A user equipment, UE, adapted to perform carrier aggregation with use of a plurality of serving cells, comprising: a processor; a memory in electronic communication with the processor, wherein instructions stored in the memory are executable to: determine a duplex method of each of the plurality of serving cells for frequency-division duplexing, FDD, and time-division duplexing, TDD, carrier aggregation, wherein the plurality of serving cells are constituted by at least one serving cell which is a TDD cell and at least one serving cell which is a FDD cell; determine, on detection of a physical downlink shared channel, PDSCH, which is received on a secondary cell during a period of one or more downlink subframes, an uplink subframe in which Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK/NACK, is transmitted, wherein when a primary cell is a TDD cell and the secondary cell is a FDD cell, the uplink subframe is determined based on a downlink, DL, association set for the secondary cell, the DL association set indicates an association between the one or more downlink subframes on which the PDSCH is detected and the uplink subframe in which the HARQ-ACK/NACK is transmitted, and the DL association set is determined based on a primary cell uplink/downlink, UL/DL, configuration; and send the HARQ-ACK/NACK, in the uplink subframe regardless of whether the HARQ-ACK/NACK, is sent on a physical uplink control channel, PUCCH, or a physical uplink shared channel, PUSCH. a processor; a memory in electronic communication with the processor, wherein instructions stored in the memory are executable to: determine a duplex method of each of the plurality of serving cells for frequency-division duplexing, FDD, and time-division duplexing, TDD, carrier aggregation, wherein the plurality of serving cells are constituted by at least one serving cell which is a TDD cell and at least one serving cell which is a FDD cell; determine, on detection of a physical downlink shared channel, PDSCH, which is received on a secondary cell during a period of one or more downlink subframes, an uplink subframe in which Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK/NACK, is transmitted, wherein when a primary cell is a TDD cell and the secondary cell is a FDD cell, the uplink subframe is determined based on a downlink, DL, association set for the secondary cell, the DL association set indicates an association between the one or more downlink subframes on which the PDSCH is detected and the uplink subframe in which the HARQ-ACK/NACK is transmitted, and the DL association set is determined based on a primary cell uplink/downlink, UL/DL, configuration; and send the HARQ-ACK/NACK, in the uplink subframe regardless of whether the HARQ-ACK/NACK, is sent on a physical uplink control channel, PUCCH, or a physical uplink shared channel, PUSCH. determine a duplex method of each of the plurality of serving cells for frequency-division duplexing, FDD, and time-division duplexing, TDD, carrier aggregation, wherein the plurality of serving cells are constituted by at least one serving cell which is a TDD cell and at least one serving cell which is a FDD cell; determine, on detection of a physical downlink shared channel, PDSCH, which is received on a secondary cell during a period of one or more downlink subframes, an uplink subframe in which Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK/NACK, is transmitted, wherein when a primary cell is a TDD cell and the secondary cell is a FDD cell, the uplink subframe is determined based on a downlink, DL, association set for the secondary cell, the DL association set indicates an association between the one or more downlink subframes on which the PDSCH is detected and the uplink subframe in which the HARQ-ACK/NACK is transmitted, and the DL association set is determined based on a primary cell uplink/downlink, UL/DL, configuration; and send the HARQ-ACK/NACK, in the uplink subframe regardless of whether the HARQ-ACK/NACK, is sent on a physical uplink control channel, PUCCH, or a physical uplink shared channel, PUSCH.