An apparatus, a method and a computer program for determining estimates is disclosed. The apparatus determines estimates for transmitted symbols from received symbols of an optical signal affected by cross-Polarization Modulation (XpolM). The apparatus groups two or more subsequent received symbols from the optical signal into a group of received symbols and estimates an optical channel based on the group of received symbols and a hypothesis on the transmitted symbols. The apparatus further determines a reliability measure for a combination of the group of received symbols, the channel estimate, and the hypothesis, and determines the estimates for the transmitted symbols based on reliability measures for multiple hypotheses on the transmitted symbols.