Abstract: An apparatus for use by a coherent receiver is configured to perform obtaining a first set of coefficients indicating a first filter in an adaptive equalizer of the coherent receiver, wherein the first filter is an adaptive Multi-Input Multi-Output, MIMO, filter with N taps in each branch, wherein N is an integer number; determining a further set of coefficients indicating a MIMO filter with a single tap in each branch based on the first set of coefficients; determining a first matrix based on the further set of coefficients; orthogonalizing the first matrix, thereby obtaining an orthogonalized matrix; determining a state-of-polarization based on at least part of the orthogonalized matrix.