## 4 Simulation Model of DQPSK Modulated Data Transmission
DQPSK is the first step to higher spectral efficiency, which can transmit 2 bits/symbol by utilizing a pair of nested MZMs called I&Q (in-phase and quadrature) modulator. Several relative advantages of using DQPSK modulation are higher data rate, reduction in the required OSNR, increasing spectrum efficiency, resistance to the signal power fluctuation, robust to nonlinear distortion, and improving linear sensitivity that enhances the overall system performance [31; 32]. Return-to-zero (RZ) pulse shape has superior performance compared to non-return-to-zero (NRZ) pulse shape because RZ is inherently more tolerant to inter-symbol interference (ISI), chromatic dispersion (CD), and polarization mode dispersion (PMD). Moreover, RZ pulse shape has an excellent nonlinear performance on high power signal, which is mandatory for ultra-long span applications with high nonlinear Kerr distortion [33]. The simulation model in this study uses RZ-DQPSK transmitter consisting of a laser diode at a wavelength of 1550 nm, which is externally modulated to form an RZ pulse train. The implementation of RZ-DQPSK transmitters most conveniently use dual stages Mach-Zehnder modulators (MZMs) as a phase modulator and pulse carver [32]. The phase modulator works as a data modulator driven by the incoming electrical data stream. The optical channel power into the fiber launch point is set to 0 dBm (1 mW) to balance between providing a sufficient OSNR and reducing the impact of the nonlinear effect to achieve optimum BER performance [34; 35]. Figure 7 shows a schematic diagram of the simulation setup for the ultra-long span transmission system, which was configured without an active repeater or optical line amplifier (OLA) component along the 390-km transmission link. This unrepeetered system consists of a bidirectional pumped distributed Raman amplifier to compensate transmission fiber loss, chromatic dispersion compensation fiber (DCF), and EDFA which is mainly to compensate DCF loss at the receiver side. A pulse broadening technique is used to reduce the Kerr nonlinear distortion by broadening the initial pulse shape using a 6-km dispersion compensation fiber (DCF1) which is placed prior to launching the signal into the fiber span. Moreover, in order to maintain high OSNR and to compensate the DCF1 loss, a relatively high forward Raman pump is installed right before the launch point. The signal power in the fiber propagation follows a profile of optical gain/loss in which the bidirectional distributed Raman amplifications provide distributed gain along the fiber link and counterbalance the passive attenuation of the fiber, resulting in acceptable signal power at the receiver point, \(z=390\) km. When the optical signal reaches the received side, the optical power is still strong enough due to 5 dB Raman net gain provided by the bidirectional Raman system. The following step is a dispersion compensation process to counter balance the accumulated fiber anomalous dispersion by cascading DCF2 and DCF3 with normal dispersion, which equally can be substituted using electrical dispersion compensation. Since the length of the span is very long, the amount of the dispersion to be compensated becomes very high. Compensating for the dispersion on a single DCF pool will drop the signal power to an extremely low level. Therefore, we chose to split the DCF and put the EDFA in between to compensate for the attenuation of the two DCFs, to maintain optical power at a moderate level, and finally to provide enough optical power to be detected by the receiver photodiode [10; 36]. This transmission system does not have a ROPA along the propagation fiber or a forward error correction (FEC) component on signal recovery subsystem.