A method, apparatus, and computer program for estimating a rank, i.e. the number of uncorrelated spatial channels, of a radio channel are provided. The rank estimation is carried out by examining mutual proportions of a first eigenvalue and a second eigenvalue of a two-by-two channel matrix representation with a test function including elements of a solution of roots of a quadratic characteristic equation of the two-by-two channel matrix representation, wherein the test function comprises a discriminant of the solution of the roots of the quadratic characteristic equation without a square root operation.