## 2 Proposed Model Layout  
This section clarifies the framework of the proposed system and performance against RIrNLs. The increase in transmission channels gives rise to further distortion effects including channel-to-channel interference or beating noise, saturation of EDFA due to distribution of the overall transmit power to multiple channels, wavelength dependent chromatic dispersion interaction with several wavelengths as compared to the single channel. The WDM system is idealized by using 4 to 32 number of continuous wave (CW) laser sources to investigate the corresponding increase in non-linear effects due to the multiple channel transmission. An advanced duo-binary modulation technique is implemented at the transmitter side, whose function is to adjust the spectral width of the input optical signal, as presented in Figure 1. Each channel is modulated with a 3.125 Gb/s data rate signal which provides an aggregate data rate of 100 Gb/s for 32 numbers or channels.  
Duo-binary modulation is implemented by using an electrical time delay line with an exclusive OR gate, which encloses pseudo-random binary sequence (PRBS) generator and uses two LiNb Mach-Zehnder modulators (MZMs) as a simulation model in Optisystem software. This model implements duo-binary modulator through a duo-binary precoder, an electrical subtractor, \(\chi\), and delay, \(\tau\), to generate duo-binary signal. After modulation from each channel, the data are multiplexed using dense wavelength division multiplexer (DWDM) which has \(-100\) dB threshold noise and 3 dB dynamic noise. After multiplexing, optical signals are transmitted over SMF. Above 500 km, SMF is used with five spansof 100 km each. To minimize RIrNLs, linear dispersion and attenuation, each span is connected with EDFA and semiconductor optical amplifier (SOA). At the receiver, \(R_{x}\), side, the data is de-multiplexed (DeMUX) and optical signals are converted to electrical form using avalanche photo diode (APD) with 1 A/W responsivity and \(100\times 10^{-24}\) W/Hz thermal power density. The gain of diode is maximized several times and enables greater sensitivity as compare to conventional photo diode (PD). The signal then proceeds to microstrip Chebyshev low pass filter to minimize the factors of chromatic dispersion (CD), and amplified spontaneous noise (ASE). The filtered signals are then purified from RIrNLs by employing limiting RF amplifier and enhanced DSP system. The proposed DSP receiver includes a FD-MIMO equalizer, synchronizing block to balance the flow of multiple received wavelengths and time domain equalizer (TDE) to support the phase behavior of the received signals. The internal procedure of FDE-MIMO equalizer is presented in Figure 2, employing overlap save technique. Moreover, 3R-regenerator component is installed for re-shaping, re-timing and re-amplification of the received signals, where they are linked to a BER analyzer to investigate the received pulse property. The list of parameters with their values for the simulation are described in Table 1. As compared to [19; 20; 21], the authors have investigated the performance of duo-binary modulation for accumulative distortion effects caused due to transmission of large number of WDM channels over long-haul distances, while varying the wavelength dependent chromatic dispersion and refractive index of the optical fiber.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Description** & **Magnitude** \\ \hline Reference wavelength & 1550 nm \\ \hline Attenuation & 0.2 dB/km \\ \hline Input power & \(-6\) dBm to 6 dBm \\ \hline Received power & \(-27\) dBm to \(-16\) dBm \\ \hline Channel spacing & 50 GHz to 200 GHz \\ \hline Length of fiber & 500 km \\ \hline Nonlinear effective area & 80 \(\mu\)m\({}^{2}\) \\ \hline Number of Channel & 4–32 \\ \hline Nonlinear dispersion & \(-3\) ps\({}^{3}\)/km \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters with their values and units.  
Figure 1: WDM based OTN model for mitigation of RIrNLs.