Small Cell Forum does the splits for 5G small cells

Small Cell Forum says its internal RAN interfaces can be a split option for 5G Open RAN small cells.

The Small Cell Forum is calling for those developing Open RAN specifications, including the ORAN Alliance, TIP and 3GPP, to adopt existing interfaces it has developed between small cell network elements and network layers.

SCF says that the industry could speed up adoption of 5G Open RAN by adopting two interfaces it has developed for split architecture small cells (ie the architectural model whereby the radio and baseband units are separated at some level).

Small Cell Forum’s MAC-PHY split (highlighted)

SCF’s 5G-FAPI (Femto Application Platform Interface) defines the split option known in 3GPP specifications as Option 6, or MAC-PHY. Option 6 defines the “split” between central and remote units so that the MAC resides within the central unit (with other functions such as RRC, PDCP, RLC) and the upper and lower PHY layers reside on the remote unit.

Prabhakar Chitrapu, who chairs SCF’s TECH Group said, “Split RAN/Small Cell architectures have seven options, as identified by 3GPP. Of these, 3GPP has focused on Option-2 (RLC-PDCP) and ORAN on Option-7.2 (PHY-PHY). Option-6 (PHY-MAC) is not being addressed by any of these organisations. SCF seeks to fill this gap.”

So why does Chitrapu think it is important to have Option 6 on the table, given that other parties are proceeding on different splits?

“The PHY-MAC interface is important for the industry because it is an interface that has been highly successful in the 4G world, where it is called FAPI and nFAPI. It is therefore considered very important that we extend these interface specifications for 5G, as 5G-FAPI and 5G-nFAPI.

Chitrapu added, “FAPI helps Equipment Vendors to mix PHY & MAC Software from different suppliers via this open FAPI interface. So, FAPI is an “internal” interface.”

“5G-nFAPI (network FAPI) is a “network” interface and is between a Distributed Unit and Centralised Unit  of a Split RAN/Small Cell network solution. An open specification of this interface (nFAPI) will help network architects by allowing them to mix distributed and central units from different vendors.”

One aspect of SCF’s plea is that it says adoption of Option 6 will reduce fragmentation of standards in the market. That may appear counter intuitive as it is asking others to increase the number of splits they support. However, the SCF splits are designed specifically for split small cells, and it wants to see those interfaces go forward into 5G small cells. If vendors start developing products designed for dense network environments (say on a 5G grid at higher spectrum bands) with different specs, then the ability of operators to mix and match vendors in the RAN reduces, as does the ability of OEMs themselves to integrate technology from different suppliers.