Small Cell Forum releases inter-layer API for 5G small cells

Taking the building block of system interoperability into 5G small cells.

The Small Cell Forum has made available a 5G version of its Femto API (FAPI), known as PHY API.

FAPI is a set of APIs that defines the interface between layers within a small cell – ie between hardware PHY and RF layers and upper layer protocol stacks. Adopted by small cell vendors since 2008, it enables manufacturers to use interoperable parts from different hardware and software suppliers.

The definition of nFAPI (Network FAPI) then extended that API so that it would work across a network interface between two parts of a disaggregated small cell, where the PHY and MAC functional layers are split at different ends of that interface between a Distributed and Central Unit (DU-CU).

For 5G, this “split” is known as 3GPP Option 6 – and it is interworking across this split that SCF has made available as a defined API for 5G – PHY API. The idea is to enable operators to deploy multi vendor networks, with an interoperable interface between the Central Unit (upper layers) and Distributed Unit (lower layers).

O-RAN Alliance is defining the DU-CU fronthaul interface for a split known as 7.2x – which is a lower level split option. Within this split the FAPI would be deployed within the CU itself, and not across the frontnhaul interface.

Split Option 2 would see the FAPI remain within the DU itself, with the F1 interface defined between the DU and CU

Clare Somerville, Senior Wireless Architect, Intel and Head of the 5G FAPI project at SCF, says in this video, that 5G FAPI has to capture more complexity than its LTE equivalent. That’s because it introduces support for enhanced physical aspects such as beam forming, as well as increased spectrum support for sub 6GHz and mmWave bands. The 5G air interface is also more flexible in areas like variable frame rates – so an API also needs to be able to encompass that flexibility.