A user equipment, UE, for performing carrier aggregation, comprising: a processor; memory in electronic communication with the processor, wherein instructions stored in the memory are executable to: send Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK, information on a primary cell, in response to Physical Downlink Shared Channel, PDSCH, transmission; wherein, in a case that the UE is configured to use at least one frequency division duplexing, FDD, cell and at least one time division duplexing, TDD, cell, the UE is configured to use an FDD cell as the primary cell, the UE is configured to use a TDD cell as a secondary cell, and both the PDSCH transmission and physical downlink control channel, PDCCH, transmission indicating the PDSCH transmission occur in a subframe n-4 of the secondary cell, the HARQ-ACK information is sent in a subframe of the primary cell in accordance with PDSCH HARQ-ACK associations, in response to the PDSCH transmission, the PDSCH HARQ-ACK associations are associations between a) PDSCH HARQ-ACK transmissions in HARQ-ACK reporting subframes and b) PDSCH transmissions in subframes including a downlink subframe and a special subframe, an interval between the PDSCH HARQ-ACK transmissions and the PDSCH transmissions is fixed to 4 subframe interval, and subframe n is determined as the subframe for sending the HARQ ACK information based on the fixed 4 subframe interval. a processor; memory in electronic communication with the processor, wherein instructions stored in the memory are executable to: send Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK, information on a primary cell, in response to Physical Downlink Shared Channel, PDSCH, transmission; wherein, in a case that the UE is configured to use at least one frequency division duplexing, FDD, cell and at least one time division duplexing, TDD, cell, the UE is configured to use an FDD cell as the primary cell, the UE is configured to use a TDD cell as a secondary cell, and both the PDSCH transmission and physical downlink control channel, PDCCH, transmission indicating the PDSCH transmission occur in a subframe n-4 of the secondary cell, the HARQ-ACK information is sent in a subframe of the primary cell in accordance with PDSCH HARQ-ACK associations, in response to the PDSCH transmission, the PDSCH HARQ-ACK associations are associations between a) PDSCH HARQ-ACK transmissions in HARQ-ACK reporting subframes and b) PDSCH transmissions in subframes including a downlink subframe and a special subframe, an interval between the PDSCH HARQ-ACK transmissions and the PDSCH transmissions is fixed to 4 subframe interval, and subframe n is determined as the subframe for sending the HARQ ACK information based on the fixed 4 subframe interval. send Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement, HARQ-ACK, information on a primary cell, in response to Physical Downlink Shared Channel, PDSCH, transmission; wherein, in a case that the UE is configured to use at least one frequency division duplexing, FDD, cell and at least one time division duplexing, TDD, cell, the UE is configured to use an FDD cell as the primary cell, the UE is configured to use a TDD cell as a secondary cell, and both the PDSCH transmission and physical downlink control channel, PDCCH, transmission indicating the PDSCH transmission occur in a subframe n-4 of the secondary cell, the HARQ-ACK information is sent in a subframe of the primary cell in accordance with PDSCH HARQ-ACK associations, in response to the PDSCH transmission, the PDSCH HARQ-ACK associations are associations between a) PDSCH HARQ-ACK transmissions in HARQ-ACK reporting subframes and b) PDSCH transmissions in subframes including a downlink subframe and a special subframe, an interval between the PDSCH HARQ-ACK transmissions and the PDSCH transmissions is fixed to 4 subframe interval, and subframe n is determined as the subframe for sending the HARQ ACK information based on the fixed 4 subframe interval.