ed in TX_Data.request using the control information provided in DL_TTI.request and transmits the resultant data over the air to the UE. The UE will return an ACK/NACK message indicating whether it correctly received the data in slot N + K, where for 5G the value K is flexible and can be different for each UE. The MAC schedules the ACK/NACK reception by sending an UL_TTI.request message to the PHY. If the UE is transmitting uplink data then the ACK/NACK is indicated in a PUSCH PDU, and if there are no uplink data a PUCCH PDU is used. 4.6 Small Cells sd DL-SCH LI PHY L2/L3 software (from Actors) (from FAPI) (from Actors) DL_TTI.request(PDSCH and PDCCH in slot N) TX_Data.request(MACPDU) MAC PDU sent to UE() alt Methods for defining ACK/NACK response to POSCH UL_TTI.request(.DUSCH PDU in slot N+K) UL_TTI.request(PUCCH PDU in slot N+K) ACK/NACK response from UE() UCI. indication() Figure 4.6.5 FAPI downlink data procedure. (Source: Reproduced by permission of © Small Cell Forum). The PHY uses this information to locate and decode the ACK/NACK received from the UE. The received ACK/NACK is sent to the MAC in the UCI.indication message. The FAPI procedure to receive data on the UL-SCH is shown in Figure 4.6.6. Assuming the uplink transmission needs to be scheduled the procedure starts with downlink mes- sages. The MAC sends a UL _DCI.request message to the PHY. In this message there will be PDCCH PDUs that specify DCIs sent to the UE to indicate data are scheduled in the uplink. It should be noted that not every UL-SCH transmission requires a DCI; for example, configured grant. The UE will transmit the uplink data in slot N+K and when that slot arrives the MAC schedules the UL-SCH reception by sending an UL_TTI.request message to the PHY including a PUSCH PDU for each UE scheduled to transmit. The PHY uses this information to locate and decode the MAC PDU received from the UE. The received data are sent to the MAC in the RX_Data.indication message and the success or failure of the decoding is sent to the MAC in the CRC.