## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Modulation** & **Spectral** & **Transmission** & **Power** & **Transmitter** & **Receiver** \\
**Format** & **Efficiency** & **Reach** & **Consumption** & **Simplicity** & **Simplicity** \\ \hline NRZ & – & – & + & ++ & ++ \\ EDB & + & ++ & – & – & – \\ PAM-4 & ++ & ++ & — & — & — \\ PAM-8 & +++ & + & — & — & — \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pros and cons of transceivers exploiting the considered intensity modulation direct detection (IM/DD) schemes. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Signals** & **w/o** & **4-FF** & **9-FF\&5-FB** & **43-FF\&21-FB** \\ \hline
28 Gbaud OOK & - & 29 dB/5.1 dB & 31 dB/7.1 dB & 31 dB/7.1 dB \\
28 Gbaud EDB & 27 dB/3.1 dB & 29 dB/5.1 dB & 30 dB/6.1 dB & 30 dB/6.1 dB \\
14 Gbaud PAM-4 & 26 dB/2.1 dB & 27 dB/3.1 dB & 27 dB/3.1 dB & 27 dB/3.1 dB \\
14 Gbaud PAM-8 & - & - & - & - \\
28 Gbaud PAM-4 & - & - & 22 dB/\(-\)1.9 dB & 23 dB/\(-\)0.9 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Optical power budget and available power margin for the explored alternatives and post-equalizer structures (number of FF and FB taps). The number of taps is chosen in a way to maximally improve the system's performance by tackling the bandwidth limitations due to the receiver's side bandwidth and chromatic dispersion. Further increase of the filter tap number does not improve BER values but only adds the complexity. Note that linear equalization using FF taps only is implemented to overcome bandwidth limitations and ISI, whereas FB taps are added to the structure to improve the performance in the presence of noise. The results show that such post-equalization (43-FF&21-FB taps) significantly improve the BER performance, and thus, it enhances the optical power budget as compared to the previous case without the post-equalization. The optical power budget of the 14 Gbaud PAM-4 is increased by 1 dB. The corresponding value for the 28 Gbaud EDB is 3 dB. More importantly, the post-equalization enables more options of the modulation format alternatives. The HD-FEC requirement is met for the 28 Gbaud OOK and 28 Gbaud PAM-4 modulations. Moreover, the signal quality is so good that it allows achieving the optical power budget of 31 dB and 23 dB, respectively. Although the BER improvement is not enough to meet the HD-FEC requirements for the 14 Gbaud PAM-8 signals, its BER curve approaches to the threshold and a slight SNR improvement, e.g., using a nonlinear equalization, would aid in meeting this requirement. Figure 2d shows the corresponding eye diagrams captured for received optical power values that allow detecting signals with the BER below \(3.8\times 10^{-3}\) or close to it as in the 14 Gbaud PAM-8 case. Figure 2: Quality of transmission (QoT) characteristics for the 28 Gbaud OOK, 28 Gbaud EDB, 14 Gbaud PAM-4, 14 Gbaud PAM-8, and 28 Gbaud PAM-4 signals in the IM/DD PON system: (**a**) BER vs. received optical power (ROP) before the digital post-equalization; (**b**) BER vs. ROP after the digital post-equalization employing 43-FF&21-FB taps; (**c**) eye diagrams captured in the ONU at the highest ROP and before the digital post-equalization; and (**d**) eye diagrams captured in the ONU and ROP values that ensure BER \(<3.8\times 10^{-3}\) after the digital post-equalization. Next and to better explore the impact that post-equalization has on the optical power budget and to emphasize its importance, we use three equalizer configurations and compare the OPB for each of them: (i) without post-equalizer, (ii) post-equalizer with only 4-FF taps, (iii) 9-FF&5-FB taps. The obtained results are benchmarked to the previous case, i.e., with the equalizer having 43-FF&21-FB taps. Figure 3 presents the results for scenarios whose combination of data rates and modulation formats ensures line rates no higher than 28 Gbps--28 Gbaud OOK/EDB and 14 Gbaud PAM-4, while Figure 4 includes the 14 Gbaud PAM-8 and 28 Gbaud PAM-4.