Initiatives toward Open RAN

2.1 Three elements of Open RAN

Open RAN consists of the following three elements (Fig. 1):

Fig. 1. Main elements of Open RAN.

2.2 Effects of Open RAN

The base-station components for Open RAN can be separated into three units—radio unit (RU)^*3, distributed unit (DU)^*4, and central unit (CU)^*5—each of which can be connected via standardized interfaces. This means that telecom operators can free themselves from vendor lock-in, shorten the time to commercial deployment, and adopt a better equipment configuration to provide optimized services to consumers. The virtualization of a RAN means that costs can be decreased through the use of general-purpose hardware and that flexibility and scalability can be enhanced. Envisioning an increasingly complex mobile network, manual operation of the network as has been the practice will become all the more difficult, but this problem can be solved through the intelligent control of a RAN.

4.1 NTT DOCOMO multi-vendor network initiative

In conventional networks, a single vendor provides base stations for both baseband signal processing units (BBUs) and RUs in a single-vendor configuration. One advantage of having a single vendor is that the operator can rely on that vendor for a range of services from deployment to maintenance in a one-stop manner. Since the interfaces between equipment are vendor-proprietary interfaces, a disadvantage is that the operator cannot upgrade to equipment from other vendors, decreasing flexibility.

NTT DOCOMO took the lead in achieving a multi-vendor network before the coming of 5G. Specifically, NTT DOCOMO enabled connections between different vendors by prescribing original interfaces between the BBU and RU, achieving a flexible base-station configuration (Fig. 2). Since there were now multiple options for selecting base-station-equipment vendors, it became possible to select the most optimal vendors in terms of cost and performance, lowering the cost of deploying equipment.

Fig. 2. Single-vendor and multi-vendor networks.

4.2 NTT DOCOMO Open RAN initiatives in 5G

When launching 5G commercial services in 2020, NTT DOCOMO was the first in the world to achieve Open RAN in a commercial network using interfaces conforming to O-RAN ALLIANCE standards. All 5G base stations that NTT DOCOMO is now rolling out conform to O-RAN ALLIANCE fronthaul and X2 specifications. Since this is Open RAN, NTT DOCOMO has been gradually expanding equipment vendors and the variation in equipment ever since the 5G pre-commercial service period.

Specifically, NTT DOCOMO has come to support millimeter-wave (mmW)^*9 capabilities, sub-6^*10 inter-band carrier aggregation (CA)^*11, and stand-alone (SA)^*12 operation. While only small RUs (SRUs) for small-cell use of both the antenna-separated and antenna-integrated types were deployed at the launch of 5G commercial services, NTT DOCOMO has since expanded the equipment lineup to include regular power RUs (RRUs) for macrocell use and 5G fronthaul multiplexers (FHMs)^*13. NTT DOCOMO is therefore adopting new vendors for CU/DU and RU.

NTT DOCOMO considers its multi-vendor initiative between equipment through open interfaces that it has been implementing as being its first step in Open RAN deployment. As the next step, it will promote studies on achieving the remaining two elements of Open RAN described above, i.e., virtualization and intelligent control (Fig. 3). This will be achieved through NTT DOCOMO’s 5G Open RAN Ecosystem initiative described in the next section.

Fig. 3. NTT DOCOMO Open RAN deployment steps.

5.1 Founding of 5G Open RAN Ecosystem

In February 2021, NTT DOCOMO established the 5G Open RAN Ecosystem together with 12 other companies with the aim of accelerating the adoption of Open RAN by operators. Through this ecosystem, NTT DOCOMO aims to accelerate vRAN testing. It also intends to make state-of-the-art RAN into a commercial package based on the requirements of mobile operators that are studying the introduction of Open RAN and to offer Open RAN deployment, operation, and maintenance services. By making use of the Open RAN knowledge it has built up over many years, NTT DOCOMO will work to promote the 5G Open RAN Ecosystem and provide high-quality and flexible networks.

5.2 Open RAN problems and solutions

As described above, Open RAN has advantages but also has a number of problems that must be solved. We give an overview of the main problems in deploying Open RAN and describe how the 5G Open RAN Ecosystem plans to solve these problems.

(1) Performance

In vRAN, one of the elements of Open RAN, general-purpose servers are used as hardware, but when running RAN applications on such servers, radio characteristics, accommodation capacity, etc. may deteriorate. As a solution to this problem, end-to-end (E2E) vRAN testing using an accelerator is being promoted within the 5G Open RAN Ecosystem to achieve performance 2–3 times that of current performance. The 5G Open RAN Ecosystem Whitepaper including these performance targets was released in June 2021, so we ask the reader to refer to that material as well [2].

(2) Integration

In Open RAN, base-station components can be separated, but an issue that arises is how to integrate those components. In vRAN, hardware and software can be separated, so it is believed that components from different vendors can be integrated and provided accordingly. In this case, the number of interfaces needed for integration increases compared with conventional RAN. As a solution to this problem, an Open RAN testing environment, which is described in the next section, is being set up within the 5G Open RAN Ecosystem to provide multi-vendor integration testing to overseas operators who are testing and operating RAN.

(3) Other issues

Costs, automation, and equipment rollout are also problems in Open RAN, and NTT DOCOMO solutions to these problems are described in other feature articles in this issue [3, 4].

5.3 Sharing of Open RAN testing environment

As described above, using multi-vendor products means that products must be appropriately selected and integration testing conducted. However, test cases increase as the number of products and interfaces increases, and having each operator prepare an environment tailored to its testing needs is inefficient.

Against the above background, NTT DOCOMO set up an open testbed in Japan as part of the 5G Open RAN Ecosystem. One of the main functions of this testbed is enabling overseas mobile operators to remotely control it as if it was a testbed in their own laboratories. This testbed can be connected to an operator’s core network^*14, making it easy to conduct tests of vRAN equipment using products from multiple vendors. Thus, an operator can dramatically decrease the time and costs incurred by testing, so it is thought that this testbed can contribute to the timely deployment of Open RAN. The testbed began operation in summer 2021. Products from 5G Open RAN Ecosystem partners have been provided since October 2021 and testing has begun. This testbed was released in February 2022 as a shared open laboratory providing an environment for operators around the world to test vRAN while based overseas. NTT DOCOMO is using this shared open laboratory to deepen its ties with a wide range of stakeholders including other operators and to contribute to the establishment of technologies and expertise toward the early diffusion of an open network, especially Open RAN and vRAN, that can respond flexibly and rapidly to diverse needs.