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SCF’s first open 5G interface will lower barriers to densification

The open, multivendor RAN has been discussed for over a decade, but not achieved outside of WiFi (and only imperfectly there). In 5G, traditional operators and emerging private service providers and neutral hosts are more urgently focused on the issue because of the twin needs to reduce the cost of building another new network, and to be far more flexible in supporting specific industries and use cases.

Much of the early work is being done in small cells – they have a broader ecosystem anyway, and most at-scale deployments will be greenfield and so less challenging in terms of opening up or migrating a legacy network.

A breakthrough came this week with Small Cell Forum’s release of its 5G FAPI (functional application programming interface) specifications, the first in a series of 5G APIs which extend work already done for 3G and 4G small cells and their chipsets.

The latest API addresses the fact that 5G networks will be very different from their predecessors. They will often be disaggregated, with baseband processes split in various ways between the cloud and the cell site, depending on the services to support. They will be very dense, with large numbers of small cells enhancing capacity and coverage to enable a new class of applications. And if the operators have their way, they will be far more open.

Open APIs at every level – chip, network and application – are central to the vision that, in the 5G era, mobile platforms will be interoperable, mixing elements from multiple vendors and from open source projects, and will lower barriers to entry both for suppliers and network deployers.

This means the first in SCF’s family of 5G FAPI common interfaces is very much in the zeitgeist. FAPI is a Forum initiative, first established for 3G small cells, to provide a common API around which suppliers of platform hardware, platform software and application software can compete.

The first 5G FAPI release is the 5G PHY API. This is an extension of the original 3G and LTE FAPI PHY API specifications, which are incorporated in most small cell chips today. The PHY API specs define internal interfaces between the two chip-level network layers, the PHY and the MAC (medium access control). The former links the radio channel to the MAC, while the MAC supports functions like error correction and scheduling. Three logical interfaces are defined:

  • P5 – the PHY mode control interface
  • P7 – the main data path interface
  • P19 – the RF control interface

These open interfaces allow for interoperability between the PHY and the MAC – in 3G, 4G and now 5G – in a small cell design, and they have secured backing from a wide range of influential companies, including both Intel and Qualcomm, as they address a current gap in 3GPP standards.

Small Cell Forum believes that the 5G FAPI family, which will roll out over the coming year, will play an important role in bringing to cellular the open economics and scalability that are seen in markets where open PHY/MAC interfaces prevail, notably Ethernet.

In turn, that will help to lower the cost and time to market for small cell designers, and enable the scale economics which will be essential to enable 5G requirements for affordable densification and ubiquitous coverage.

In subsequent releases, SCF will address other 5G FAPI interfaces, including those to support a split MAC and PHY in a disaggregated small cell network (5G Network FAPI); plus releases addressing the Front End and Network Monitor user cases.

Innovations likes these are very important to operators. In a survey of over 70 MNOs, conducted by Rethink Technology Research, over two-thirds of respondents said that opening up their supply chain, and achieving multivendor systems, were among their top 10 commercial goals for 5G. However, over half of those operators added that they lacked confidence in being able to achieve the goal before 2025.

The 5G release of nFAPI (network FAPI) will be even more significant to open networks. It will extend the concept to virtualized small cell networks and provide an interface between the remote radio unit (RRU), and a centralized baseband unit (BBU), on which some or all of the baseband functions are virtualized. In indoor enterprise networks – the focus of many challenger operators focused on enterprise, industrial, IoT and edge services – nFAPI will be essential to support the most common architecture, a group of cells controlled by a central, virtualized unit.

To date, deployment of this virtualized RAN (vRAN) has been impeded partly by the lack of a fully unified fronthaul interface between the RRU and BBU. SCF has already made considerable progress on this, in a small cell context, and is also addressing aspects of the interface which are not a central focus for other initiatives (FAPI itself, for instance, fills a key gap in 3GPP standards).

Densification absolutely requires an open ecosystem to make deployment of large numbers of cells cost-effective. These interfaces are the enabler of the innovation and price competition that comes when operators can select equipment from many suppliers.

Once standard interfaces between the radio and the controller are supported, along with open baseband virtual network functions (VNFs), the economic argument for densification will be far stronger, and a key disadvantage vis-à-vis WiFi (its open ecosystem) will be removed.

nFAPI moved beyond the SCF when an open initiative, the open-nfapi project, was announced in 2017. Led by Cisco, the project worked to implement the 4G nFAPI disaggregated base station architecture in a set of libraries and simulators. As Mark Grayson, distinguished consulting engineer at Cisco, wrote in a blog post, Cisco was drawn to nFAPI because it supports active RAN sharing, and therefore neutral host or multi-operator small cell networks – which will be essential for many enterprise and vertical industry deployments in the 5G era, which cannot be tied to a single MNO.

“We concluded that the Small Cell Forum’s multivendor nFAPI split architecture, together with its neutral host management model, offered a new approach to active sharing of an LTE RAN based on a multivendor CU-DU implementation,” wrote Grayson.

He highlighted the challenges facing many would-be open standards, particularly in getting sufficient real world adoption when there are many open, and even open source, projects to choose from. He continued: “Even though Cisco has licensed the nFAPI libraries under the permissive Apache 2.0 licence – enabling them to be integrated by the widest possible set of stakeholders, including closed source proprietary RAN products – we do not underestimate the real challenges in getting such capabilities implemented.”

To help address the issue, Cisco joined the Open Air Interface (OAI) open source initiative and said it would use its membership to demonstrate how to integrate the open-nFAPI libraries into an existing LTE RAN protocol stack; the integration between the lower nFAPI libraries and the PHY layer, implemented on a software defined radio platform; and the integration between the upper nFAPI libraries and the MAC and RRC layers. This will help to lower the barriers to active RAN sharing in small cell environments.

Such efforts show how specifications like nFAPI will certainly be working in a friendlier environment in the 5G era, with the whole industry shifting towards a more open focus, and with enterprise interest in 5G driving operators towards a more neutral host approach.

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