A cellular multiple-input and multiple-output, MIMO, system comprises a base station having a plurality of antennas. Pilot signals are received by the base station in each frame of a sequence of frames (60). The base station analyzes the pilot signals to determine radio channel properties of radio channels between respectively each one of a plurality of terminals and the base station. The sequence of frames (60) comprises frames of a first frame type (61, 63, 64, 68) and frames of a second frame type (62, 69). Each frame of the second frame type (62, 69) have a greater number of time slots in which the base station receives the pilot signals than each frame of the first frame type (61, 63, 64, 68). The frames of the second frame type (62, 69) are included periodically in the sequence of frames (60).