Data signals transmitted by a plurality of transmitting antennas over a radio channel are demodulated. The method comprises receiving (202) on a plurality of receiving antennas, a data signal and a reference signal, the contents of the reference signal being known a priori to the receiver. The contents of the reference signal are used for calculating (204) an estimated polynomial channel matrix. A polynomial pre-filter matrix is calculated (206, 208) by a decomposition of the estimated polynomial channel matrix into a product of a paraunitary polynomial matrix and an upper triangular polynomial matrix with minimum phase filters on its main diagonal, where the polynomial pre-filter matrix is obtained by calculating the paraconjugate of the paraunitary polynomial matrix. The received data signal is demodulated (212) where the received data signal is multiplied with the calculated polynomial pre-filter matrix.