## 4 Application Scenarios for Different Network Topologies  
Different application scenarios exist with various network topologies. For each network topology, there is a different approach to apply the demonstrated method, as depicted in Figure 9. For most of \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value and Range** \\ \hline Receiver Optical Power (dBm) & \(-18\) \\ Transmitter Optical Power (dBm) & \(1\) \\ Optical Signal to Noise Ratio (dB) & \(16\) \\ Baud Rate (GBd) & \(30.1\) \\ Chromatic Dispersion (ns/nm) & \(40\) \\ Polarization Mode Dispersion (ps) & \(25\) \\ Polarization Dependent Loss (dB) & \(3\) \\ Signal Bandwidth (GHz) & \(20\) to \(50\) \\ Rx FIR Peaking Frequency (GHz) & \(13\) to \(19\) \\ Rx FIR Peaking Amount (dB) & \(1\) to \(5\) \\ Tx FIR Peaking Amount (dB) & \(1\) to \(9\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental parameters for the Rx FIR filter and the Tx FIR filter. Figure 8: Tx pre-compensation is achieved through the adjustment of FIR filter. The spectra above show the output from the coherent transmitter at different peaking values from \(1\) dB to \(9\) dB at \(2\) dB step size. the scenarios in the network, there are bi-directional links. Here, the transmitter of the network node one on the east end goes to the receiver network node two on the west end, while the transmitter of the network node two on the west end goes to the receiver of the network node one on the east end. The links in both directions go through the same number of ROADMs. The ROADMs in both directions are usually of the same type, from the same vendor, and under the same ambient environment. Thus, we can assume that the bandwidth narrowing effect in the east-west direction is the same as that in the west-east direction. In this case, one can estimate the bandwidth from the Rx of node 2 on the west end and use this information to adjust the FIR filter on the Tx of node 2. Hence, the characteristics of the west-east link are improved. This approach is shown in Figure 9a. Furthermore, one can also send the estimated information of the signal's bandwidth to the network management layer. The network layer will relay the information to the other end of the link accordingly. This approach is shown in Figure 9b. In addition, another approach is to send the information of the bandwidth in the reverse direction through the in-band signal, which can be embedded in Internet protocol (IP) packet or optical transport network (OTN) frame. This approach is shown in Figure 9c. Once the information is received by the other end, the FIR filter on the Tx side can be configured accordingly. For example, the Rx side of node two estimates the signal bandwidth in the east-west direction. This information is sent through the Tx of node two to the Rx of node one in the west-east direction through two approaches above. Once the node one receives the bandwidth information, it can adaptively adjust its Tx FIR for the pre-compensation that improves the performance of the  
Figure 9: Application scenarios for different network topologies. (**a**) The estimated signal bandwidth is directly fed back from the coherent receiver to the coherent transmitter in a bi-directional link. (**b**) The estimated signal bandwidth is fed back through the network management layer. (**c**) The estimated signal bandwidth is fed back through the in-band signal. east-west link. These two approaches can extend the application to any network scenario, although it will take longer time to adjust the Tx FIR to the appropriate setting. A typical process is depicted below. Initially the FIR filters at both the Rx and the Tx sides are loaded with the default tap coefficients to compensate the channel loss of the analog interfaces. Next, the bandwidth of the received signal is estimated, and the Rx FIR is adaptively adjusted.