Udalcovs, A., V. Bobrovs, and G. Ivanovs. “Investigation of Allowed Channel Spacing for Differently Modulated Optical Signals in Combined HDWDM Systems”. Elektronika Ir Elektrotechnika, vol. 112, no. 6, June 2011, pp. 19-24, doi:10.5755/j01.eee.112.6.437.