## 3 Results and Discussion
### Results on Metro-Access Network Convergence  
After characterizing the transceivers, we perform an analysis useful for the merged metro and access network's joint design. As mentioned in the Introduction section, thanks to the LPDC codes used, the current PON specification uses a BER target value of \(10^{-2}\). Focusing on this BER target performance in the Figure 4 inset, this can be guaranteed only for a subset of receiver optical powers and transmitter OSNR joint values. Referring to Figure 2, we define the \"metro path\" as the path from transmission to the ROADM first block. Within this path, there is a VOA used to emulate the metro link; its setting produces different signals with different OSNRs. However, this VOA setting also affects the optical signal power loss in the metro path, which we can call the \"metro optical power budget\" (\(OPB_{Metro}\)). Similarly, we define an \"access path\" from the ROADM output to the VOA used to emulate the PON splitter (placed  
Figure 5: BER curves versus received optical power with OSNR varying from 20.4 to 36.7 dB for the two pre-commercial transceivers, in the case of the laboratory test-bed shown in Figure 3, with a transmitted optical power of 4 dBm. just before the coherent receiver) and the fiber link loss. This VOA setting changes the received optical power in the access segment but also affects the optical power loss within the \"access path\", which we can call the \"optical power budget\" of the access segment (\(OPB_{Access}\)). Considering these parameters, we can estimate the received optical power and OSNR satisfying the \(10^{-2}\) BER target of the Figure 4 inset, this evaluating the \(OPB_{Metro}\) and \(OPB_{Access}\), producing the left plot in Figure 6.  
The graph on the left in Figure 6 illustrates a joint design between access and metro segments as a function of the two path losses. The results indicate that an optical power budget in the access network can exceed the current class N1 requirement of 29 dB. In particular, the figure demonstrates that if the power budget requirement for the access network increases in the future, there is sufficient margin to accommodate this change. Specifically, achieving a power budget of 34 dB or higher is possible by reducing the metro loss to 30.8 dB or lower. The access network optical power budget can also be expressed in terms of OSNR (at the metro segment output) instead of the metro optical power budget (on the right in Figure 6). This graph shows that an access optical power budget in excess of 33.8 dB can be achieved if the OSNR of the metropolitan network is greater than 20 dB. Similarly, we perform the same analysis for the pre-commercial transceivers, thus focusing on the Figure 5 inset, estimating the subset of receiver optical powers and transmitter OSNR values guaranteeing the BER target, and finally, the \(OPB_{Access}\) evaluation inside the access network (defined as the optical power difference from the transmitter to the receiver, blue in Figure 3). We also obtain a similar plot to that on the right in Figure 6 for the pre-commercial transceivers. Overlaying the optical power budget results for the three transceivers in a single graph, we obtain the graph on the left in Figure 7.  
The graph on the left in Figure 7 illustrates the optical power budget (OPB) of the access network relative to the OSNR values of the metro network for the three transceivers. As shown, all three transceivers meet the current minimum acceptable optical power budget N1 class requirement of 29 dB when the OSNR of the metro network exceeds 25 dB. Additionally, depending on the transceiver, an higher optical power budget can be achieved. Specifically, the plot indicates that transceiver number 2, depicted by the red curve, has a 1 dB higher optical power budget compared to transceiver number 1 in the green curve. Furthermore, the first transceiver in blue reaches the current highest PON OPB of 35 dB (guaranteeing PON class E2 [2]) if the OSNR increases above 25 dB.