## 1 Introduction  
A recent Cisco's forecast report stated that the number of devices connected to the internet will be more than three times the worldwide population by 2023 [1]. Therefore, the increase in demand for unlimited bandwidth has led to the evolution of high-capacity and high-speed optical transmission systems to satisfy the ever-growing capacity requirements. A variety of multiplexing techniques have been used to provide the required high-speed transmission. For instance, speed is an advantage obtained from using the time division multiplexing (TDM) system. However, TDM uses one wavelength for downstream and another wavelength for upstream, limiting the optimum bandwidth for a given user. Therefore, the bandwidth of the single fiber is not fully utilized. To compensate for this, the wavelength division multiplexing (WDM) system, which is considered the most successful technique in optical networks [2, 3], assigns a specific wavelength for a given subscriber. Despite this advantage in increasing capacity, WDM lacks wavelength sharing and suffers from nonlinear effects. On the other hand, optical code division multiple accesses (optical CDMA) have several key features, most notably, their highly flexible and efficient asynchronous access for multiple users in a busy network, fast scheduling without buffering time, and easier expansion in network implementation. However, optical CDMA suffer from various types of noise, one of which is multiple access interference (MAI), the main source of bit error. To deal with the limitations and inefficiency that arises from each system mentioned above, hybrid systems have been proposed. The hybrid system of optical CDMA-dense wavelength division multiplexing (optical CDMA/DWDM) combines the advantages and strengths of both multiplexing schemes [2, 4, 5, 6, 7] but still suffers from limitations such as chromatic dispersion (CD), laser phase noise (LPN) and nonlinear effects as do all the previous systems. These limitations have a huge impact on the signal quality and degrade system performance. A variety of methods have been proposed to compensate for chromatic dispersion, such as using digital filters including the time-domain least mean square adaptive filter, a time-domain fiber dispersion finite impulse response filter, and a frequency-domain blind look-up filter [7] and by controlling the grating length of the fiber Bragg grating (FBG) [8; 9]. The phase noise, which occurs due to transmit and receive lasers, has a huge impact on coherent systems employing high order modulation formats. The appearance of transmit and receive phase noises make it very difficult to discriminate between them. [PERSON] et al. proposed a digital coherence enhancement (DCE) technique using an interferometric device along with a very simple electronic processing, which significantly reduced the phase noise of transmit or receive lasers [10]. However, when the interactions between the chromatic dispersion and the laser phase noise occurred, they produced a new issue that was very difficult to compensate for. The interaction is called equalization enhanced phase noise (EEPN) [11]. A deep investigation on the impact of EEPN on nonlinear optical fibers through split-step Fourier simulations has been conducted and analytical model predictions have been proposed for single and multiple channels of a dual-polarization DP-16 QAM system over the long haul. The investigation proved that the reducing the EEPN had a huge impact on improving the system performance [12]. In addition, nonlinear effects such as inter- and intra-channel Four Wave Mixing (FWM) are considered a major source of impairments in high data rate and high-capacity optical networks [12; 13]. In fact, inter- and intra-channel FWM effects have been widely investigated in pure WDM systems and are considered one of the foremost application issues in optical fiber networks [14; 15; 16; 17; 18]. On the other hand, the effects of inter- and intra-channel FWM in the hybrid optical CDMA/DWDM system have not been addressed.