Determining transmitter antenna weights at a base station having more available transmit antennas than the available number of reference signals can be performed by transmitting reference signals and receiving channel feedback data derived by a mobile terminal from the reference signals. The reference signals are each assigned to one of two or more antenna groupings, wherein at least a first one of the antenna groupings includes two or more transmit antennas, and transmitted using at least one transmit antenna from the corresponding antenna grouping. A first beam-forming vector for the first one of the antenna grouping is determined, and mapping the one or more data streams to the transmit antennas according to a final precoding matrix that depends on the channel feedback data and the first beam-forming vector is performed, to obtain a weighted transmit signal for each of the antennas.