is 5G Open RAN?

Traditionally, the proprietary network equipment used in cellular radio access networks (RAN) comes from a small number of network equipment manufacturers. However, with the development of the wireless communications industry, this model can no longer meet the needs well. Instead, it has caused mobile network operators (MNOs) to be "locked" into proprietary RANs and restricted in many ways.

Later, the rise of software-defined networking (SDN) and network function virtualization (NFV) brought greater flexibility and cost efficiency to the network core. However, RAN is still a single-vendor system.

In recent years, mobile network operators around the world have been pushing for 5G to adopt the Open RAN architecture (also known as O-RAN). The Open RAN architecture is expected to make 5G networks more flexible while promoting innovation, but it also brings additional technical complexity and testing requirements.

This article explores the advantages, principles, structural components of 5G Open RAN architecture, and the importance of consistency and interoperability testing of Open RAN components.

O-RAN Alliance

The O-RAN Alliance, which consists of 25 MNOs and nearly 200 contributing organizations from across the wireless landscape, has been developing open, intelligent, virtualized and interoperable RAN specifications since its founding in 2018.

The Telecom Infra Project (TIP) is an independent consortium of hundreds of members from across the infrastructure equipment space that maintains an Open RAN project to define and build 2G, 3G and 4G operational solutions based on common hardware-neutral hardware and software-defined technologies. Earlier this year, TIP also launched the Open RAN Policy Coalition, an independent group under TIP dedicated to promoting policies to accelerate and stimulate the adoption and innovation of Open RAN technologies.

In February 2020, the O-RAN Alliance and TIP announced a collaboration agreement to jointly develop interoperable Open RAN technologies, including information sharing, reference specifications, and joint testing and integration efforts.

The O-RAN Alliance defines the O-RAN architecture for 5G and defines a 5G RAN architecture that divides the RAN into several parts, called the O-RAN Radio Unit (O-RU), Distribution Unit (O-DU), and Centralized Unit (O-CU). The interfaces between these units are defined by open, interoperable standards that enable mobile network operators to mix and match RAN components from multiple different vendors for the first time. The O-RAN Alliance has created more than 30 specifications, many of which define the interfaces between the units.

Interoperable interfaces are a core principle of Open RAN

Interoperable interfaces allow smaller vendors to quickly introduce their own services. They also enable mobile network operators to adopt multi-vendor deployments and allow them to customize their networks to meet their own needs. Mobile network operators can choose the products and technologies they want to use in their networks, regardless of the vendor. As a result, mobile network operators will have the opportunity to build more robust and cost-effective networks.

By creating a more competitive supplier ecosystem for MNOs, enabling smaller suppliers to quickly launch services, it will also improve cost efficiency, thereby reducing the cost of 5G network deployment. Operators who were previously "locked" in proprietary RAN had limited negotiating power, while an open environment makes competition more fair, stimulates market competition and reduces costs.

Innovation is another important advantage of Open RAN. The development of open interfaces stimulates innovation, allowing smaller and more agile competitors to develop and deploy breakthrough technologies, which not only creates the potential for more innovation, but also accelerates the development of breakthrough technologies, as small companies tend to move faster than large companies.

A notable example is fronthaul, the transport network of the C-RAN (Cloud-RAN) architecture, which connects the RRHs at the cell site to the baseband units (BBUs) of the centralized baseband controller at some distance. In the O-RAN Alliance reference architecture, the IEEE Radio over Ethernet (RoE) and open enhanced public radio interface (eCPRI) protocols can be used on top of the O-RAN fronthaul specification interface to replace the bandwidth-intensive and proprietary CPRI.

By using Ethernet, operators can adopt virtualization technology and use off-the-shelf network equipment to switch front-end transmission between physical nodes. Virtualized network elements allow for more customization.

