5G FAPI specs updated with richer 3GPP feature set

5G FAPI suite continues to evolve to meet industry needs and 3GPP alignment

SCF has updated the 5G FAPI and 5G nFAPI specs, extending the feature sets – including NR positioning and 7.2x FH status awareness. This update demonstrates the ongoing commitment of the ecosystem to continually improve the standard, adding new features and maintaining existing ones.


SCF’s latest FAPI release; critical for 5G small cells to cross the chasm

In the early days of any new technology, there is a critical period that comes between the first deployments, and large-scale adoption. The time it takes to cross that chasm towards mass uptake is critical to the commercial success of the technology and its supporters. That is why a key role of industry bodies like Small Cell Forum is to create conditions that shorten the wait for critical mass as much as possible.

The 5G small cell network is now at that point. There are some cutting edge deployments of dense 5G networks in several parts of the world, and the industry is holding its breath for mass adoption. Small Cell Forum’s release of the first 5G members of its FAPI (functional application programming interface) family will be an important springboard for vendors to cross that chasm, as will a broader programme of work to define common 5G small cell platforms.

There are two fundamental enablers of a high volume market – common product specifications, from chips to software, which can be the foundation of a broad ecosystem; and fully open interfaces at each level, to ensure interoperability between components and products from different suppliers. Since its foundation, SCF has worked on many such common specifications, including the FAPI open interfaces, which allow for interoperability between two chip-level network layers, the PHY (physical) and the MAC (medium access control) in a small cell design – in 3G, 4G and now 5G.

With support across the small cell semiconductor market, FAPI enables a level of openness that has been near-impossible in the macro RAN to date,  allowing operators to combine elements from different suppliers to achieve the best solution for their use case, and to introduce significant price competition into the ecosystem. This encourages competition and innovation, lowering barriers to entry for suppliers because common interfaces reduce the level of customized engineering required.

The latest FAPI specifications are the result of intensive activity by SCF’s TECH working group, which includes representatives from across the range of small cell vendors, component providers and operators. They add 5G capabilities to the widely deployed 3G and 4G specs, and  provide common logical interfaces for the main components of a 5G small cell, namely:

  • 5G implementation of PHY-API, which aligns to 3GPP P5 and P7 interfaces (the PHY links the radio channel to the MAC, which then supports functions like error correction and scheduling).
  • the RF and digital front end control interface (3GPP P19), including 5G enhancements to support tight timing requirements imposed by the use of millimeter wave spectrum.
  • the network monitor/listen mode API (3GPP P4).


Internal architecture of small cell showing 5G FAPI APIs interfacing L1 hardware components with L2/L3 software


The specs are the first step towards a unified, open platform, providing common interfaces which everyone can support in order to reduce time to market and ensure interoperability.

The next step – which will ensure wide adoption of the specs and so reduce the time for a mass market to emerge – is to make them as easy as possible for stakeholders to embrace. Open source implementations and reference designs are important ways to make new technologies as simple and cost-effective as possible to incorporate in market-ready products.

The Forum also has considerable experience in this area. It works with a variety of open industry alliances as well as with certification labs and standards bodies, to push its interfaces into the ecosystem as quickly as possible. For 5G FAPI, SCF will expand an existing partnership with OpenAirInterface.org (OAI) to build open source reference implementations.

Another important success factor for an open platform is that it can support a huge diversity of designs and form factors, without the risk of fragmentation. Another important SCF initiative is currently underway, to define a baseline set of small cell product configurations targeted at different small cell network scenarios, such as the level of disaggregation of the cells, and the environment in which they will be deployed. These configurations will provide common foundations on which vendors can innovate, creating diverse products for different 5G small cell network scenarios, while retaining interoperability and scale economics. The Forum is currently undertaking a detailed survey of operators and vendors to establish the most important deployment priorities, and therefore the configurations that will provide the biggest short-to-medium term opportunities for members.

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.

Figure 1 illustrates the difference that open platforms and interfaces make to the scale of deployment. Based on SCF forecasts, the total installed base of small cells across all environments could be over 130% larger by the end of 2025 if open interfaces, such as FAPI and 5G FAPI, are widely implemented, greatly improving the economics of small cells and the breadth of the ecosystem.

Figure 1. Global installed base of small cells across all environments end 2020 to end 2026, with and without adoption of open platforms (,000s of radio units).

Further details, including links to the three specifications, can be found on the FAPI landing page.