## 4 Discussion and Conclusions  
We demonstrated through simulations the feasibility of 200 Gbps per \(\lambda\) PAM-8 downstream PON transmission in C-band over up to 20 km SMF, preserving DD scheme, achieving 29 dB maximum ODN loss with different RX DSP options. We analyzed the impact \begin{table}
\begin{tabular}{c c c} \hline \hline
**RX DSP Options** & **NL Memory of VNLE** & **MPS** \\ \hline SQRT \({}^{1}\) + FFE \({}^{2}\) + DFE \({}^{3}\) & - & 127 \\ \hline \multirow{4}{*}{VNLE \({}^{4}\)} & 5 & 256 \\  & 10 & 891 \\  & 15 & 2401 \({}^{*}\) \\  & 20 & 5161 \({}^{*}\) \\ \hline \multirow{4}{*}{SQRT \({}^{1}\) + VNLE \({}^{4}\)} & 5 & 258 \\  & 10 & 893 \({}^{*}\) \\ \cline{1-1}  & 15 & 2403 \({}^{*}\) \\ \cline{1-1}  & 20 & 5163 \({}^{*}\) \\ \hline \hline \end{tabular} \({}^{\dagger}\) SQRT requires two additional MPS. It is implemented by using the second order polynomial function as shown in [16]. \({}^{2}\) For FFE, MPS equals the number of FFE taps \(N_{FFE}\). \({}^{3}\) For DFE, MPS equals the number of DFE taps \(N_{DFE}\). Note: DFE requires additional implementation MPS due to the feedback loop. \({}^{4}\) The MPS of VNLE is evaluated by using the method in [34]. \({}^{*}\) 29 dB maximum ODN loss can be achieved with 50G-O/E devices.  
\end{table} 
Table 1: Complexity comparison in terms of multiplication per sample (MPS) for different RX DSP options. Figure 7: Maximum ODN loss as a function of fiber length targeting \(BER_{T}\) = 10\({}^{-2}\), for 50G-O/E devices. The normalized amplitude \(V_{ppJ}/V_{\pi}\) and the pre-compensated length \(L_{\text{C}}\) are optimized for each case. The transmitted optical power \(TOP\) = 15 dBm. Solid: VNLE, Dashed: SQRT + VNLE. of the peak-to-peak amplitude of the signals, which must be optimized to obtain the best system performance. The VNLE and the SQRT can partially compensate for the nonlinear distortions, and allow a higher optimum normalized amplitude \(V_{pp,J}/V_{\pi}\), resulting in a higher OMI and better system performance. Using the SQRT + FFE + DFE method, without VNLE, 29 dB maximum ODN loss cannot be reached even considering a very large 70 GHz TX and RX bandwidth. For 20 km transmission with 50G O/E devices, when the NL memory of the VNLE is higher than 12 samples, 29 dB maximum ODN loss can be reached by using the VLNE alone. The minimum required NL memory is reduced to 9 samples when the SQRT is combined with the VNLE. Although the reduction in terms of NL memory is relatively small, the resulting reduction in RX DSP complexity (evaluated in terms of MPS) is remarkable. The SQRT + VNLE requires 706 MPS, whereas 1369 MPS are needed for the VNLE alone. This confirms that the SQRT can be used in combination with VNLE to decrease the NL memory (and eventually the RX DSP complexity) while preserving the required link loss budget. We also studied the impact of different RX DSP options for different fiber length. Our findings show that, 200 Gbps PAM-8 communication can be achieved with 29 dB maximum ODN loss by using a variable number of NL memory equal to 5 samples and 10 samples in the 5-14 km and 15-20 km link length ranges, respectively. Conceptualization, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; methodology, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; software, P.T.-F. and H.W. ; validation, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; formal analysis, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; investigation, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; resources, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; data curation, H.W. ; writing--original draft preparation, H.W. ; writing--review and editing, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; visualization, R.G., V.F., R.M., G.R., P.T.-F. and H.W. ; supervision, R.G., R.M. and V.F. ; project administration, R.G., R.M. and V.F. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable.