A terminal apparatus comprising: a receiver configured to receive a Downlink Control Information, DCI, format being used for scheduling of a Physical Downlink Shared CHannel, PDSCH, and a higher layer parameter including a parameter subframeAssignment-r15 that indicates a configuration of a downlink subframe, a special subframe, and an uplink subframe for a continuous 10 subframes; and a transmitter (613, 614) configured to transmit a Hybrid Automatic Repeat request ACKnowledgement, HARQ-ACK, corresponding to the PDSCH, wherein in a case that a duplex mode of a primary cell is Frequency Division Duplex, FDD, whether a number of bits of a HARQ process number field in the DCI format is three bits or four bits is determined, based on whether Evolved universal terrestrial radio access network New radio Dual Connectivity, EN-DC, is configured, whether the higher layer parameter including the parameter subframeAssignment-r15 is configured, and whether the DCI format is mapped to a user equipment, UE, specific search space, USS, given by a Cell Radio Network Temporary Identifier, C-RNTI; wherein the number of bits of the HARQ process number field in the DCI format is four bits, in a case that the duplex mode of the primary cell is FDD, the EN-DC is configured, the higher layer parameter including the parameter subframeAssignment-r15 is configured, and the DCI format is mapped to the USS given by the C-RNTI; and the number of bits of the HARQ process number field in the DCI format is three bits, in a case that the duplex mode of the primary cell is FDD, the EN-DC is configured, the higher layer parameter including the parameter subframeAssignment-r15 is configured, and the DCI format is mapped to a Common Search Space, CSS. a receiver configured to receive a Downlink Control Information, DCI, format being used for scheduling of a Physical Downlink Shared CHannel, PDSCH, and a higher layer parameter including a parameter subframeAssignment-r15 that indicates a configuration of a downlink subframe, a special subframe, and an uplink subframe for a continuous 10 subframes; and a transmitter (613, 614) configured to transmit a Hybrid Automatic Repeat request ACKnowledgement, HARQ-ACK, corresponding to the PDSCH, wherein in a case that a duplex mode of a primary cell is Frequency Division Duplex, FDD, whether a number of bits of a HARQ process number field in the DCI format is three bits or four bits is determined, based on whether Evolved universal terrestrial radio access network New radio Dual Connectivity, EN-DC, is configured, whether the higher layer parameter including the parameter subframeAssignment-r15 is configured, and whether the DCI format is mapped to a user equipment, UE, specific search space, USS, given by a Cell Radio Network Temporary Identifier, C-RNTI; wherein the number of bits of the HARQ process number field in the DCI format is four bits, in a case that the duplex mode of the primary cell is FDD, the EN-DC is configured, the higher layer parameter including the parameter subframeAssignment-r15 is configured, and the DCI format is mapped to the USS given by the C-RNTI; and the number of bits of the HARQ process number field in the DCI format is three bits, in a case that the duplex mode of the primary cell is FDD, the EN-DC is configured, the higher layer parameter including the parameter subframeAssignment-r15 is configured, and the DCI format is mapped to a Common Search Space, CSS.