This form of the electrical (not optical) SNR implies a number ofconstraintsonthevarianceofthenonlinearinterferenceσ2NL; it must be statistically independent of the ASE (with variance σ2NL) [3] and Gaussian-distributed. Approximate expressions for σ2NL have been developed by a number of methods. For example, using a “Gaussian Noise” (GN) model based on a FWM-like approach and relying the observation that after DSP the statistical distribution of received constellation points is approximately Gaussian distributed,Carenaetal.[3]andPoggiolinietal.[5],[6]developed integralexpressionsforthenonlinearinterference.Furthermore, it has been shown that experimental observations [7] of scaling with link length conform to a partially coherent addition of interference between spans. Analytic expressions for maximum capacity and optimum launch power have been developed by Mecozzi et al. [8] using a perturbation approach (the GN model is also a perturbation approach).Oneoftheearliestefforts[9],expoundeduponin[10] and in [11], derived an analytical expression for the intensity fluctuations of cross-phase modulation (XPM) noise. Another derivation of (nonlinear) maximum capacity for various fiber types utilized the Volterra series expansion [12]. Independent of the specific modeling methods, it is necessary to include the three primary characteristics that define the fiber channel: loss, dispersion, and nonlinearity. Each of these is necessary to accurately account for the nonlinear impairments of deterministic signals in neighboring channels.