A mobile communications system is disclosed. The mobile communications system includes: a first user apparatus for two-way communicating using frequency division duplex (FDD); a second user apparatus for two-way communicating using time division duplex (TDD); and a base station for communicating with the first and second user apparatuses. The base station distinguishes among the simultaneous transmitting and receiving schemes of the user apparatuses based on user-apparatus specific user identification information, flag information showing a simultaneous transmitting and receiving scheme that is included in an uplink control channel, or signatures, a proper use of which is made according to the simultaneous transmitting and receiving scheme.