The following figure shows the layers of the wireless protocol stack and the main architectural components of 4G LTE RAN and 5G Open RAN. The CPRI data rate between the BBU and RRH is sufficient for LTE due to the total bandwidth required and the fewer antennas involved. In 5G, massive multiple-input/multiple-output (MIMO), higher data rates, and an increase in the number of antennas mean that more data is passed back and forth on the interface. Also, note that the main components of the LTE RAN, the BBU and RRH, are replaced in the O-RAN architecture by the O-CU, O-DU, and O-RU, as described below.

The main components of 4G LTE RAN and 5G O-RAN show that the BBU is replaced by the O-RU, O-DU and O-CU, which provides greater flexibility in network architecture.

Open RAN Architecture

As mentioned above, a core principle of the Open RAN architecture is openness, particularly in the form of open, interoperable interfaces that enable MNOs to build RANs with technology from multiple vendors. The O-RAN Alliance is also committed to integrating open source technologies when appropriate and maximizing the use of off-the-shelf general-purpose hardware and commercial silicon while minimizing the use of proprietary hardware.

As stated by the O-RAN Alliance, the second core principle of Open RAN is to integrate higher intelligence. The increasing complexity of networks requires the combination of artificial intelligence (AI) and deep learning to create self-driving networks. By embedding AI into the RAN architecture, MNOs can increasingly automate network functions and minimize operational costs. AI also helps MNOs improve network efficiency through dynamic resource allocation, traffic control, and virtualization.

The three main components used for 5G O-RAN are O-CU, O-DU and O-RU.

• The O-CU is responsible for the Packet Data Convergence Protocol (PDCP) layer of the protocol.
• The O-DU is responsible for all baseband processing, scheduling, radio link control (RLC), medium access control (MAC) and the upper part of the physical layer (PHY).
• The O-RU is the component responsible for the underlying physical layer processing, including the analog components of the radio transmitter and receiver.

Among them. Two components, O-CU and O-DU, can be virtualized. O-CU is a component of RAN, which is always centralized and virtualized, and usually runs on x86-based computers. O-DU is usually a virtualized component, and virtualization of O-DU requires some hardware acceleration help in the form of FPGA or GPU. Currently, the prospect of O-RU virtualization is still far away, but a working group of the O-RAN Alliance is planning to implement white box radio using off-the-shelf components. White box can build O-RU without proprietary technology or components.

Interoperability testing requirements

While Open RAN offers many benefits to MNOs, making it work properly means that rigorous testing requirements must be adopted. Open RAN and distributed RAN require that each component of the RAN be tested individually for compliance with standards and that combinations of components be tested for interoperability.

Why test for both conformance and interoperability? In the O-RAN era, it is imperative to determine whether components independently comply with the corresponding standards and whether they work together as a unit. Skipping the conformance test step and only performing interoperability testing is like an aircraft manufacturer building an aircraft using untested parts and then only checking whether it can fly.

Conformance testing is usually performed first to ensure that all components meet the interface specifications between units. Testing each component individually requires test equipment that simulates the surrounding network to ensure that the component meets all functions of the interface protocol.

Conducting component conformance testing independently has several benefits. On the one hand, conformance testing allows for negative testing to check the component's response to invalid inputs, which is not possible with interoperability testing alone. In addition, conformance testing enables test engineers to exercise protocol features that cannot be controlled during interoperability testing.

The conformance test specification developed by the O-RAN Alliance Open Fronthaul Interface Working Group consists of multiple parts with many test categories to test almost all 5G O-RAN elements.

Interoperability testing for 5G O-RAN is similar to interoperability testing for 4G RAN. However, conformance testing for 5G O-RAN is very different and requires more equipment. The following figure shows the test equipment radio in the O-RAN conformance test specification.

Testing for O-RAN conformance specifications requires emulators for each part of the Open RAN 5G network

Open RAN is gaining momentum and delivering significant benefits in terms of improved efficiency, reduced costs, and increased innovation. However, testing and validating multi-vendor Open RAN is not an easy task.