# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 4 BVT with Switching Nodes Adopting SOA Technology: Experimental Setup and Optimization
is placed for being able to ensure a constant power at the input of the photodetector. The sample rate of the ADC is 80 GS/s. The OSNR is measured within 0.1 nm and the target BER is set to 3.8 \(\times\) 10\({}^{-3}\). The fiber spans are Single Standard Mode Fiber (SSMF) of 25 km each, with 0.2 dB/km of attenuation. A probe signal modulated with uniform loading (4-QAM) is sent to the channel to estimate the SNR profile at the receiver side. To calculate the SNR, a sliding window method is used. We name the number of samples taken after or before the subcarrier considered, as Window Size (WS). The total number of the subcarriers considered for calculating the mean is given by TW = WS 2 + 1 where TW is the total size of the window. Figure 4 shows the optimization for 75 km (3 spans of 25 km of SSMF). We can see that the minimum BER is achieved by a WS of 4 (meaning 4 subcarriers taken after and 4 subcarriers taken before the subcarrier considered) that corresponds to a TW size of 9. As an example, the SNR estimation for the different scenarios with and without SOAs and with SOA-based OAD is shown in Figure 5a considering two spans of 25 km for a total of 50 km of fiber. There it is possible to observe a degradation of the SNR around the 156 th subcarrier, which is due to the CD. This attenuation depends on the fiber length and appears at certain frequencies that are given by the expression  
\[f^{n}=\sqrt{\frac{\frac{c(2n-1)}{2\lambda^{2}}}{LD}} \tag{1}\]  
being \(c\) the speed of light, \(\lambda\) the center wavelength, \(L\) the fiber length, \(D\) the dispersion parameter and \(n\) the \(n\)-th attenuation peak (positive integer). When the SOAs are included, this peak suffers a shift of around 10 subcarriers towards the lower frequencies. The maximum obtained SNR decreases 1 dB, but the SNR profile is similar. This frequency shift, with respect to the theoretical frequency, is due to the variation of the injected current of the SOAs to find the best working point. In our case study, the best conditions are achieved for a SOA input power of -8 dBm and an injected current within 60 mA and 80 mA. When the SNR profile is estimated for the case using SOA-based OAD nodes, the maximum SNR decreases down to 14 dB, and the fading frequency peak is shifted to higher frequencies, where the minimum SNR is about 6.5 dB. In this case, the cascading of network elements, in particular the presence of filtering elements, causes the reduction of the obtained SNR for all the subcarriers and the shift of the fading peak. Indeed, for this case, the peak fading is not so pronounced as in the case  
Figure 4: BER versus WS for 75 km (3 spans of 25 km of SSMF) and OSNR of 36 dB without considering any SOA placed inline. without filtering elements and it covers a greater number of subcarriers (from the 120 th to the 200 th approximately). According to the SNR profile, the subcarriers for each scenario are modulated at the transmitter side using BL/PL algorithms. Figure 5b shows the bit distribution according to the SNR estimation in Figure 5a and considering the MA criterion with a fixed gross bit rate of 28 Gb/s at 34 dB of OSNR. It can be seen that the modulation format order decreases with the reduction of the SNR values and increases with the increase of the SNR values. For the OAD scenario, since the SNR profile is more affected by the fiber impairments (CD) and the filtering elements, lower bits per symbol are loaded onto the subcarriers. In Figure 5c, the PL distribution according to the SNR profile is presented. It can be seen that the subcarriers corresponding to the lower SNR values for the case with SOAs do not have power assigned. For the OAD scenario, all the subcarriers have power assigned and the power variation corresponds to the change from one modulation format to another. In Figure 5d, the BER per subcarrier for the different analyzed cases is reported. We can observe that for the OAD scenario, the subcarriers affected by CD present high number of errors.