## 2 Proposed Network Architecture  
The aim of this work is to compensate the effect of the nonlinearities while using a cost effective model. This section includes the proposed architecture of the long haul high capacity optical network, for which the considered downlink setup is shown in Figure 1.  
Figure 1a shows a WDM transmitter consisting of an array of independent free running lasers separated by a fixed channel spacing (25 or 50 GHz). External modulation has been used in this work to achieve high extinction ratio with minimum nonlinear effects from the transmitter side. The use of an electroabsorption modulated laser (EML) causes nonlinear effects due to the limited linear range of operation and transient chirp of the laser. On the other hand, directly modulated laser (DML) based systems experience further degradation in performance due to the intrinsic adiabatic chirp. To compensate the chromatic dispersion effects, duo-binary modulation has been used in this work, which is a three level signal in electrical form (instead of two as used in many other modulation formats and waveform). The optical form of the duo-binary modulation is achieved by over-driving a Mach-Zehnder modulator (MZM) after producing the driver signal by passing the output of the pseudo random bit sequence (PRBS) module through a low pass filter. The low pass filter acts as a duo-binary encoder and it produces the same effect in frequency domain as a one-bit delayed line in electrical domain to produce a set of correlated signal levels for the duo-binary modulation. The duo-binary modulation occupies less spectrum and therefore is more tolerant the fiber dispersion effects, which is very useful for long haul transmission. A modulator driver is used with the MZM to control the bias voltage and ensure a full swing of 2V\({}_{pi}\) swing is available for the modulation. A WDM Mux is installed to transmit optical signals over single mode fiber by combining different wavelengths with an insertion loss of 3 dB. The transmission fiber consist of 0.2 dB/km constant loss and, to compensate  
Figure 1: (**a**) Considered setup for the optical fiber communication system for beyond 100 Gbps data rates; (**b**) detailed block diagram of the proposed DSP receiver. the power loss, an EDFA amplifier is used in the system after each 100 km of transmission loop. At the receiver side, after de-multiplexing, the optical signals are converted into electrical signals by employing the photo detectors for direct detection that converts the duo-binary modulated data to a simple NRZ demodulation. The BER analyzer is used to quantify the performance of the received signals. In the Figure 1, a sub-figure is also presented to depict how the transmitted various pulses spread inside single mode fiber. The performance is evaluated by keeping \(5\times 10^{9}\) symbol/s symbol rate for eight and 16 number of channels, thus providing aggregate data rate of 40 Gbps and 80 Gbps. For 32 number of channels, symbol rate has to be dropped to \(3.2\times 10^{9}\) symbol/s to satisfy the BER limit of \(3.8\times 10^{-3}\), which provides over 100 Gbps data rate. The simulations are performed in a high performance software Optisystem, which is a well-known tool for optical communication system analysis. Optisystem provides a large set of optical and RF components library to enable users for modeling of a complete downlink or uplink system. The range of signal visualizers and analysis tools makes it a useful resource to investigate the practical system designs. Figure 1b shows the details of the proposed DSP receiver. To improve the performance of the system in the presence of second-order non-linearities, a cascade operation of LMS based linear transversal filter (seven-tap) and RLS filter (11-tap) is used. This function combines the advantages of the two approaches by improving the convergence time, even in the presence of non-stationary impairments in the channel.