A user equipment configured to operate in a wireless communication system, the user equipment comprising: a communication module; and a processor that controls the communication module, wherein the processor is configured to: receive a plurality of first physical downlink control channels, PDCCHs, for downlink scheduling, wherein each first PDCCH includes an Nc-bit count downlink assignment indicator, c-DAI, related to a scheduling order of a corresponding downlink scheduling, and Nc is one of 1 and 2; receive a second PDCCH for scheduling a physical uplink shared channel, PUSCH, wherein the second PDCCH includes 2-bit uplink DAI, UL-DAI, related to a total number of the downlink scheduling; and transmit a hybrid automatic repeat request, HARQ,-acknowledge, ACK, codebook for the downlink scheduling via the PUSCH, wherein a size of the HARQ-ACK codebook is associated with a value O satisfying: O = 4 * floor j * C / 4 + Q + V, where - j is a counter value for a case that a Nc-bit c-DAI has a value of less than or equal to a previous Nc-bit c-DAI within a plurality of received Nc-bit c-DAIs, - C is a value of 2^Nc, - Q is 0 or 1, - V is a value of the 2-bit UL-DAI, and in a range of 1 to 4, and - floor is a flooring function. a communication module; and a processor that controls the communication module, wherein the processor is configured to: receive a plurality of first physical downlink control channels, PDCCHs, for downlink scheduling, wherein each first PDCCH includes an Nc-bit count downlink assignment indicator, c-DAI, related to a scheduling order of a corresponding downlink scheduling, and Nc is one of 1 and 2; receive a second PDCCH for scheduling a physical uplink shared channel, PUSCH, wherein the second PDCCH includes 2-bit uplink DAI, UL-DAI, related to a total number of the downlink scheduling; and transmit a hybrid automatic repeat request, HARQ,-acknowledge, ACK, codebook for the downlink scheduling via the PUSCH, wherein a size of the HARQ-ACK codebook is associated with a value O satisfying: O = 4 * floor j * C / 4 + Q + V, where - j is a counter value for a case that a Nc-bit c-DAI has a value of less than or equal to a previous Nc-bit c-DAI within a plurality of received Nc-bit c-DAIs, - C is a value of 2^Nc, - Q is 0 or 1, - V is a value of the 2-bit UL-DAI, and in a range of 1 to 4, and - floor is a flooring function. receive a plurality of first physical downlink control channels, PDCCHs, for downlink scheduling, wherein each first PDCCH includes an Nc-bit count downlink assignment indicator, c-DAI, related to a scheduling order of a corresponding downlink scheduling, and Nc is one of 1 and 2; receive a second PDCCH for scheduling a physical uplink shared channel, PUSCH, wherein the second PDCCH includes 2-bit uplink DAI, UL-DAI, related to a total number of the downlink scheduling; and transmit a hybrid automatic repeat request, HARQ,-acknowledge, ACK, codebook for the downlink scheduling via the PUSCH, wherein a size of the HARQ-ACK codebook is associated with a value O satisfying: O = 4 * floor j * C / 4 + Q + V, where - j is a counter value for a case that a Nc-bit c-DAI has a value of less than or equal to a previous Nc-bit c-DAI within a plurality of received Nc-bit c-DAIs, - C is a value of 2^Nc, - Q is 0 or 1, - V is a value of the 2-bit UL-DAI, and in a range of 1 to 4, and - floor is a flooring function. - j is a counter value for a case that a Nc-bit c-DAI has a value of less than or equal to a previous Nc-bit c-DAI within a plurality of received Nc-bit c-DAIs, - C is a value of 2^Nc, - Q is 0 or 1, - V is a value of the 2-bit UL-DAI, and in a range of 1 to 4, and - floor is a flooring function.