## 5 The CDS case study for the OFDM-Based Long-Haul Standard Single-Mode Fiber system
It may be noted that the BER for the case of 48 Gb/s with CDS (Figure 16, \(k=1\)) is lower than the BER of the conventional system at the same data rate (Figure 15) because of the Bayesian approach (see Section 3.1.2). The CDS uses 2 Gb/s discretization step for data rate and a step of 1 dB for the launch power. In the next cycle (\(k=2\)), the executive predicts the BER for increasing the data rate to 50 Gb/s using the model \(P(\hat{Y}_{n}^{1}\,|\hat{X}_{n}^{2})\) (see Equations (7) and (8)) and is shown in Figure 17. As can be seen in Figure 17, the predicted BER is lower than the FEC threshold and the action of enhancing the data rate is communicated to the transmitter using a low data rate link and transmitter enhances the data rate. Since there is no model in the model library for the case of the data rate of 50 Gb/s, the perceptor will switch to the prediction mode also, and predicts \(P(\hat{X}_{n}^{2}\,|\hat{Y}_{n}^{2})\) based on the model for 48 Gb/s (i.e., \(P(\hat{Y}_{n}^{1}\,|\hat{X}_{n}^{1})\)) and the BER is computed. It is found that the BER is under the FEC threshold as predicted by the executive (See Figure 17, \(k=2\)). Since the BER is now below the FEC threshold, the preceptor will operate in BER improvement mode and it will extract the model for \(k=2\) (\(P(\hat{Y}_{n}^{2}\,|\hat{X}_{n}^{2})\)). Therefore, the BER can be improved further (Figure 16), for example, if we compare the BERs in Figures 16 and 17, when \(k=2\), we find that the BER is lower in the BER improvement mode (See Figure 16) as compared to that in the prediction by the executive (see Figure 17). Since the objective is to enhance the data rate (with BER below the FEC threshold), in the next cycle (\(k=3\)), the executive will predict the BER for the enhanced data rate of 52 Gb/s and the predicted BER is shown in Figure 17 (\(k=3\)) and this process continues. This process of cycling through the BER improvement mode and prediction mode will continue and the executive will increase the data rate up to 56 Gb/s (\(k=5\) in Figure 17). At this point, the executive finds that the predictive model cannot bring the BER under the FEC threshold (see Figure 17). Therefore, it will decrease the data rate and the steady-state data rate will be 54 Gb/s. The system will continue to operate at 54 Gb/s until there is a disruption to the system. When the disturbance arises, the BER increases and the executive will decrease the data rate, and the perceptor will operate in the prediction mode and choose the closest model from the model library to predict the transmitted data. Figure 14: The OFDM-based fiber optic system with simplified CDS. For example, if the gain of an inline amplifier of a fiber optic link drops from 18 to 10 dB, the BER would increase. The BER is sent from the preceptor to the executive that takes action to lower the data rate. The CDS cycles through the prediction mode and BER improvement mode (see Figures 16 and 17) and finally settles down at a data rate that is achievable for the current state of the channel without interruption of the service. The CDS monitors the BER continuously, and if the  
Figure 16: The simulation of a fiber optic system with CDS with the known model (BER improvement mode). Figure 17: The simulation results for the prospective BER predicted in executive (CDS in prediction mode). Figure 15: The simulation of a conventional system with a linear equalizer. amplifier is repaired, then the CDS automatically will retrieve the original data rate. Additionally, if there is a cut in a fiber-optic network, the data will be re-routed over a fiber optic link that may have different characteristics such as fiber length, dispersion, and nonlinear coefficient. The CDS senses this change due to the change in BER and again adapts itself to the new environment. Now, we show the improvement of the BER for the OFDM fiber optic link upgraded with the CDS.