One way in which 5G promises to be very different from its predecessors is in its focus on open platforms and interfaces above the physical layer. Several high profile events in the past weeks have shone the spotlight on the essential requirement for standard interfaces throughout the 5G network, and this work is increasingly being driven by open initiatives as well as by conventional standards bodies.
The Facebook-initiated Telecom Infra Project (TIP) and Linux Foundation-hosted ORAN Alliance are both working on common ways to interface the remote radio head, in a disaggregated RAN, with the virtualized baseband. There were strong hints, at the recent TIP Summit in London, that the two might converge their efforts, which are largely complementary, driving a de facto standard which could run with any fronthaul technology and many of the 3GPP’s eight defined splits between virtualized and physical network functions.
Small Cell Forum’s nFAPI has a key role to play in 5G:
However, a great deal of work on standard interfaces has been pioneered in the enterprise or small cell worlds. Small Cell Forum has worked on open interfaces between different elements of the network 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 out-door 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 in-door 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 which are not a central focus for other initiatives, filling gaps in work by 3GPP or the open projects.
Densification absolutely requires an open ecosystem to make deployment of large num-bers 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-a -vis WiFi (its open ecosystem) will be removed.