Why Constant-Composition Codes Reduce Nonlinear Interference Noise

A time-domain perturbation model of the nonlinear Schrödinger equation is used to study wave-length-division multiplexed communication over a single polarization. The model explains (a) why constant-composition codes offer an improvement in signal-to-noise ratio compared with independent and uniform selection of constellation points and (b) why similar gains are obtained using carrier recovery algorithms even without using constant-composition codes.