Method for transmitting an optical digital WDM signal over an optical transmission link or a passive optical network, the optical WDM signal (S WDM) including N optical channels matching a given optical frequency grid, the method comprising the steps of: (a) supplying the WDM signal (S WDM) to a near end of the optical transmission link or a near end of the passive optical network; (b) receiving the WDM signal (S WDM) at a far end of the optical transmission link or at one or more far ends of the passive optical network; (c) separating the optical channel signals (s i (t)) by splitting and bandpass filtering the WDM signal (S WDM) received; and (d) converting the optical channel signals (s i (t)) into electrical channel signals using direct optical detection; (e) wherein pre-distortion is applied for creating the optical channel signals (s(t)), the method being characterized in (f) that the pre-distortion is applied by adding a pre-distortion signal (s pd,i (t)) to a respective digital modulation data signal (s mod,i (t)) which is used for creating the respective optical channel signal (s i (t)), the pre-distortion signal (s pd,i (t)) including at least one pre-distortion component depending on the optical channel signal (s i±j (t)) of a neighboring channel, (g) that the at least one pre-distortion component is dependent on the overlap of the spectral optical filter function (F i (f)) used at a far end of the optical transmission link or at one or more far ends of the passive optical network to extract the respective optical channel signals (s i (t)) from the WDM signal (S WDM) received and on the spectrum of the optical channel signal (s i±j (t)) of the respective neighboring channel, and (h) that the optical WDM signal (S WDM) is created such that neighboring channel signals (s i (t)) are orthogonally polarized. (a) supplying the WDM signal (S WDM) to a near end of the optical transmission link or a near end of the passive optical network; (b) receiving the WDM signal (S WDM) at a far end of the optical transmission link or at one or more far ends of the passive optical network; (c) separating the optical channel signals (s i (t)) by splitting and bandpass filtering the WDM signal (S WDM) received; and (d) converting the optical channel signals (s i (t)) into electrical channel signals using direct optical detection; (e) wherein pre-distortion is applied for creating the optical channel signals (s(t)), (f) that the pre-distortion is applied by adding a pre-distortion signal (s pd,i (t)) to a respective digital modulation data signal (s mod,i (t)) which is used for creating the respective optical channel signal (s i (t)), the pre-distortion signal (s pd,i (t)) including at least one pre-distortion component depending on the optical channel signal (s i±j (t)) of a neighboring channel, (g) that the at least one pre-distortion component is dependent on the overlap of the spectral optical filter function (F i (f)) used at a far end of the optical transmission link or at one or more far ends of the passive optical network to extract the respective optical channel signals (s i (t)) from the WDM signal (S WDM) received and on the spectrum of the optical channel signal (s i±j (t)) of the respective neighboring channel, and (h) that the optical WDM signal (S WDM) is created such that neighboring channel signals (s i (t)) are orthogonally polarized.