Open and interoperable networks have always been a key goal of the mobile networking community. 3GPP standards provide a baseline of course, and at the start of the 5G era, there is unprecedented focus in driving open interfaces at every layer of the network from the radio to the applications. Amid all the excitement about virtualized networks, it can be easy to overlook the most foundational interfaces of all – those between different components in a chipset. This is the critical issue that Small Cell Forum addresses with the latest members of 5G FAPI, its landmark family of chip-level interfaces.

FAPI is an API (application programming interface) to support interoperability between hardware components that implement 3GPP physical layer (PHY) functions, and the higher layer functions that are implemented in software stacks.

Contribution to open standards and interoperability has been at the heart of Small Cell Forum’s work since its foundation and these are more urgently needed than ever in the 5G era. The diversification of small cell use cases, form factors and spectrum bands is attracting new innovators into the supply chain, but also risks fragmentation if all the organizations cannot work within a common framework. This is vital for companies that produce components for system-on-chip (SoC) and circuit board platforms – complex, highly integrated sets of components that are crucial for making small cells compact, power-efficient and affordable.

But if an SoC cannot be assembled using components from different vendors, and work with software stacks from yet more suppliers, the OEMs and operators will miss out on many potential innovations and technology combinations. The goal of the FAPI family is to provide the open platform on which all component makers can develop and work together.

FAPI’s adoption in the vast majority of 2G, 3G and 4G products highlights the strong support that the platform has always had from chip providers, and in 2019 and 2020, the first two releases of 5G specs were published, with similarly enthusiastic backing from the industry. Now SCF is significantly expanding that family with four new 5G specifications, all designed to drive innovation and price competition for base station components.

Although FAPI was, of course, conceived with small cells in mind, it is increasingly important, in the 5G era, for vendors to be able to develop a whole range of base stations, from ultra-small to macro, that have common interfaces in the physical layer. 5G FAPI specs can, then, be applied to any class of base station to enable flexibility and interoperability.

This third release highlights how modern standards must evolve constantly to meet the changing requirements of 5G networks and users. Key elements of the latest release includes even tighter integration between different specs in the family to improve flexibility and efficiency. The result of intensive work and cooperation between many SCF members from across the ecosystem, they provide common logical interfaces for the main components of a 5G small cell.

The four specs are:

• ‘5G FAPI: PHY API’ – main data path (P7) and PHY mode control (P5) interface [SCF222]
• ‘5G FAPI: RF and Digital Front End Control API’ – (P19) for Front end Unit control [SCF223]
• ‘Network Monitor Mode API’ – (P4) for 2G/3G/4G/5G [SCF224]
• ‘5G nFAPI Specification’ [SCF225]

These specs will be crucial in the wider industry quest for open, interoperable networks that span all sizes of base station and many spectrum bands and architectures. Without agreed interfaces between components and software stacks down at the physical layer, it will be impossible to achieve a fully multivendor platform, and the innovation, competition and supplier diversity that enables. The ability to build on the mass deployments of previous FAPI generations will provide a high level of confidence in the new specs, and these will play a vital role in the progress towards the open RANs of the 5G era.