A base station used in a packet transmission system in which shared user data to be selectively received by a user are transmitted using a shared data channel from one or more base stations to one or more mobile stations is provided. The base station includes means configured to provide user control data associated with transmission of the shared user data; means configured to provide notification data that include notification information used to select the shared user data and group identification information for specifying the notification information; and means configured to generate an associated control channel that is associated with the shared data channel and includes the user control data and the notification data, the associated control channel and the shared data channel being transmitted from the base station to the mobile station. The generated associated control channel is transmitted from the base station to the mobile stations.