There is provided beam forming using an antenna array. The antenna array is configured to transmit across an angular sector. The angular sector is divided into at least two sub-sectors by respective port weights being applied to respective physical antenna ports of the antenna array. A respective set of virtual antenna ports is determined for each sub-sector. A first set of reference signals for acquiring channel state information on the set of virtual antenna ports is alternatingly transmitted in each one of the at least two sub-sectors.