## 3 Results and Discussion  
In this section, we reveal how the signal post-equalization impacts the achievable optical power budget of the IM/DD PON system. Before moving to the optical power budget analysis, we summarize the pros and cons of transceivers exploiting the modulation formats of the interest (see Table 1). We judge aspects such as spectral efficiency of the modulation scheme, power consumption (as a line  
Figure 1: Experimental setup of the 25+ Gbps IM/DD PON system used for the power budget comparison: AWG—arbitrary waveform generator, B2S—bit-to-symbol mapping, CW—continuous wave laser, DAC—digital-to-analog convertor, EA—electrical amplifier, MZM—Mach–Zehnder modulator, SMF—single-mode fiber, SOA—semiconductor optical amplifier, VOA—variable optical amplifier, PIN—(positive-intrinsic-negative) photodiode, DSO—digital storage oscilloscope, DSP—digital signal processing. card must supply enough electrical power), transmission reach (considering an MZM with non-zero chirp factor), and simplicity of the transmitter and receiver (considering the complexity of the driving circuits and pre-/post-processing requirements). For the optical power budget assessment, we use the conventional BER curves showing how a pre-FEC BER values change with the received optical power. Two different a pre-FEC BER criteria are considered: (i) a 7% overhead (OH) HD-FEC threshold of \(3.8\times 10^{-3}\) and (ii) a 20% OH soft-decision (SD) FEC threshold of \(2\times 10^{-2}\). If the first one is used as a benchmark for the performance and optical budget comparison, then the second one is mainly used for illustration purposes. We assume that all errors can be corrected by a FEC code for the pre-FEC BER below \(3.8\times 10^{-3}\), which leads to the background block error BBE = 0 and thus no service disruption. Additionally, we present eye diagrams so the reader could make the judgment about the signal quality in a such bandwidth-limited IM/DD PON solution. Figure 2 shows BER curves and eye diagrams for the 28 Gbaud OOK/EDB/PAM-4 and 14 Gbaud PAM-4/8 modulated signals in the IM/DD PON systems. First, we analyze the system performance without any post-equalization (Figure 2a,c), and then, we focus on improvements that the linear post-equalization offers (Figure 2b,d). The optical power budget values (together with the corresponding values of the available power margin) obtained for all considered cases are summarized in Table 2. For the power margin calculation, we assume the insertion loss of 4.4 dB for the feeder fiber, 17.3 dB for the 1:32 optical power splitter, and 2.2 dB for the drop fiber. A negative power margin (as for the 28 Gbaud PAM-4) means that the splitting ratio of 1:32 is not supported. Note that the average insertion loss of 1:16 splitter is at least 3 dB lower, which enables the 28 Gbaud PAM-4 alternative. In the configuration without the post-equalization, the HD-FEC requirements are met for two cases: (i) 28 Gbaud EDB and (ii) 14 Gbaud PAM-4. These signal formats allow achieving the HD-FEC requirements and the optical power budget of 27 dB and 26 dB, respectively. Such performance could be considered as expected, but the level distribution between eyes in the eye diagrams shows evidence of driver's linearity. Even for the 14 Gaud PAM-8, the linearity is acceptable, and only a high signal-to-noise ratio (SNR) requirement hinders to reach the HD-FEC performance. For the 28 Gbaud OOK and PAM-4, a dispersion-induced power fading degrades the performance, and chromatic dispersion compensation or signal equalization must be applied to reach the BER threshold of \(3.8\times 10^{-3}\). To test this, we have chosen a linear equalizer whose structure consists of 43-FF&21-FB taps.