The idea of the invention is to allocate channels within a predetermined group of cells with a dynamic channel allocation method and to equalize interference caused to the cell group by connections outside the cell group e.g. through frequency or time slot hopping. Cell groups may be superimposed and they may be formed e.g. by the cells of an office system or a broad-coverage mobile communications system, by the cells of a mobile communications network of two different operators, by the cells of a network according to two different systems or by the cells of a network operated by one operator. If the cell groups are formed of cells in one network, a suitable size may be chosen for the cell group so that the computing and signalling load required by centralized dynamic channel allocation in the cell group will remain easily controllable.