The authors have investigated the impact of nonlinear distortions to optical signal transmission over proposed configuration of combined wavelength division multiplexed (WDM) fiber-optic transmission system (FOTS). In chosen model of ultra-dense combined WDM system optical signals are transmitted with two different per channel bitrates (i.e., 10 and 40 Gbit/s) and three different optical signal modulation formats are used for the encoding of transmitted data body (i.e., non-return to zero encoded on-off keying or NRZ-OOK, orthogonal polarization shift keying or 2-POLSK and NRZ encoded differential phase shift keying or NRZ-DPSK) and in addition “non ITU-T” defined minimum and equal frequency intervals are used for the channels separation.