To feedback MIMO channel conditions, a codeword from a codebook is selected. To reduce signalling, the codewords are organized into codeword subsets. The receiver signals an index of a codeword into a current 5 codeword subset previously made known to the transmitter. The current codeword subset is adaptively selected based on a threshold criterion. For example, if the best codeword from the current codeword subset is not sufficiently similar to the best codeword in the full codebook, a switch in the codeword subset is made.