# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 6 Conclusions  
In this paper, the use of S-BVTs, based on DMT modulation and DD with adaptive loading capabilities has been proposed as a flexible and adaptive cost-effective solution for future optical metro networks adopting SOA-based switching nodes. A preliminary assessment of this system has been analyzed considering two scenarios: with or without filtering elements. Using loading algorithms for both scenarios, the limitations due to transmission impairments (caused by the CD, DAC bandwidth or the network elements) can be mitigated enabling higher transmission rates and reach. When BL and PL algorithms are implemented using the MA criterion, the results show that the BER performance obtained for a 25 km of fiber link with an SOA-based OAD node has similar BER performance than the case of 50 km (2 spans of 25 km) of fiber with 2 SOAs acting as switching nodes, or the case of 75 km (3 spans) of fiber. Thus, the introduction of the SOAs or the OAD node, in terms of BER performance, results in an additional span of 25 km of fiber is included. On the other hand, using the RA criterion, the net bit rate performance improves including SOA elements in the transmission system, compared to the case of simply adding fiber spans. This is due to the possibility of controlling the SOA current to face the transmission impairments. These impairments include the losses due to the fiber links, the SOAs as well as the WDM elements or the splitter/combiners used in the experiments [21]. These losses can be up to 4 dBm for each WDM element. Thus, as the number of SOA-based switching nodes increases, more SOA elements can be controlled, to ensure a better optimization of the system and an improvement of the performance overcoming the power losses due to the different elements in cascade. However, adding SOAs or/and filtering elements limits the maximum achievable OSNR and thus the number of cascading nodes that  
Figure 8: Net bit rate versus OSNR for (**a**) 25 km and (**b**) 50 km for the different scenarios at the target BER. can be traversed by the adaptive signal generated by the S-BVT. In particular, 27 Gb/s connections can be supported up to 75 km including 3 SOAs acting as switching nodes without any filtering element, and while 26 Gb/s for 50 km is the maximum achievable reach including 2 OAD nodes. The experimental assessment demonstrates that thanks to the possibility of controlling the injected current to the SOAs and the application of loading schemes at the adaptive transceivers, high flexibility, scalability, and adaptability can be obtained. However, despite the possibility of compensating the losses thanks to the SOAs, the maximum transmission distance with 2 OAD nodes was 50 km, due to the introduction of the WDM elements and the splitters/combiners. These losses are around 4 dBm for each WDM element and 1 dBm considering each splitter/combiner, meaning that the current injected into the SOA needs to be efficiently controlled to avoid them. Different alternatives must be exploited for this setup to improve the attainable distance including the OAD nodes. OFDM based on SSB or VSB can be adopted with DD to simply maintain architecture [7; 20; 21; 24]. To enhance the impairment tolerance and the achievable distance, coherent detection can also be used as in [25; 26]. Monitoring techniques and the SDN paradigm play a key role in enabling these features. The combination of cost-effective implementation of programmable (SDN-enabled) BVT and SOA-based switching nodes seems to be promising for further investigation. In fact, future work in this direction will be carried out to improve the proposed system and target future optical metro networks. Investigation, L.M.G. ; Methodology, L.M.G., S.v.d.H. and X.X. ; Resources, S.v.d.H., X.X., J.v.W., N.C. and C.O. ; Software, L.M.G. and J.M.F. ; Supervision, N.C., C.O., M.S.M. and J.M.F. ; Validation, L.M.G., S.v.d.H., J.v.W., N.C. and C.O. ; Visualization, S.v.d.H., J.v.W., N.C. and C.O. ; Writing--original draft, L.M.G. ; Writing--review & editing, L.M.G. and M.S.M.