The invention relates to the technical field of radio communications, and in particular to an antenna device for a radio base station, and a method of operating an antenna device in a Multiple-Input Multiple-Output system. Embodiments of the invention disclose a secondary precoder (24) in series with a Multiple-Input Multiple-Output precoder (22). The Multiple-Input Multiple-Output precoder (22) has a first plurality of inputs (26, 28) for one or more Multiple-Input Multiple-Output data streams, and a first plurality of outputs (30, 32) for the one or more Multiple-Input Multiple-Output data streams. The secondary precoder has a second plurality of inputs (34, 36) and a second plurality of outputs (38, 40). The first plurality of outputs (30, 32) are in communication with the second plurality of inputs (34, 36). The antenna device further comprises a controller device (64), and the antenna device is operable to control an output power value of the one or more data streams at the second plurality of outputs (38, 40).