Patent ID: 7376120

Claim:
A mobile communication system comprising: a plurality of radio base stations whose system types are different each other; mobile phones wirelessly connected with the plurality of radio base stations; and a private branch exchange wired with the plurality of radio base stations, wherein an identical calling code is provided to each reference radio base station designated as belonging to a predetermined radio base station group, and a minimum additional ID is provided to said each reference radio base station designated as belonging to the predetermined radio base station group, where the minimum additional ID provided to said each reference radio base station is different from any other minimum additional ID provided to other reference radio base stations belonging to the predetermined radio base station group; a given reference radio base station is adapted to receive a frame signal transmitted from another reference radio base station, where the frame signal contains radio synchronous conditions including the calling code of the another reference radio base station which transmitted the frame signal, a reception threshold level which indicates a predetermined satisfactory reception level of the frame signal, and the minimum additional ID of the another reference radio base station which transmitted the frame signal, when the given reference radio base station receives the frame signal containing a control CH signal from the another reference radio base station, the given reference radio base station detects the calling code, the reception threshold level and the minimum additional ID of the another reference radio base station which transmitted the frame signal, on the basis of control information contained in the control CH signal of the frame signal, and determines the another reference radio base station which transmitted the frame signal as a radio base station to be synchronized-to, if all three conditions of: the calling codes of the another reference radio base station which transmitted the frame signal and the given reference radio base station are identical, the reception threshold level is equal to or more than a predetermined level, and the minimum additional ID level of the another reference radio base station which transmitted the frame signal is smaller than the minimum additional ID level of the given reference radio base station, are met, and responsive to the conditions being met, matches a transmission timing of the frame signal of the given reference radio base station with a transmission position of the control CH signal in the frame signal transmitted from the another reference radio base station, whereby a frame synchronization in a radio section between the given reference radio base station and the another reference radio base station is established.