What is 5G femto application platform interface (5G FAPI)?

The 5G femto application platform interface (5G FAPI, or simply FAPI) published by the Small Cell Forum (SCF) is a suite of specifications that enable small cells to be built up piece-by-piece using components from different suppliers. It can be viewed as a subset of the network functional application platform interface (nFAPI), also published by the SCF.

5G new radio (NR) is optimized for different use cases than today’s 4G LTE. While 4G LTE was designed to enhance users’ multimedia experiences, 5G NR is being optimized to support a substantially higher density of users with high bandwidths, lower latencies, and higher reliability. Instead of focusing on the delivery of multimedia, 5G NR is designed to be flexible and deliver three types of service, including enhanced mobile broadband (eMBB), ultra-reliable low latency communication (URLLC), and massive internet of things (mIoT). As a result, flexibility is a key feature of 5G NR, and 5G FAPI is designed to support the need for flexibility.

The multiple service types envisioned for 5G NR are causing mobile network operators (MNOs) to look for different cost models that can be optimized based on interoperability and an open and competitive hardware environment. The disaggregation of networks is making the interface between a distributed unit (DU) for radio functions and a centralized unit (CU) for protocol stacks and baseband functions more critical. As an open specification, FAPI is designed to enable MNOs to mix and match protocol stacks, baseband, and radios from different suppliers, enabling the deployment of disaggregated virtualized radio area network (vRAN) systems.

5G FAPI is intended to stimulate competition between 5G small cell hardware, software, and other equipment suppliers. 5G FAPI provides common APIs agnostic to the type of small cell radio area network (RAN) architecture. It is designed to support interoperability between 5G small cell hardware components and software layers to prevent network fragmentation.

The 5G FAPI Suite
The 5G FAPI suite includes three specifications covering the following APIs:

• PHY API – main data path (P7) and PHY mode control (P5) interface [SCF222]
• RF and Digital Front End Control API –frontend unit control (P19) [SCF223]
• Network Monitor Mode API – for 2G/3G/4G/5G (P4) [SCF224]

When defining the architecture of a 5G network to identify the use of the 5G FAPI, only two elements are considered: the 5G Core consisting of the access and mobility function (AMF) and user plane function (UPF); and the 5G node B (gNB). The 5G FAPI is used in the gNB, and the standardized interfaces are called NG, Xn, and F1, as illustrated below.

The protocol model for the gNB is defined in the 5G-RAN architectural standard [TS38.401]. It involves the separation of control- and data-plane information, which is maintained throughout the 5G NR network. Control and data-plane information are both passed through the PHY API. Each API message contains either control- or data-plane information, but not both.

PHY API
If carrier aggregation is supported, then one instance of the PHY API exists for each component carrier. Three categories of procedures use the PHY API, including configuration procedures, slot procedures, and beamforming/RF procedures. Configuration procedures occur least frequently and handle the general management of the PHY layer. Slot procedures occur much more frequently; they determine each slot’s structure and operate based on the subcarrier spacing, such as periods of 125us, 250us, 500us, or 1ms.

Beamforming/RF procedures are used for defining precoding and beamforming parameters. They are flexible and are defined in tables loaded into the PHY at configuration time. A precoding index and/or beamforming index is included in each protocol data unit (PDU) in slot messages.

Precoding and digital beamforming can be viewed as a cascade of two matrix multiplications. For precoding operation, an index to a pre-stored precoding matrix PM(idx) is specified in the message. A matrix DB represents digital beamforming. For efficient messaging, columns of DB are picked from a pre-stored digital beamforming table (DBT). It suffices to specify the beam’s index to be applied for each input port in the message. Baseband ports are logical entities that may be further processed by a digital frontend (DFE) unit. Within the context of 5G NR a baseband port typically corresponds to a component carrier that is handled by a specific RF path.

RF and Digital Front End Control API
In response to the need for close coordination between the front-end unit (FEU) and the stack, 5G FAPI has developed the P19 interface specification dedicated to the control and configuration of the FEU. The use of P19 is different than the interface used in the 4G counterpart specification. The P19 interface specification serves as a companion to the 5G FAPI P5/P7 specification. In addition to supporting fast dynamic control of the FEU, P19 supports network virtualization by allowing stack and FEU solutions from different suppliers to bind together easily. The 5G FAPI approach applies to all-in-one small cell solutions and disaggregated solutions where the upper and lower layers and the FEU may reside at different physical locations.

The capability and flexibility of P19 results from a large set of APIs that allows the upper stack to configure the RF, the DFE, and the ABF blocks in a wide range of configurations and to monitor and adjust various system parameters quickly as required by 5G NR. However, the scope of P19 is limited to the control path. P19 does not mandate the protocols or interface standards for the data flow between the DFE, RF, or the PHY.

Network Monitor Mode API
The FAPI Network Monitor Mode (NMM) P4 API interface operates and configures network monitoring functions within the PHY(s) capability. The NMM specification defines the procedures, messages, and structures to implement NMM functions for 5G 3GPP release 15 and legacy LTE, UTRAN, GERAN, and NBIoT.

The ability to monitor the radio environment in the vicinity of a small cell and optimize the RAN deployment parameters accordingly is a key factor that enables self-organizing networks (SON). To implement network monitoring, also called network listening, a small cell acts as a passive UE receiver for local RF signals as well as searching and decoding system parameters from nearby cells.

For example, NMM can configure the PHY to scan nearby cells, identify potential reference sources for synchronization, and create a map of the interference spectrum. Typical applications for NMM include SON spectrum sharing deployments in dynamic RF environments that require monitoring and 4G and 5eG SON configurations such as small office and residential environments where network planning overhead can be minimized.

What is nFAPI?
FAPI is a set of APIs for small cell components. It is defined by component suppliers and adopted by software stack vendors and small cell integrators. It is applicable for both integrated and disaggregated base stations for all split options. nFAPI ‘wraps’ these APIs to make them transportable over network connections. nFAPI is an external network interface between S-RU and S-DU network nodes. It is defined by small cell components and system suppliers for deployers and operators in building open RAN small cell networks.

5G nFAPI is a network interface which leverages the widely adopted FAPI system-on-a-chip (SoC) PHY API used in most existing SoC-based small cells. 5G nFAPI 1.0 has been released to enable early implementations for evaluation by MNOs and other small cell deployers. 5G nFAPI 1.0 adds a network transport wrapper around the 5G FAPI PHY API to create the split option-6 interface between S-RU and S-DU network nodes:

• The S-RU (SCF Remote Unit) is the small cell or physical network function (PNF).
• The S-DU (SCF Distributed Unit) can be a virtual network function (VNF) or physical.
• A CU (Central Unit) may or may not be co-located with the S-DU. Along with mobile core network aspects, the CU is out of scope for the first release of nFAPI and will be added in future releases.

Summary
5G FAPI is a suite of specifications that enable small cells to be built up piece-by-piece using different suppliers’ components. It can be viewed as a subset of the network functional application platform interface (nFAPI). 5G NR is designed to be flexible and deliver three types of service, including enhanced mobile broadband (eMBB), ultra-reliable low latency communication (URLLC), and massive Internet of things (mIoT). As a result, flexibility is a key feature of 5G NR, and 5G FAPI is designed to support that need for flexibility.

References
About 5G nFAPI, Small Cell Forum
5G FAPI: Network Monitor Mode API, Small Cell Forum
5G FAPI: PHY API, Small Cell Forum
5G FAPI: RF and Digital Frontend Control API, Small Cell Forum
5G nFAPI specifications, Small Cell Forum
The 5G FAPI Suite, Small Cell Forum