A communications system includes a base station; and a mobile communications terminal, where the base station and the mobile communications terminal execute a random access procedure, and perform radio communication. The base station is configured to assign to the mobile communications terminal, in the random access procedure, an identifier that corresponds to preamble information, the base station assigning the identifier based on first correspondence information that indicates a given correspondence relation of the preamble information to be transmitted by the mobile communications terminal and the identifier assigned to mobile communications terminal. The mobile communications terminal is configured to execute the random access procedure with the base station by transmitting to the base station, the preamble information that corresponds to the identifier assigned by the base station.