A user equipment (UE) for discontinuous reception (DRX) operation having a Downlink DRX Hybrid Automatic Repeat reQuest (HARQ) Round-Trip Time Timer (drx-HARQ-RTT-TimerDL) is disclosed. The UE comprises one or more non-transitory computer-readable media having computer-executable instructions embodied thereon; at least one processor coupled to the one or more non-transitory computer-readable media, and configured to execute the computer-executable instructions to perform the DRX operation, wherein the DRX operation comprises: starting the drx-HARQ-RTT-TimerDL from an initial value in a first symbol after an end of a corresponding transmission carrying a DL HARQ feedback, the DL HARQ feedback corresponding to downlink data. The initial value is represented in number of symbols of a BWP (bandwidth part) where the downlink data is received.