## 4 Conclusions  
In this study, the feasibility of Bi-OCDMA with a phase modulator for polarization coding in short-to-medium range FSO environments was successfully implemented and validated. The simulation results showed that the proposed Bi-OCDMA could appropriately decode the original signal from its corresponding encoder. The proposed system was designed based on the original SAC-OCDMA technique, which theoretically relieves the MAI effect. Compared with the previous Bi-OCDMA scheme with an optical switch [21] and dual EOM [22], the proposed Bi-OCDMA scheme with a phase modulator improved the transmission rate and power efficiency. The performance measurements were simulated under extreme weather conditions, first for the AWGN channel, and second for the AWGN and turbulence-induced fading channels, by utilizing four different OCDMA codes. The proposed Bi-OCDMA system was further tested under different turbulence conditions. The simulation results revealed that the Walsh-Hadamard code had the best performance among the codes. The simulations also showed that in terms of the FSO range, the MD code was the most resistant to the increase in FSO distance, indicated by the lowest performance deterioration, whereas the modified M-sequence code suffered the highest performance degradation in almost all extreme weather and turbulence conditions. This result indicates that the MD code can be used for medium-range FSO applications in the proposed Bi-OCDMA system, whereas the modified M-sequence code can be applied to short-range FSO. In the case of stormy weather conditions and channels with intense turbulence, the Walsh-Hadamard code suffered the highest performance deterioration, although it still achieved superior performance, showing that Walsh-Hadamard code performance is most affected by optical signal attenuation caused by weather and turbulence conditions. Future work can apply the proposed Bi-OCDMA technique in long-range wireless optical communication channels in correlation with different atmospheric conditions for higher multi-user schemes. Performance measurement with some parameters, such as BER, Q-factor, and the eye diagram, can be undertaken, including of the performance improvements resulting from several optimization techniques. Furthermore, Bi-OCDMA can be investigated for higher-order intensity modulation signals with different codes. Conceptualization, [PERSON] and [PERSON]-M.H. ; methodology, [PERSON] and [PERSON]-M.H. ; software, E.W. ; validation, [PERSON] and [PERSON]-M.H. ; formal analysis, [PERSON] and C.-M.H. ; data curation, E.W. ; writing--original draft preparation, E.W. ; writing--review and editing, [PERSON] and C.-M.H. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Weather** & **Code** & **Min. Log of BER (a.u.)** & **Max. Q-Factor (a.u.)** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Weak \\ Turbulence \\ \end{tabular} } & MD & 0.7285 & 0.17811 \\ \cline{2-4}  & MQC & 1.5086 & 0.35096 \\ \cline{2-4}  & Modified M-Sequence & 1.5746 & 0.3357 \\ \cline{2-4}  & Walsh-Hadamard & 1.1005 & 0.2018 \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} Medium \\ Turbulence \\ \end{tabular} } & MD & 1.0025 & 0.24587 \\ \cline{2-4}  & MQC & 1.767 & 0.41239 \\ \cline{2-4}  & Modified M-Sequence & 2.1138 & 0.4531 \\ \cline{2-4}  & Walsh-Hadamard & 1.8359 & 0.3382 \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Intense \\ Turbulence \\ \end{tabular} } & MD & 3.7143 & 0.94502 \\ \cline{2-4}  & MQC & 4.0829 & 0.98137 \\ \cline{2-4}  & Modified M-Sequence & 7.2407 & 1.6425 \\ \cline{2-4}  & Walsh-Hadamard & 8.5379 & 1.6617 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Performance degradation with the increasing of FSO range in different turbulence conditions. **Funding:** This research was funded by the Ministry of Science and Technology, Taiwan (grant numbers MOST 108-2221-E-150-041 and MOST 107-2218-E-150-008-MY2). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.