Toward Open and Intelligent Wireless Networks
Initiatives toward Open RAN

The 5G mobile network will need to support a wide range of services compared with past networks. To meet this need, it will be necessary to construct a flexible network that can accommodate all kinds of services. Open RAN can make this a reality. This article provides an overview of the Open RAN concept, presents the current state of the O-RAN ALLIANCE that promotes Open RAN standards, and describes the 5G Open RAN Ecosystem, a new NTT DOCOMO Open RAN initiative.

1. Introduction

To respond in a more suitable manner to customer needs, all mobile operators are required to add new network equipment and replace existing equipment to expand network functions on a continual basis. The network will therefore have to be capable of such scalability in a flexible and agile manner. To this end, NTT DOCOMO has been promoting open interfaces that will enable operators to add new equipment or components or replace existing ones whenever needed. In this way, an operator can freely select and adopt optimal solutions from a variety of vendor products.

This article provides an overview of the Open Radio Access Network (RAN)^*1 and touches on the current state of the O-RAN ALLIANCE^*2. It also describes the 5G Open RAN Ecosystem launched by NTT DOCOMO to globally accelerate Open RAN and the approach that it takes to solve issues related to performance, integration^*3, and interoperability testing in Open RAN.

2. Open RAN

1) Three elements of Open RAN

The open radio access network (hereinafter referred to as “Open RAN”) can be broadly divided into the following three elements (Figure 1):

1. (1) Open interfaces that combine RAN equipment from a variety of vendors
2. (2) Virtualization (i.e., virtualized RAN (vRAN)^*4) that enables hardware and software inside RAN equipment to be separated
3. (3) Intelligent control that optimizes and automates RAN operation

2) Effects of Open RAN

Open RAN can be separated into multiple components—Radio Unit (RU)^*5 and Distributed Unit (DU)^*6 and Central Unit (CU)^*7)—each of which can be connected via standardized interfaces. For telecom operators, this means that they can free themselves from vendor lock-in^*8, shorten the time to commercial deployment, and adopt a better equipment configuration to provide optimized services to consumers. In addition, the virtualization of RAN means that costs can be decreased through the use of general-purpose hardware and that flexibility and scalability can be enhanced. Furthermore, envisioning an increasingly complex mobile network in the future, manual operation of the network as has been the practice so far will become all the more difficult, but this problem can be solved through the intelligent control of RAN.

In short, these three elements are essential to meeting the demand for a more diverse RAN industry and supporting an increasingly complex mobile system.

Figure 1 Main elements of Open RAN

3. Current Status of Open RAN Standardization

In February 2018, NTT DOCOMO joined up with other industry operators to establish the O-RAN ALLIANCE as an industry organization with the aim of promoting open and intelligent RAN. Since an overview of the O-RAN ALLIANCE has already been given in a past article of the NTT DOCOMO Technical Journal [1], we here introduce new initiatives at this organization from 2019 up to the present.

To begin with, there were 19 operators and 55 vendors and institutions serving as members of the O-RAN Alliance as of 2019, but this has expanded to 31 operators and 294 vendors and institutions as of February 22, 2022.

Additionally, while only fronthaul^*9 specifications from Work Group 4 (WG4) had been released as of 2019, new specifications had been released from each WG and Focus Group (FG)^*10 as of February 22, 2022. Among these, the following introduces newly released specifications from WG4 and WG5 for which NTT DOCOMO has been serving as co-chair.

In WG4, in addition to fronthaul specifications consisting of the Control，User and Synchronization Plane (CUS-Plane)^*11 and Management Plane (M-Plane)^*12, the following three types of specifications have been newly released with version upgrades already being made.

• Test specifications
  Open Fronthaul Conformance Test Specification
  Fronthaul Interoperability Test (IOT) Specification
• Cooperative Transport Interface (CTI) specifications
  Fronthaul CTI Transport Control Plane Specification

In constructing an Open RAN, testing must be performed to confirm whether equipment from different vendors conforms to the interface specifications of the O-RAN Alliance and whether multi-vendor connections can actually be made. In other words, it is not just fronthaul specifications that are needed but the above test specifications as well.

In WG5, the following specifications have been newly released with version upgrades being made.

• X2^*13 specifications
  New Radio (NR)^*14 Control plane (C-plane)^*15 profile
  NR User data plane (U-plane)^*16 profile
• Transmission path specifications
  Transport Specification
• Monitoring control specifications
  O1 Interface specification for O-CU-UP and O-CU-CP
  O1 Interface specification for O-DU
• IOT specifications
  Interoperability Test Specification

Furthermore, to make the vision of an open and intelligent RAN a reality, a number of new WGs and FGs have been established, so we again summarize WG/FG activities in Table 1.

The WGs and FGs that have been added since the founding of the O-RAN Alliance in 2018 are WG9, WG10, Security FG (SFG), Test & Integration FG (TIFG), Open Source FG (OSFG), and Standard Development FG (SDFG).

Security concerns, in particular, have been raised as a key issue in Open RAN, and at the O-RAN Alliance, SFG is conducting security risk analyses and countermeasure studies.

