A method in a first communication device, for assignment of uplink, UL, channel resources by determining a first downlink, DL, Hybrid Automatic Repeat request, HARQ, timing for a DL Frequency-Division Duplexing, FDD, carrier and a second DL HARQ timing for the DL FDD carrier, for enabling a second communication device to provide HARQ feedback on a physical UL shared channel in a communication system based on Time-Division Duplexing, TDD, and FDD carrier aggregation of at least a TDD carrier and a DL FDD carrier and an UL FDD carrier, wherein the method comprises: determining the first DL HARQ timing for the DL FDD carrier by utilizing an association which determines which UL subframes in the TDD carrier are defined for transmitting HARQ feedback on an UL control channel for an associated set of DL subframes in the DL FDD carrier, and wherein at most N -1 of N consecutive DL subframes in the DL FDD carrier are associated with one or more UL subframes in the TDD carrier, and wherein the method further comprises enabling, for the non-associated DL subframe/s in the DL FDD carrier, HARQ feedback transmission only on the physical UL shared channel on the UL FDD carrier; determining the second DL HARQ timing for the DL FDD carrier by utilizing an association which determines which UL subframes in the FDD carrier are defined for transmitting HARQ feedback on the physical UL shared channel on the UL FDD carrier for an associated set of DL subframes in the DL FDD carrier, whereby said determination results in a first set of UL subframes for which the physical UL shared channel on the UL FDD carrier is enabled to comprise HARQ feedback, and a second set of UL subframes for which the physical UL shared channel on the UL FDD carrier is not comprising HARQ feedback; and assigning UL channel resources for the HARQ feedback on the physical UL shared channel on the UL FDD carrier in said first set of UL subframes, according to said second DL HARQ timing, when there is no UL control channel assigned in the TDD carrier according to said first DL HARQ timing. determining the first DL HARQ timing for the DL FDD carrier by utilizing an association which determines which UL subframes in the TDD carrier are defined for transmitting HARQ feedback on an UL control channel for an associated set of DL subframes in the DL FDD carrier, and wherein at most N -1 of N consecutive DL subframes in the DL FDD carrier are associated with one or more UL subframes in the TDD carrier, and wherein the method further comprises enabling, for the non-associated DL subframe/s in the DL FDD carrier, HARQ feedback transmission only on the physical UL shared channel on the UL FDD carrier; determining the second DL HARQ timing for the DL FDD carrier by utilizing an association which determines which UL subframes in the FDD carrier are defined for transmitting HARQ feedback on the physical UL shared channel on the UL FDD carrier for an associated set of DL subframes in the DL FDD carrier, whereby said determination results in a first set of UL subframes for which the physical UL shared channel on the UL FDD carrier is enabled to comprise HARQ feedback, and a second set of UL subframes for which the physical UL shared channel on the UL FDD carrier is not comprising HARQ feedback; and assigning UL channel resources for the HARQ feedback on the physical UL shared channel on the UL FDD carrier in said first set of UL subframes, according to said second DL HARQ timing, when there is no UL control channel assigned in the TDD carrier according to said first DL HARQ timing.