A method is implemented in a base station used in a wireless communications system that supports flexible-TDD (flexible-time division duplex) UL-DL (uplink-downlink) configuration. The method includes configuring a first type of user equipment (UE) with a first type of configuration; configuring a second type of UE with a second type of configuration; and receiving from a user equipment a HARQ-ACK (hybrid automatic repeat request-acknowledgement) signal according to a DL association set. The DL association set includes: a first DL association set for a UL-DL TDD configuration used by the first type of UE; and a second DL association set for a reference UL-DL TDD configuration including a third DL association set for PUCCH resource mapping used by the second type of UE.