A network (30, 200) for handling a downlink, DL, transmission in a frequency-division duplexing, FDD, system, comprising: a storage unit for storing instructions of: selecting a first reference configuration for a DL transmission of a communication device (202, 204) from a plurality of configuration candidates, wherein the plurality of configuration candidates comprises at least one configuration for hybrid automatic repeat request, HARQ, acknowledgement, ACK/negative acknowledgement, NACK, reporting in a time-division duplexing, TDD, system, and at least one timing reference for HARQ ACK/NACK reporting in a FDD-TDD carrier aggregation system; transmitting information of the first reference configuration to the communication device (202, 204); performing the DL transmission with the communication device (202, 204) according to the first reference configuration; and receiving a HARQ feedback corresponding to the DL transmission from the communication device (202, 204) according to the first reference configuration, when a device-to-device, D2D, communication occurs in a first subframe which is four subframes after a second subframe where the DL transmission is performed, wherein a HARQ timing of the network (30, 200) receiving the HARQ feedback corresponding to the DL transmission to the communication device (202, 204) is determined based on both the first reference configuration and a second reference configuration when the network (30, 200) is reconfiguring from the first reference configuration to the second reference configuration, wherein the HARQ timing and a timing of the corresponding DL transmission to the communication device (202, 204) satisfy both the first reference configuration and the second reference configuration; and a processing means, coupled to the storage unit, configured to execute the instructions stored in the storage unit. a storage unit for storing instructions of: selecting a first reference configuration for a DL transmission of a communication device (202, 204) from a plurality of configuration candidates, wherein the plurality of configuration candidates comprises at least one configuration for hybrid automatic repeat request, HARQ, acknowledgement, ACK/negative acknowledgement, NACK, reporting in a time-division duplexing, TDD, system, and at least one timing reference for HARQ ACK/NACK reporting in a FDD-TDD carrier aggregation system; transmitting information of the first reference configuration to the communication device (202, 204); performing the DL transmission with the communication device (202, 204) according to the first reference configuration; and receiving a HARQ feedback corresponding to the DL transmission from the communication device (202, 204) according to the first reference configuration, when a device-to-device, D2D, communication occurs in a first subframe which is four subframes after a second subframe where the DL transmission is performed, wherein a HARQ timing of the network (30, 200) receiving the HARQ feedback corresponding to the DL transmission to the communication device (202, 204) is determined based on both the first reference configuration and a second reference configuration when the network (30, 200) is reconfiguring from the first reference configuration to the second reference configuration, wherein the HARQ timing and a timing of the corresponding DL transmission to the communication device (202, 204) satisfy both the first reference configuration and the second reference configuration; and selecting a first reference configuration for a DL transmission of a communication device (202, 204) from a plurality of configuration candidates, wherein the plurality of configuration candidates comprises at least one configuration for hybrid automatic repeat request, HARQ, acknowledgement, ACK/negative acknowledgement, NACK, reporting in a time-division duplexing, TDD, system, and at least one timing reference for HARQ ACK/NACK reporting in a FDD-TDD carrier aggregation system; transmitting information of the first reference configuration to the communication device (202, 204); performing the DL transmission with the communication device (202, 204) according to the first reference configuration; and receiving a HARQ feedback corresponding to the DL transmission from the communication device (202, 204) according to the first reference configuration, when a device-to-device, D2D, communication occurs in a first subframe which is four subframes after a second subframe where the DL transmission is performed, wherein a HARQ timing of the network (30, 200) receiving the HARQ feedback corresponding to the DL transmission to the communication device (202, 204) is determined based on both the first reference configuration and a second reference configuration when the network (30, 200) is reconfiguring from the first reference configuration to the second reference configuration, wherein the HARQ timing and a timing of the corresponding DL transmission to the communication device (202, 204) satisfy both the first reference configuration and the second reference configuration; and a processing means, coupled to the storage unit, configured to execute the instructions stored in the storage unit.