Method for setting up a group call in a cellular radio system

A method for setting up a group call potentially involving a plurality of mobile radio stations, in a mobile radio system in which a geographical area covered by the system is divided into a plurality of radio cells, at least some of which overlap one another in at least one respective overlapping peripheral area, each cell having at least one base station provided with a respective multichannel transceiver, each base station being assigned at least one radio frequency channel for speech connections, the radio frequency channels for speech connections being different from one another in neighboring ones of said cells, each said base station also having a radio frequency channel for control, there being at least two mobile radio stations within said geographical area, each registered with a respective base station of a respective cell, but being free to roam within said geographical area providing that when transitioning beyond a respective said overlapping area from one said cell to another said cell, the respective mobile radio station changing registration from the respective cell departed to the respective cell entered, comprising:

• a) programming into each of said mobile radio stations an identification of at least one group to which each mobile radio station belongs;
• b) maintaining a first file listing which of the base stations in said plurality of radio cells belong to an operation area of the group, said operation area being smaller than said geographical area covered by said mobile radio system,
• c) maintaining a second file listing which of said base stations in said plurality of radio cells are in radio cells not belonging to but abutting base stations belonging to said operation area of the respective said group,
• d) initiating a group call, initiating a signalling message by a user's respective said mobile radio station indicating that a group call is requested and the identity of the desired said group,
• e) retrieving from said first file information as to which of said base stations serve cells belonging to said operation area of the respective said group, and allocating traffic channels for said group call on each of the respective base station multichannel transceivers belonging to said operation area,
• f) transmitting on a control channel of said multichannel transceivers of said base stations which belong to said operation area of the respective said group, to the mobile radio stations of the respective said group, a first group call paging message containing an instruction to switch to said group call on the respective said allocated traffic channel of a respective one of said base stations belonging to said operation area of the respective said group call,
• g) receiving said first group call paging message at said mobile radio stations which are registered to any of said base stations belonging to said operation area of the respective said group, and switching said mobile radio stations to said group call on the respective said allocated traffic channel of a respective one of said base stations belonging to said operation area of the respective said group call,
• h) retrieving from said second file information listing which of said base stations in said plurality of radio cells are in radio cells not belonging to, but abutting base stations belonging to said operation area of the respective said group,
• i) transmitting on a control channel of said multichannel transceivers of said base stations in said abutting cells, to mobile radio stations of the respective said group, a second group call paging message which contains the respective said group identification and an instruction to mobile radio stations receiving said second group call paging message and belonging to the respective said group, to switch to a respective said allocated traffic channel of a respective adjoining one of said base stations belonging to said operation area of the respective said group call,
• j) receiving said second group call paging message at mobile radio stations registered to any of said base stations in said abutting radio cells and belonging to the respective said group, and switching said mobile radio stations to said group call on the respective said allocated traffic channel of a respective adjoining one of said base stations belonging to said operation area of the respective said group call.