A User Equipment (UE) for performing carrier aggregation is described. The UE includes a processor and instructions stored in memory that is in electronic communication with the processor. The UE determines a duplex method of each serving cell for frequency-division duplexing (FDD) and time-division duplexing (TDD) carrier aggregation. At least one serving cell is a TDD cell and at least one serving cell is a FDD cell. The UE also determines physical downlink shared channel (PDSCH) Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement (HARQ-ACK) transmission timing for a serving cell. When a primary cell is a TDD cell, the PDSCH HARQ-ACK transmission timing for the serving cell is determined based on a downlink (DL) association set for the serving cell. The UE further sends PDSCH HARQ-ACK information based on the PDSCH HARQ-ACK transmission timing.