## 1 Introduction
Since our objective is to design an ultra-long span system without an in-line active device, a reasonable target distance would be between 365 and 402 km. This target distance shows improvement over the results of previous studies on cases span configuration without in-line active devices and approaching the results on cases span configuration  
Figure 1: Signal power profile for various Raman pump power ratios, showing the upper and lower signal power boundaries as two limiting factors. Based on [8] with modification to show the profile for propagation distance of 390 km. with in-line active devices. Our preliminary analytical model on unrepeated ultra-long span system using a bidirectional pumped distributed Raman amplifier was described in [12], the impact of the size of the fiber effective core area on the transmission performance was analyzed in [13], and the transmission simulation of various data rates was reported in [14]. Our previous report [14] showed the simulation results for an overall 390-km length DBPSK-modulated signal on the conventional single-segment single fiber effective core area structure. Therefore, in this article, we use a 390-km span length and explore the benefits of using a three-segment hybrid fiber effective core area structure, and we extend the transmission capacity by using DQPSK-modulated signals. The proposed three-segment hybrid fiber core structure is an alternative option as a simple solution for the deployment of a new transmission link in a green field but will have high costs for upgrading from existing fiber links. Other solutions commonly used to overcome fiber nonlinearity in ultra-long span system are high-order (second-order or third-order) distributed Raman amplification schemes that have the ability to deliver Raman pump power that penetrates deeper into the fiber along the transmission link. As reported in [15], the optical signal is counter-directionally amplified by a powerful third-order Raman fiber laser (RFL) located at the receiver side. In the experiment, the RFL delivers 4.9 W at 1276 nm as the primary pump wavelength. The pump energy is transferred from the initial wavelength to a band around 1360 nm and then to 1425 and 1455 nm as amplification wavelength along the fiber transmission line through a cascaded Raman amplification scheme. In this backward pump configuration, the optical pump power at 1425 and 1455 nm provides 27 dB Raman gain to the optical signal. Using a backward pumping only configuration has a drawback on the increase in signal power variation and noise. As reported in [16], the experimental results show that bi-directionally pumped second-order Raman amplification scheme based on a random distributed feedback (DFB) fiber laser configuration can achieve low signal power variation and improves transmission performance compared to conventional Raman amplification schemes. There is no relative intensity noise (RIN) increase on the signal using this scheme even with bidirectional pumping. Another technology enabling extended reach is remote amplification. Recent experimental work [17] has shown that single-carrier 400 Gb/s unrepeated transmission can achieve 443.1 km by using bidirectional pumping remote optically pumped amplifiers (ROPAs) supported by parallel fibers for pump power delivery. ROTA is typically placed 60 to 140 km away from the receive site and also transmit side in some cases. The addition of a ROTA within the span at a distance from the Raman pumps improves the OSNR significantly and reduce the signal power variation along the fiber where the signal power is very low near the receiving end. In a conventional configuration, the energy necessary for creating optical amplification is brought to the ROTA by optical pump waves launched backward into the line fiber from the receiving end. Actually, this is the residual pump power that was not consumed to build the distributed Raman gain inside the line fiber that is used to pump the ROPAs.