# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 5 Results and Discussion  
As shown in Figure 3 and detailed in Section 4, the experimental analysis considers two scenarios: the first using simple SOAs acting as switching nodes and the second adopting OAD nodes based on SOA technology. Fiber spans of 25 km until a maximum of 4, for a total path of 100 km, are considered. In particular, to observe the impact of adopting the SOAs in the system, we place an SOA after each  
Figure 5: (**a**) SNR profile, (**b**) BL assignment, (**c**) PL assignment and (**d**) BER per DMT subcarrier considering MA transmission at 28 Gb/s over two SSMF spans of 25 km, with and without SOAs and for SOA-based OAD, at 34 dB of OSNR. span of fiber and compare the results with the transmission over the fiber link without them. To analyze the second scenario, we consider SOA-based switching nodes based on the OAD node architecture of Figure 2. For both scenarios, the current injected into the SOA is properly varied to compensate the losses due to the fiber impairments or/and the different elements (splitters, combiners, filtering elements) of the OAD node. First, we analyze the case at fixed bit rate (28 Gb/s) using the MA BL/PL algorithm for maximizing the performance. The BER performance at the varying of the OSNR is presented in Figure 6. It is interesting to observe that a similar behavior is obtained for 75 km (3 spans) of fiber without SOAs, 50 km (2 spans) of fiber with an SOA after each span and one fiber span of 25 km with an OAD node, being the difference between these cases lower than 1 dB at the target BER of 3.8 \(\times\) 10\({}^{-3}\). Similar results are obtained when we compare the BER curves for 100 km (4 spans) of fiber, 75 km (3 spans) of fiber with an SOA per each span and 50 km (2 spans) of fiber including 2 OAD nodes (one after each span), being as well the penalty less than 1 dB, at the target BER. Up to 100 km (4 fiber spans) can be reached by cascading 4 SOAs with an OSNR value of 33 dB. It was not possible to retrieve results after 75 km (3 spans) with the OAD nodes because of the losses accumulation, mainly due to the filtering elements, and the noise introduced by the SOAs. The introduction of OAD nodes involves losses around 4 dBm per each WDM element and 1 dBm per each combiner/splitter. Then, we have studied the maximum achievable net bit rate, adopting the RA BL/PL algorithm, for different transmission distances and scenarios considering at a fixed target BER of 3.8 \(\times\) 10\({}^{-3}\). In Figure 7, the results at 33 dB of OSNR, are presented. We can see that, as the fiber length increases, the maximum net bit rate decreases, being more pronounced for the case of fiber transmission without SOAs. In fact, for the 50 km (2 spans) of fiber, the maximum net bit rate is obtained when 2 SOAs or 2 SOA-based OAD nodes are included in the optical path, obtaining 26.5 Gb/s. The value of the net bit rate when only 2 spans of fiber are placed inline, is lower, being its value 26.1 Gb/s. Therefore, the introduction of the SOAs, as switching nodes themselves or either as part of the OAD node, improves the net bit rate performance at least for fiber links higher than 50 km. In fact, there is an  
Figure 6: BER versus OSNR (with and without SOAs and including SOA-based OAD nodes) using BL and PL under the MA criteria for a fixed gross bit rate of 28 Gb/s. improvement of 2 Gb/s when 75 km of fiber links and 3 SOAs are included, with respect to the case when the fiber links do not include the SOAs. This is due to the possibility of better optimizing the transmission by controlling the current injected into the SOAs, in combination to the transceiver adaptability by means of the BL/PL algorithm. According to the obtained results, a 26 Gb/s connection can be supported up to 50 km considering 2 cascading SOA-based OAD nodes and a maximum of 27 Gb/s for 75 km considering 3 cascading SOAs without filtering elements. Table 1 shows the maximum achievable net data rate at different transmission distances for the considered scenarios adopting RA BL/PL.