A mobile station is connected to a multi-carrier cellular communication system having a plurality of sub-carriers. The sub-carriers are classified into K frequency sub-bands, channel state information (CSI) of the frequency sub-bands is represented by matrices Wi (i=0 . . . K−1), where K is an integer grater than 1. The mobile station determines a first sub-index k1 for the K matrices Wi (i=0 . . . K−1), and a second sub-index k2 for each one of the K matrices Wi(i=0 . . . K−1). The first sub-index k1 is common for all frequency sub-bands, and the second sub-index k2 is specific for the indexed matrix that corresponds to one frequency sub-band. The mobile station reports to a base station of the multi-carrier cellular communication system the first sub-index k1 and at least one second sub-index k2.