# Nonlinear Effect and MAI Impact on SAC-OCDMA System Based on 2D Multi-Diagonal Code and Laser Array
## 5 Results and Discussion
### Second Scenario: With Considering MAI Effect  
In this section, we consider all the effects including the MAI on SAC-OCDMA system performance. Figure 6, presents the system setup for four users. This system is composed of a CW laser array used to generate the three wavelengths required by the SAC-OCDMA encoder to produce the user signature for each network subscriber. Furthermore, user bits are generated by a pseudo-random bit sequence (PRBS) with a sequence length of 214\(-\)1 followed by an NRZ block to produce a line code. Then, a MZM external modulator is used to modulate the electrical user signal using the optical carrier generated by the CW laser. A SAC-OCDMA encoder is employed to multiply the generated '1' user bit by the 2D-MD signature. Then, all user contributions are combined in one composite signal transferred into the optical channel. At the reception side, a SAC-OCDMA decoder is employed to extract the desired user signal from the received signal. Then, a photo-detector PIN is used to convert the optical desired signal into the electrical one. Furthermore, a Bessel filter, with a cutoff frequency equal to 0.75 muliplied by the bit rate, is employed to limit the signal into the desired frequency band. Finally, a BER analyzer is used to measure the quality of the received user signal in terms of Q-factor and eye diagram. According to Figure 4 (i.e., Q-factor versus effective area), we have to choose an aggregate bit rate less than 30 Gb/s to obtain a Q-factor higher than 15.5 dB. Indeed, we select a bit rate equal to 5 Gb/s per user to fulfill the high-quality transmission and to respect the user requirement for access network in terms of QoS. Figure 5: SAC-OCDMA system performance versus fiber length. Figure 7 shows the received electrical eye diagram for the different simultaneous users at 5 Gb/s per user and over 50 km of fiber transmission, while considering the optical coherent source noise, the linear effects (fiber loss, GVD, DGD), the nonlinear effects (SPM, SRS and \(\gamma\)) and the photodetector noise (i.e., thermal and shot noises). According to Figure 7, the system can accommodate four users leading to a total aggregate bit rate of 20 Gb/s with an open eye diagram. Additionally, the open eye diagram of the four simultaneous users indicates a high transmission quality at an aggregate bit rate of 20 Gb/s. Figure 6: Simulation setup of 2D-MD SAC-OCDMA system supporting four users (MZM: Mach-Zehnder Modulator, OF: Optical Fiber, DCF: Dispersion Compensating Fiber, EDFA: Erbium Doped Fiber Amplifier, BOF: Bessel Optical Filter, PPIN: Photodiode PIN, LPBF: Low Pass Bessel Filter). Figure 7: _Cont._  
Figure 7: _Cont_. Furthermore, Table 5 illustrates a Q-factor greater than 15.5 dB for all users considering linear and nonlinear effects for two effective area values (Aeff = 93 \(\upmu\)m\({}^{2}\)_and_ 23 \(\upmu\)m\({}^{2}\)). In this simulation, the optical launch power was fixed to \(-\)3.4 dBm per user. However, a degradation of the Q-factor is observed while increasing the number of users. Indeed, this degradation is due to the fact that increasing the number of users leads to the increasing of MAI and as a consequence the system performance decreases significantly. For one user (i.e., example user 1) the difference in Q-factor when nonlinear effect is counted (column 3 compared to column 4) is equal to (31.7 \(-\) 29.9) = 1.8 dB. So, in this scenario the nonlinear effect is a dominant factor compared to MAI. On the other hand, above one user (i.e., example user 2, user 3 and user 4 are considered) the difference in Q-factor when nonlinear effects is counted becomes limited and less than 1 dB. So, in this case MAI is more dominant compared to the nonlinear effects. In addition, when the effective area decreases from 93 \(\upmu\)m\({}^{2}\) to 23 \(\upmu\)m\({}^{2}\), the nonlinear effects increase proportionally.