# Bidirectional Atmospheric Channel Reciprocity-Based Adaptive Power Transmission
## 3 Simulation and Investigation
### The BER Performance of Conventional FTD and ATD  
Fixed threshold decision is frequently used in traditional communication technology because it has benefits like simple operation and intuitive results. In the FSO communication system, the FTD technique is to take the average fixed threshold for judgment after acquiring the channel state, as the name implies. Figure 3a shows the performance of the FTD under six-channel settings and various turbulence intensities. With the exception of channel circumstances \(\sigma^{2}=0.0596\), the BER performs nearly linearly across a range of SNRs. In other turbulent channel circumstances, the curve progressively reduced out at SNR > 16 dB, the BER gradually hit the lower limit, and as the severity of the turbulence rose, so did the BER's lower limit. The BER was roughly 0.2, especially when the turbulence intensity reached 0.5250. The FTD approach cannot be used to transfer the signal properly in this situation. The reason for this is that the fixed threshold for averaging is no longer able to send the signal adequately when the turbulence is high since the optimal judgment threshold for signal transmission is changing over time. When the turbulence intensity was significant, the FTD technique was unable to successfully transmit information. In order to complete transmission, adaptive transmission technology was created that modifies the judgment threshold in accordance with the signal that was influenced by turbulence. Additionally, Figure 3b illustrates how well the ATD approach performs in turbulent channels of varying intensities. As seen in Figure 3b, the ATD technology performed better than the FTD technology at transmitting signals in various turbulence scenarios, and the BER performance of the ATD transmission technology had increased by several orders of magnitude to \(1\times 10^{-7}\). The BER curves of ATD technology under various turbulence circumstances were comparable; as an illustration, in the turbulent channel of \(\sigma^{2}=0.1896\), the BER gradually declined as the SNR rose. Although conventional ATD performed better than FTD, the good BER at \(\sigma^{2}=0.5250\) was only \(1.5\times 10^{-5}\), which still has room for improvement. The OOK signal is utilized by the transmitter to control the optical signal transmission by switching the sinusoidal carrier on and off using a series of unipolar non-return-to-zero codes. To examine the performance of the signal over various turbulent channels, Figure 4 displayed the time domain performance of the real transmitted OOK signal. The MATLAB simulation software used the mean square error (MSE) function to calculate the error generated between the sent signal and the received signal more precisely. Figure 4 displayed the OOK signal's time-domain performance as it moved through several turbulent channels. In Figure 4, it could be observed that as the signal passed through the channel with higher turbulence intensity, the more intense the fluctuation of the time domain profile and the more severe the signal distortion. As shown in Figure 4b,c, the fluctuation of the time-domain curves embodied by the signal passing through the turbulence intensity and \(\sigma^{2}=0.0684\) channels were minor, and it could be seen that the optical signal l was less affected by the turbulence noise. The numerical values of MSE were 0.047 and 0.051, along with the original signals that might be easily recuperated. As shown in Figure 4d,e, the time-domain curve produced distortion. For channels with scintillation intensity \(\sigma^{2}=0.1002\), the MSE value was 0.067, while for channels with \(\sigma^{2}=0.1896\), it was 0.114. Anti-noise measures must be used during communication in such channels in order to recover the signal transmission's original content. As turbulence intensity rises, the signal ups and downs became more pronounced, and MSE values rose as a result.