## 4 Conclusions  
The optical power budget is analyzed for the bandwidth-limited IM/DD PON exploiting the digital signal post-equalization at ONUs. We compare the optical power budget values and the available power margins for the conventional TDM-PON configuration (20 km long feeder fiber, 1:N passive optical power splitting before ONUs) with NRZ-OOK, EDB, PAM-4, and PAM-8 formats at net-rates above 25 Gbps. These signal formats are chosen thanks to their simplicity, the possibility to re-use some of the transceiver's elements (e.g., electrical drivers, amplifier, and photodiodes), and compatibility with the deployed PON infrastructure. We consider only a downstream transmission over a single wavelength in the C-band. To investigate whether components intended for the 10G operation can be used for 25+ Gbps operation, we limit the receiver's bandwidth to 8 GHz before the estimation of the optical power budget. The obtained results show that (i) 10G PON component bandwidth is enough to achieve net-rates above 25 Gbps; (ii) linearity of the optoelectronic components allows operating them using schemes with multilevel modulation formats; and (iii) in combination with the digital signal post-equalization, the obtained optical power budget is compliant with E1 class (OPB = 18-33 dB). More specifically, we reveal that the 28 Gbaud EDB and 14 Gbaud PAM-4 schemes could be used in PONs even without any post-equalization. The signal quality is such that it fulfills the HD-FEC requirement and ensures the power margin of 3.1 dB. For the 28 Gbaud NRZ-OOK, an equalizer with FB taps is required; a 9-FF&5-FB tap equalizer helps to overcome ISI in the presence of noise, which leads to the optical power budget of 29 dB. Finally, line-rates up to 56 Gbps can be supported by the PON employing the PAM-4 modulation. Although a 1:32 splitting ratio might not be feasible due to high insertion loss of such a splitter, it provides the optical power budget of 23 dB. Therefore, the considered IM/DD PON alternatives together with digital signal equalization has the potential to ensure the sustainability for future PONs with line rates well beyond 25 Gbps in the access segment of optical networks.  
**Author Contributions:** Conceptualization, A.U. and L.Z.; methodology, A.U., A.D., S.P., and O.O.; software, A.U., L.Z., and X.P.; validation, T.S., S.S., and V.B.; formal analysis, T.S., S.S., and V.B.; investigation, A.D. and X.P.; resources, S.P. and O.O.; data curation, A.U.; writing--original draft preparation, A.U. and T.S.; writing--review and editing, A.U., T.S., L.Z., X.P., A.D., S.S., V.B., S.P., and O.O.; visualization, A.U.; supervision, O.O.; project administration, A.U.; funding acquisition, T.S., S.S., and O.O. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the ERDF-funded NG-FAST project (1.1.1.2/VIAA/1/16/044) within activity no. 1.1.1.2 Post-doctoral Research Aid, the Doctoral Grant programme of Riga Technical University in Latvia, the Swedish Research Council (Vetenskapsradet) within the project PHASE (2016-04510) and the project (2019-05197), and the Fundamental Research Funds for the Central Universities (no. 2020 QNA5012) in China.  
**Conflicts of Interest:** The authors declare no conflict of interest.