A communication device for assigning transmission timing of Physical Uplink Control Channel, PUCCH, configured with jointly aggregated Time Division Duplex, TDD, and Frequency Division Duplex, FDD, component carriers, CC, comprising: a processing means; and a storage unit, coupled to the processing means, for storing a program code, wherein the program code instructs the processing means to execute the following steps characterized by: receiving a first downlink, DL, transmission on a first cell; determining a first uplink, UL, transmission timing in a first subframe for a corresponding feedback in response to the first DL transmission in a second subframe on the first cell, where the second subframe is k subframes prior to the first subframe; 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; receiving a second DL transmission on a third cell; and determining a second UL transmission timing of second DL transmission of the third cell independently, irrespective of the first UL transmission timing of the first DL transmission of the first cell; wherein 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; wherein 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; and wherein the third cell is a TDD CC and has different TDD configuration than the first cell. a processing means; and a storage unit, coupled to the processing means, for storing a program code, wherein the program code instructs the processing means to execute the following steps characterized by: receiving a first downlink, DL, transmission on a first cell; determining a first uplink, UL, transmission timing in a first subframe for a corresponding feedback in response to the first DL transmission in a second subframe on the first cell, where the second subframe is k subframes prior to the first subframe; 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; receiving a second DL transmission on a third cell; and determining a second UL transmission timing of second DL transmission of the third cell independently, irrespective of the first UL transmission timing of the first DL transmission of the first cell; wherein 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; wherein 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; and wherein the third cell is a TDD CC and has different TDD configuration than the first cell. receiving a first downlink, DL, transmission on a first cell; determining a first uplink, UL, transmission timing in a first subframe for a corresponding feedback in response to the first DL transmission in a second subframe on the first cell, where the second subframe is k subframes prior to the first subframe; 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; receiving a second DL transmission on a third cell; and determining a second UL transmission timing of second DL transmission of the third cell independently, irrespective of the first UL transmission timing of the first DL transmission of the first cell; wherein 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; wherein 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; and wherein the third cell is a TDD CC and has different TDD configuration than the first cell.