A method of assigning PUCCH transmission timing for a mobile device configured with jointly aggregated TDD and FDD CCs includes determining a UL transmission timing in a first subframe for a corresponding feedback in response to a first DL transmission in a second subframe on a first cell, where the second subframe is k subframes prior to the first subframe; and transmitting the corresponding feedback of the first DL transmission in the first subframe on a second cell according to the first UL transmission timing of the first DL transmission of the first cell; where the first cell is on a TDD CC and the first cell has no PUCCH to transmit the corresponding feedback of the first DL transmission, and the second cell is on an FDD CC and the second cell uses PUCCH to transmit the corresponding feedback of the first DL transmission of the first cell.