  In a nonlinear optical fiber communication (OFC) system with signal power
much stronger than noise power, the noise field in the fiber can be described
by linearized noise equation (LNE). In this case, the noise impact on the
system performance can be evaluated by moment-generating function (MGF) method.
Many published MGF calculations were based on the LNE using continuous wave
(CW) approximation, where the modulated signal needs to be artificially
simplified as an unmodulated signal. Results thus obtained should be treated
carefully. More reliable results can be obtained by improving the CW-based LNE
with the accurate LNE proposed by Holzlohner et al in Ref. [1]. In this work we
show that, for the case of linearized noise amplified by EDFAs, its MGF can be
calculated by obtaining the noise propagation information directly from the
accurate LNE. Our results agree well with the experimental data of multi-span
DPSK systems.
