In order to accomplish efficient communication of data between a base station and a plurality of mobile terminals, particular orthogonal codes are transmitted from the mobile stations to the base station as alert signals indicating the presence of data to be transmitted. The base station checks whether particular orthogonal codes are contained in the alert signals, and when the particular orthogonal codes are detected in the alert signals, information representing the detected orthogonal codes and the schedules for transmitting the data from the mobile terminals to the base station are transmitted from the base station to the mobile terminals as an alert response to the alert signals. When the information representing the orthogonal codes used for the alert signals are contained in the alert response transmitted from the base station to the mobile terminals, the data waiting to be transmitted is transmitted from the mobile terminals to the base station according the data transmission schedules in the alert response.