The invention concerns a melhod and a base station for controlling beam forming in a mobile cellular network: Antenna units apply a respective beam pattern sequence on each sector of a cell served by the respective base station. A base station acts as trigger base station informing all neighbored base stations of the cluster not to change their beam pattern sequences. The trigger base station changes the beam pattern sequence applied on the sectors of its cell. The trigger base station triggers one or more mobile units to calculate signal quality parameters for currently applied beam pattern sequence. The trigger base station selects an optimal beam pattern sequence from the set based on the calculated signal quality parameters and applies as a new beam pattern sequence on the respective sector.