Mutual information in nonlinear communication channel. Preliminary analytical results in large SNR and small nonlinearity limit

Applying perturbation theory to the path-integral representation for the mutual information of the nonlinear communication channel described by the nonlinear Shr\"{o}dinger equation (NLSE) with the additive Gaussian noise we analyze the analytical expression for the mutual information at large signal-to-noise ratio ($\mathrm{SNR}$) and small nonlinearity. We classify all possible corrections to the mutual information in nonlinearity parameter and demonstrate that all singular in $\mathrm{SNR}$ terms vanish in the final result. Furthermore our analytical result demonstrates that the corrections to Shannon's contribution to the mutual information in the leading order in $\mathrm{SNR}$ are of order of squared nonlinearity parameter. We outline the way for the calculation of these corrections in the further investigations.