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5G FAPI – Femtocell Application Programming Interface

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Introduction

FAPI is an interface, which originally meant Femtocell Application Programming Interface defines the interface between MAC-Layer#2 and PHY – Layer#1 for small cells. FAPI is specified by the Small Cell Forum and the original interface defined the interaction between a 3G MAC and PHY (SCF 048), Subsequently a version was defined between a 4G MAC and PHY (SCF 082), and recently the 5G version has been completed (SCF 222).

For each wireless technology FAPI defines a set of control messages for configuring a PHY and a set of messages for exchanging data between MAC and PHY, and indicates the timing constraints of these messages.

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FAPI Architecture

The architecture of FAPI is shown in figure below. FAPI is defined between one instance of MAC and one instance of PHY. In scenarios such as CA there will be multiple instances of FAPI, one per carrier. There are two types of PHY  configuration messages are defined by the logical interface as P5 and P7. P5 Logical Interface is  for PHY control configuration which is semi-static and is being generated by a PHY control entity.

The P7 logical interface is for PHY data plane messages, P7 interface data configuration is generated once per slot and consists of following:

FAPI State Machine

To support consistent PHY behavior, FAPI has defined a state machine for the PHY as shown below.

Once a PHY is operational the interaction between the MAC and PHY occurs at the periodicity of a slot. Since 5G supports several different subcarrier spacing these per-slot messages can be exchanged every 125 μs, 250 μs, 500 μs, or 1 ms. In fact, if a carrier is supporting several different bandwidth parts, with different subcarrier spacing, then the MAC may be exchanging FAPI messages with the PHY at different periodicities for different BWP.

FAPI reduces this complexity by providing all per-slot control messages in a single downlink control message, DL_TTI.request, and all downlink MAC PDUs in a single downlink data message, TX_Data.request. Similarly, for the uplink there is a single uplink control message, UL_TTI.request, and all similar uplink is grouped together into one message, RX_Data.indication, UCI.indication, SRS.indication, and RACH.indication.

P5 – Control Configuration Procedures
The Control configuration procedures supported by the L1 FAPI over P5 interface are listed below:

These procedures will move the PHY layer through the IDLE, CONFIGURED and RUNNING states

P7 – Data Plane Configuration Procedures

The P7 – Data plance configuration procuedres serve two purposes for subframes procedures. Firstly, they are used to control the DL and UL frame structures. Secondly, they are used to transfer the subframe data between the L2/L3 software and PHY. The procedures supported by the L1 FAPI over P7 interface are listed below:

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