Radio communications system comprising a plurality of cells (C1, C2, C3, C4), each containing a base station (SB1, SB2, SB3, SB4) and mobile terminals (TM1, TM2, TM3, TM4, TM5, TM6, TM7), said system being able to manage group communications between the mobile terminals (TM1, TM2, TM3, TM4, TM5, TM6, TM7), and comprising a switch connected to each base station (SB1, SB2, SB3, SB4), which is configured to: - during the establishment of a group communication, in each cell (C1, C2, C3, C4) covering the group communication: o allocate, an uplink channel contained in an uplink carrier frequency selected according to a first specific reuse pattern, the uplink carrier frequency being different for each adjacent cell (C1, C2, C3, C4) covering the group communication, the first frequency reuse pattern containing a number of cells less than the number of cells of a conventional TETRA type frequency reuse pattern, and a downlink channel contained in a downlink carrier frequency selected according to a second specific reuse pattern, the downlink carrier frequency comprising a number of downlink channels strictly greater than the number of uplink channels contained in the uplink carrier frequency and the number of uplink carrier frequencies per cell being greater than the number of downlink carrier frequencies, the downlink channel and the downlink carrier frequency being identical for all cells (C1, C2, C3, C4) belonging to a same downlink iso-frequency plane covering a part of the group communication, and being different for cells (C1, C2, C3, C4) belonging to different downlink iso-frequency planes covering the group communication; o match the uplink channel and the downlink channel allocated to each cell (C1, C2, C3, C4) covering the group communication; - at the end of the group communication, mismatch the uplink channel and the downlink channel allocated to each cell (C1-C4) covering the group communication, the mobile terminals (TM1, TM2, TM3, TM4, TM5, TM6, TM7) of the group communication being configured to exchange voice messages using the uplink channels and the downlink channels allocated to each cell (C1, C2, C3, C4) covering the group communication. - during the establishment of a group communication, in each cell (C1, C2, C3, C4) covering the group communication: o allocate, an uplink channel contained in an uplink carrier frequency selected according to a first specific reuse pattern, the uplink carrier frequency being different for each adjacent cell (C1, C2, C3, C4) covering the group communication, the first frequency reuse pattern containing a number of cells less than the number of cells of a conventional TETRA type frequency reuse pattern, and a downlink channel contained in a downlink carrier frequency selected according to a second specific reuse pattern, the downlink carrier frequency comprising a number of downlink channels strictly greater than the number of uplink channels contained in the uplink carrier frequency and the number of uplink carrier frequencies per cell being greater than the number of downlink carrier frequencies, the downlink channel and the downlink carrier frequency being identical for all cells (C1, C2, C3, C4) belonging to a same downlink iso-frequency plane covering a part of the group communication, and being different for cells (C1, C2, C3, C4) belonging to different downlink iso-frequency planes covering the group communication; o match the uplink channel and the downlink channel allocated to each cell (C1, C2, C3, C4) covering the group communication; o allocate, an uplink channel contained in an uplink carrier frequency selected according to a first specific reuse pattern, the uplink carrier frequency being different for each adjacent cell (C1, C2, C3, C4) covering the group communication, the first frequency reuse pattern containing a number of cells less than the number of cells of a conventional TETRA type frequency reuse pattern, and a downlink channel contained in a downlink carrier frequency selected according to a second specific reuse pattern, the downlink carrier frequency comprising a number of downlink channels strictly greater than the number of uplink channels contained in the uplink carrier frequency and the number of uplink carrier frequencies per cell being greater than the number of downlink carrier frequencies, the downlink channel and the downlink carrier frequency being identical for all cells (C1, C2, C3, C4) belonging to a same downlink iso-frequency plane covering a part of the group communication, and being different for cells (C1, C2, C3, C4) belonging to different downlink iso-frequency planes covering the group communication; o match the uplink channel and the downlink channel allocated to each cell (C1, C2, C3, C4) covering the group communication; - at the end of the group communication, mismatch the uplink channel and the downlink channel allocated to each cell (C1-C4) covering the group communication, the mobile terminals (TM1, TM2, TM3, TM4, TM5, TM6, TM7) of the group communication being configured to exchange voice messages using the uplink channels and the downlink channels allocated to each cell (C1, C2, C3, C4) covering the group communication.