  In this paper, we propose to use the discretized version of the so-called
Enhanced Gaussian Noise (EGN) model to estimate the non-linearity effects of
fiber on the performance of optical coherent and uncompensated transmission
(CUT) systems. By computing the power of non-linear interference noise and
considering optical amplifier noise, we obtain the signal-to-noise (SNR) ratio
and achievable rate of CUT. To allocate the power of each CUT channel, we
consider two optimization problems with the objectives of maximizing minimum
SNR margin and achievable rate. We show that by using the discretized EGN
model, the complexity of the introduced optimization problems is reduced
compared with the existing optimization problems developed based on the
so-called discretized Gaussian Noise (GN) model. In addition, the optimization
based on the discretized EGN model leads to a better SNR and achievable rate.
We validate our analytical results with simulations and experimental results.
We simulate a five-channel coherent system on OptiSystem software, where a
close agreement is observed between optimizations and simulations. Furthermore,
we measured SNR of commercial 100Gbps coherent transmitter over 300 km
single-mode fiber (SMF) and non-zero dispersion-shifted fiber (NZDSF), by
considering single-channel and three-channel coherent systems. We observe there
are performance gaps between experimental and analytical results, which is
mainly due to other sources of noise such as transmitter imperfection noise,
thermal noise, and shot noise, in experiments. By including these sources of
noise in the analytical model, the gaps between analytical and experimental
results are reduced.
