## 4 Simulation Result Analysis and Compensation Mechanism  
In this section, analysis is conducted on the simulation results of the OFDM-OCDMA system after adding the dispersion parameters, and the compensation ability of the proposed EDC equalizer is investigated. The SMF simulation parameters are presented in Table 2.  
The parameters in Table 2 were selected mainly by referring to the SMF specifications set by the International Telecommunication Union and the specification to the SMF-28-J9 sold by Thorlabs [22; 23]. Figure 9 is a receiver constellation diagram of an optical signal after having been subjected to 10 km SMF transmission and dispersion influence. The unit a.u. in Figure 9 means arbitrary units. After calculation, the BER of this constellation diagram was found to be 0.485. The transmitted optical signal causes the receiver constellation diagram to become blurred, making it impossible to modulate the increased error rate (Figure 9). \begin{table}
\begin{tabular}{c c} \hline \hline Wavelength & 1550 nm \\ Attenuation & 0.2 dB/km \\ Dispersion (\(D_{SMF}\)) & 17 ps/nm \(\times\) km \\ Dispersion slope & 0.056 ps/nm\({}^{2}\)\(\times\) km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SMF simulation parameters. Figure 8: Schematic diagram of dispersion-induced ISI effect. The main reason causing the OFDM-OCDMA system receiver constellation diagram to become blurred and concentrated is that this system utilizes multiple optical carrier waves to transmit the OFDM signal, and optical carrier waves of different wavelengths that are transmitted through an optic fiber will reach the receiver at different times because of dispersion. In addition, in the OFDM-OCDMA system, the OFDM signals entrained in each optical carrier wave are also influenced by dispersion, causing the signals to reach the receiver at inconsistent times. Therefore, when the receiver performs signal collection and balance detection on the two polarization states, the phase signals of the I-channel and Q-channel will be destroyed because of frequency dispersion. In this type of situation, simply using EDC equalizer technology cannot compensate for the signal. This demonstrates that compensation cannot be performed directly on the electrical signal when transmitting optical signals in an OFDM-OCDMA system. Therefore, compensation must first be performed in the initial stage of the optical transmission domain; the dispersion that could not be fully compensated is subsequently compensated using EDC equalizer technology. Regarding the influence of dispersion, this study adopted DCF to compensate for dispersion in the optical domain. The simulation parameters are presented in Table 3.  
The parameters in Table 3 were selected primarily by referring to the specification to the DCF with model number DCF38 sold by Thorlabs Thorlabs (1958). In addition, the system's total dispersion amount can be obtained using Equation (8). \[D_{all}=D_{SMF}\cdot L_{SMF}+D_{DCF}\cdot L_{DCF}, \tag{8}\]  
where \(D_{all}\) is the system's total dispersion amount; \(D_{SMF}\) and \(D_{DCF}\) are the SMF and DCF dispersion values, respectively; \(L_{SMF}\)is the SMF transmission distance; and \(L_{DCF}\) is the required DCF length. In addition, assuming a situation where DCF cannot completely compensate for the dispersion, setting the SMF and DCF lengths to respectively 10 km and 4 km and subsequently utilizing Equation (8) for calculation, the system's total dispersion amount can be found to be 16 ps/nm. The constellation diagram obtained after simulation is shown in Figure 10, and the BER is 0.5 in this situation. \begin{table}
\begin{tabular}{c c} \hline Wavelength & 1550 nm \\ Attenuation & 0.25 dB/km \\ Dispersion (\(D_{DCF}\)) & \(-\)38.5 ps/nm \(\times\) km \\ Dispersion slope & \(-\)0.12 ps/nm\({}^{2}\times\) km \\ \hline \end{tabular}
\end{table} 
Table 3: DCF simulation parameters. Figure 9: Receiver constellation diagram after 10 km of SMF transmission.