A method performed by a user equipment (UE) that operates according to a first indicator and a second indicator is provided. When the UE is in a radio resource control (RRC) connected state with a first communication system and is in a state of idle mode with a second communication system, the UE receives a first message including the first indicator and the second indicator from the first communication system. The first indicator instructs the UE to perform association processes with a base station of the second communication system. The second indicator instructs the UE to operate in a power saving mode with the base station of the second communication system. After the UE performs the association processes, the UE switches to an RRC-idle mode state with the first communication system and enters the power saving mode from the idle mode with the second communication system.