O-RAN ALLIANCE technology study groups

	WG/FG	Scope of Study
WG1	Use Cases and Overall Architecture	Architecture, use cases, slicing, demonstrations
WG2	Non-real-time RIC and A1 Interface	Non-RT RIC, A1, rApp
WG3	Near-real-time RIC and E2 Interface	Near-RT RIC, E2, xApp
WG4	Open Fronthaul Interfaces	Fronthaul
WG5	Open F1/W1/E1/X2/Xn Interface	Interoperability profiles such as X2, Xn, and F1, O1 to DU and CU
WG6	Cloudification and Orchestration	O-Cloud，vDU/vCU，AAL，O2
WG7	White-box Hardware	Mainly RU-hardware reference design
WG8	Stack Reference Design	DU/CU software-architecture reference design
WG9	Open X-haul Transport	Transport equipment, transport-network control/maintenance protocol
WG10	OAM for O-RAN	SMO，O1 (overall coordination)
SFG	Security	Security risk analyses and countermeasure studies for Open RAN
TIFG	Test & Integration	Compile test specifications, Plugfest, OTIC, certification and badging processes
OSFG	Open Source	O-RAN Software Community
SDFG	Standard Development	Standardization strategies, interface to other Standard Development Organizations (SDOs)

4. NTT DOCOMO Open RAN Initiatives

4.1 NTT DOCOMO Multi-vendor Network Initiative

In conventional networks, a single vendor provided base stations for both baseband signal processing units and radio units 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. On the other hand, since the interfaces between equipment are vendor-proprietary interfaces, a disadvantage is that the operator cannot upgrade to equipment from other vendors, which makes for a lack of flexibility.

NTT DOCOMO took the lead in achieving a multi-vendor network before the coming of the fifth-generation mobile communications system (5G). Specifically, we enabled connections between different vendors by prescribing original interfaces between the baseband signal processing unit and radio unit thereby achieving a base-station configuration with flexibility (Figure 2).

Additionally, 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 thereby lowering the cost of deploying equipment.

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. In addition, all 5G base stations that NTT DOCOMO is now rolling out conform to O-RAN Alliance fronthaul and X2 specifications. Furthermore, since this is Open RAN, we have been gradually expanding equipment vendors and variation in types of equipment ever since the 5G pre-service period (Figure 3).

Specifically, NTT DOCOMO has come to support millimeter Wave (mmW)^*17 capabilities, Sub 6^*18 Inter-band Carrier Aggregation (CA)^*19, and Stand Alone (SA)^*20 operation. Moreover, as to types of equipment, while only Small RU (SRU) for small-cell use of both the antenna-separated and antenna-integrated types were deployed at the launch of 5G services, we have since expanded the equipment lineup to include Regular power RU (RRU) for macrocell use and 5G FrontHaul Multiplexer (FHM)^*21 equipment. We are therefore adopting new vendors for CU/DU and RU too.

At NTT DOCOMO, we consider our multi-vendor initiative between equipment through open interfaces that we have been implementing up to now as being our first step in Open RAN. Now, as our next step, we will promote studies on achieving the remaining two elements of Open RAN described above, namely, virtualization and intelligent control (Figure 4). This will be achieved through NTT DOCOMO’s 5G Open RAN Ecosystem initiative described below.

Figure 2 Single-vendor network and multi-vendor network

Figure 3 NTT DOCOMO Open RAN initiatives to date

5. Promoting Open RAN Across the Globe

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 know-how it has built up over many years, NTT DOCOMO will work to promote the 5G Open RAN Ecosystem and demonstrate the company’s strengths in providing high-quality, flexible networks to the maximum.

5.2 Open RAN Issues and Solutions

As described above, Open RAN has its advantages, but it also has a number of issues that must be taken up. In the following, we give an overview of the main issues in deploying Open RAN and describe how the 5G Open RAN Ecosystem plans to address these issues.

Figure 4 NTT DOCOMO Open RAN deployment steps

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, the possibility exists that radio characteristics, accommodation capacity, etc. will deteriorate. As a solution to this issue, end-to-end (E2E) vRAN testing using an accelerator^*22 is being promoted within the 5G Open RAN Ecosystem with the aim of achieving various levels of performance 2 – 3 times that of current levels. In this regard, 5G Open RAN Ecosystem Whitepaper including these performance targets (Figure 5) 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 here is how to integrate those components. In vRAN, hardware and software can be separated, so it is thought 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 issue, an Open RAN testing environment as described below 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 issues in Open RAN, and NTT DOCOMO solutions to these issues are described in other special 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 performed. 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^*23 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. In other words, an operator can use this testbed regardless of its base of operations. In addition, this testbed can be connected to an operator’s core network^*24 making it easy to conduct tests of vRAN Equipment using products from multiple vendors. In this way, 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 the summer of 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 know-how toward the early diffusion of an open network and especially Open RAN and vRAN that can respond flexibly and rapidly to diverse needs.

Figure 5 Performance targets of 5G Open RAN Ecosystem

6. Conclusion

This article presented an overview of Open RAN, described the current state of the O-RAN ALLIANCE that is responsible for Open RAN standardization, and described the 5G Open RAN Ecosystem, a new NTT DOCOMO Open RAN initiative. As an Open RAN pioneer, NTT DOCOMO will continue to promote Open RAN that should be implemented not only for its own network but globally as well.

References

1. [1] S. Abeta, et al.: “O-RAN Alliance Standardization Trends,” NTT DOCOMO Technical Journal, Vol, 21, No. 1, pp. 38–45, Jul. 2019.
2. [2] NTT DOCOMO: “5G Open RAN Ecosystem Whitepaper,” Jun. 2021.
   https://ssw.web.docomo.ne.jp/orex/whitepaper/OREC_WP.pdf
3. [3] S. Mizuta et al.: “Initiatives toward vRAN,” NTT DOCOMO Technical Journal, Vol. 30, No. 1, pp. 14–26, June 2022.
4. [4] T. Katsuragawa et al.: “Initiatives toward Intelligent RAN,” NTT DOCOMO Technical Journal, Vol. 30, No. 1, pp. 27–36, June. 2022.

NTT DOCOMO technical journal Vol.30 No.1（Apr. 2022）