## 7 Conclusions  
A 40 Gb/s IM/DD Fast-OFDM-CDMA system was analytically and numerically demonstrated using 2D-OCDMA HC codes for an amplifier-free access network. Our hybrid IM/DD Fast-OFDM-CDMA system was based on a breakthrough analytical model that accounts for OFDM subcarriers overlapping, MAI and PAPR. It was shown that our proposal outperforms both the IM/DD AO-OFDM-OCDMA and 2D-OCDMA-WDM PON configurations in terms of system performance. Indeed, at 0 dBm, IM/DD Fast-OFDM-CDMA improves the Q-factor by 2.5 dB compared to AO-OFDM-OCDMA and OCDMA-WDM. On the other hand, the numerical simulation results showed the implementation feasibility of our hybrid system operating at 40 Gb/s in the PON configuration via open eye diagrams. Power budget analysis revealed that at a launched optical power of 10 dBm and with 16 simultaneous users, our proposed technique can improve the total power budget by 6.8 dB compared to IM/DD AO-OFDM-OCDMA. This leads to an extension of the maximum achievable distance of up to 142 km, compared to 108 km and 82 km for IM/DD AO-OFDM-OCDMA and 2D-OCDMA-WDM, respectively. The future directions for the extension of the access network could be the integration of new emergent technologies, such as multi-meter waves [31] and time domain equalizers, based on the OFDM technique [32], to extend the reachability distance and integration of 5G/6G non orthogonal multi-access (NOMA) in order to connect local and metropolitan networks to the broadband global access network. Finally, although an access network based on the OCDMA-PON configuration preserves data integrity, securing the access network will be a big challenge for researchers in the near future, as regards preserving data confidentiality and privacy against various attacks and threats by using Blockchain technology (BCT) and machine learning (ML)-based solutions [33].  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**System** & **Coding Scheme** & **Cardinality** & **Performance** & \begin{tabular}{c} **System** \\ **Components** \\ \end{tabular} & \
\begin{tabular}{c} **Reachability** \\ **(km)** \\ \end{tabular} \\ \hline AO-OFDM-CDMA [15] & 2D-HC & 45 & BER \(\leq\) FEC limit &
\begin{tabular}{c} DFB + SMF + \\ APD \\ \end{tabular} & 108 \\ \hline
2D-OCDMA [16] & \begin{tabular}{c} 2D-PHS \\ 2D-HC \\ \end{tabular} & \begin{tabular}{c} 190 \\ 120 \\ \end{tabular} & Q-factor = 6 & \begin{tabular}{c} DFB + SMF + \\ APD \\ \end{tabular} & \begin{tabular}{c} 120 \\ 190 \\ \end{tabular} \\ \hline CO-OFDM-CDMA [17] & \
\begin{tabular}{c} 2D-PHS \\ 2D-HC \\ \end{tabular} & 16 & BER \(\leq\) FEC limit &
\begin{tabular}{c} VCSEL + SMF \\ + APD \\ \end{tabular} & 83.5 \\ \hline
2D-OCDMA-WDM [23] & 2D-HC & 524 & BER \(\leq\) 10 \(\times\) 10\({}^{-10}\) & \
\begin{tabular}{c} VCSEL + SMF \\ + APD \\ \end{tabular} & 82 \\ \hline IMDD OOFDM-CDMA [29] & 2D-PHS & 5 & BER \(\leq\) FEC limit &
\begin{tabular}{c} VCSEL + SMF \\ + APD \\ \end{tabular} & 80 \\ \hline
2D-PV [30] & \begin{tabular}{c} 2D Permutation \\ vectors’ code \\ \end{tabular} & 110 & BER \(\leq\) 10 \(\times\) 10\({}^{-9}\) & \begin{tabular}{c} LED + SMF + \\ PIN \\ \end{tabular} & 20 \\ \hline IM/DD & \
\begin{tabular}{c} 2D-HC \\ \end{tabular} & 180 & BER \(\leq\) FEC limit &
\begin{tabular}{c} CWLaser + SMF \\ + APD \\ \end{tabular} & 142 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Relevant 2D-OCDMA systems in PON configuration.