A method for designing an optical system is disclosed. The optical system launches optical signals modulated with data into a fiber link having a property of inducing nonlinear distortion of the optical signals as a function of signal power of the optical signals and distance traversed in the fiber link. A Q-factor curve for the fiber link is determined as a function of the signal power. A signal power is preselected based on the Q-factor curve. The preselected signal power is associated with a set of channels. A coding gain is preselected based on the preselected signal power and a desired channel spacing for the set of channels.