Related content: nFAPI specifications, small cell virtualization, functional splits

It remains to be seen how far 5G, in reality, will differ from 4G. That will depend who deploys it and with which use cases in mind; how regulators allocate spectrum; and whether operators can make the IoT business case. One aspect that is already looking very different, however, is the focus on open platforms. Several high-profile events in the past weeks have shone the spotlight on the essential requirement for standard interfaces in the 5G network, and this work is increasingly being driven by open initiatives as well as by conventional standards bodies.

Small Cell Forum is in a powerful position here, because open interfaces between different elements of the network have been at the heart of its work since its formation over a decade ago. In the macro network, operators pushed for a common fronthaul connection between a baseband unit (BBU) and a remote radio unit (RRU). They ended up with CPRI, an over-complicated and semi-proprietary specification dominated by a few vendors.

At the same time, the Forum came up with Iuh, an interface between small cells and controllers or cores, and succeeded in having it adopted as a 3GPP standard. It went on to enhance the TR-069 and X2 interfaces, working with other standards bodies; and then to create FAPI and nFAPI, The next step will be to take these into the denser, more open world of 5G.

FAPI (functional application platform interface) provides a set of common APIs to support interoperability between the 3G, 4G or 5G PHY, and software elements such as the security coprocessor or scheduler. nFAPI (network FAPI) extends the concept to virtualized small cell networks and provides 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.

These two sets of APIs map to the two main approaches to dense 5G deployment. In outdoor HetNets, 3GPP is already defining some of the interfaces to connect RRUs with local or far-away BBUs, but FAPI will still be important as an internal interface between the chipset and software layers (a gap in the 3GPP or open source standards efforts). In indoor enterprise networks, nFAPI will be essential to support the most common architecture – a group of cells controlled by a central, virtualized unit.

In other words, SCF is addressing a requirement that is absolutely central to the economics of 5G, since these, for many operators, rest on the ability to disaggregate the RAN. 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, filling gaps in work by 3GPP or the open projects.

More broadly, its experience with open interfaces, specified according to a consensus of requirements from an operator-driven membership, will be highly valuable in 5G, as small cells move from the periphery of the network to the heart of the architecture. 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.

As outlined above, the move towards open interfaces has been more advanced in the small cell layer than the macro network. Several suppliers already offer architectures in which a number of small cells are clustered around a centralized, virtualized controller. 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.

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. The Telecom Infra Project (TIP), the Open Networking Foundation (ONF) and the Linux Foundation Networking Fund (LFNF) are the three main bodies working on new RAN interfaces, in particular that critical link between the RRU and the BBU in a vRAN.

The risk is that there will be too many competing interfaces, and work that was supposed to end fragmentation will actually contribute to it. Most groups acknowledge that they will need to find common ground. In October’s TIP Summit in London, the organizations’ Open RAN project group suggested that it might converge its efforts with those of the ORAN Alliance, part of the LFNF, with TIP focusing primarily on product specs and the physical layer, and ORAN defining the higher layers of a virtualized architecture.

Two critical success factors for open RANs were highlighted by that conversation – the need for cooperation between projects, and the need for this work to enable practical deployment in the near term.

Both these approaches are core to SCF’s ethos. It has an extensive partnership programme which includes several open RAN efforts, including ORAN and TIP, and the industry would certainly benefit if such groups support nFAPI. It has experience of submitting specifications successfully to 3GPP – most open source efforts agree that, while their process encourages innovation in defining interfaces, 3GPP standardization is the best way to ensure the end result is fully harmonized.

And SCF is heavily focused on commercial deployment. It kicked off its latest work programme at its Europe Plenary meeting in Amsterdam, and an important activity is to define a 5G small cell, including interfaces such as n/FAPI – not to offer the world yet another architecture diagram, but to map those functions directly to emerging commercial use cases. We will continue this work at our Americas Plenary in December.

5G, to be successful, needs to enable a far greater variety of services and network behaviors than 4G, and that will affect the deployment. A machine-to-machine service requiring ubiquitous coverage of a city, for instance, will require cells to be positioned and coordinated in a very different way from an enterprise application requiring a limited zone of very high capacity. The key will be uniform specifications for equipment and interfaces, which will enable limitless flexibility in how the cells are deployed and which form factors and suppliers are included.

All this highlights the important role that SCF in general, and n/FAPI in particular, can play in accelerating the progress towards a fully deployable, commercially viable dense 5G network. This will be critical to the small cell industry and help smooth the path to 5G densification. However, as small cells become a central element of all RANs, it will also be important for the influential new open RAN groups to get even closer to SCF, enriching their own work by tapping into SCF’s interfaces.