A base station, for integrating multiple Radio Access Technologies (RATs) using Carrier Aggregation (CA) inlcudes a WAN MAC entity, an adapter and a WLAN entity. The adapter is connected to one of a plurality of HARQ buffers and receives DL WAN MPDUs stored in the HARQ buffer, and converts the DL WAN MPDUs into a pluarlity of DL WLAN MPDUs. The WLAN MAC entity is coupled to the adapter, receives the DL WLAN MPDUs, wherein the WAN MAC entity respectively transmit the DL WAN MPDUs through transport channels, and the WLAN MAC entity transmits the DL WLAN MPDUs to be time-aligned with the DL WAN MPDUs.