# Bidirectional Atmospheric Channel Reciprocity-Based Adaptive Power Transmission
## 3 Simulation and Investigation
### The BER Performance of Proposed Adaptive Power Transfer Technique  
In the actual application of FSO communication systems, adaptive systems that change power based on the bidirectional atmospheric channel state information differ from FTD and ATD techniques in that they are sensitive to background noise produced by sunlight as well as other background light in addition to being affected by turbulence effects produced by the atmosphere. The AWGN function can be used to incorporate noise into system modeling, and in the following, \(\mathrm{N_{b}}\) will be used in place of background noise to highlight how it differs from the thermal noise produced by the photographic detectors. In a bidirectional atmospheric channel without the interference of background noise, it is ideal to utilize the same channel for forward and inverse transmissions, and the channel reciprocity is \(\rho=1\). The performance of the FTD, ATD, and CR-APT transmission mechanisms was tested under these circumstances. In the meantime, two channels with turbulence intensities of \(\sigma^{2}=0.0596\) and \(\sigma^{2}=0.5250\) were chosen for simulated communication in order to compare the performance of each technique in various channels more succinctly and intuitively. As observed in Figure 5a, the CR-APT technique, under the ideal channel reciprocity of \(\rho=1\), obtained the BER of \(1\times 10^{-7}\) when SNR = 18 dB. CR-APT outperformed FTD and ATD while delivering better performance at lower SNR. In channels with a turbulence intensity of \(\sigma^{2}=0.5250\), the CR-APT technique had good performance and obtained a better BER at a lower SNR, in contrast to the FTD technique, which had failed and ATD technique that had produced a high BER. Figure 5 demonstrates  
Figure 4: Plot of the time-domain performance of the signal as it passes through different turbulent channels; \(\sigma^{2}\) represents channel turbulence intensity. (**a**) OOK signal, (**b**) \(\sigma^{2}=0.0596\), (**c**) \(\sigma^{2}=0.0684\), (**d**) \(\sigma^{2}=0.1002\), (**e**) \(\sigma^{2}=0.1896\), (**f**) \(\sigma^{2}=0.3299\), (**g**) \(\sigma^{2}=0.5250\). that, under ideal circumstances, the CR-APT technique still performed well regardless of the kind of turbulence that the signal is delivered over. Due to their varying heights and reception orientations, forward and inverse broadcasts in the FSO transmission system's bidirectional atmospheric channel have various effects on the transmission channel when background noise is present. When the signal's effective power is known, the background noise is assessed in terms of SNR, which can be learned from Equation (6) in Part II. The lesser the noise power, the bigger the SNR, and therefore, the larger the \(\mathrm{N_{b}}\). Consequently, the BER of the received signal and the turbulence channel reciprocity for forward transmission and inverse transmission were examined in six cases where the background noise was, from small to large, \(\mathrm{N_{b}}=30\) dB, \(\mathrm{N_{b}}=25\) dB, \(\mathrm{N_{b}}=20\) dB, \(\mathrm{N_{b}}=15\) dB, \(\mathrm{N_{b}}=10\) dB, and \(\mathrm{N_{b}}=5\) dB in order to investigate the effect of background noise on the adaptive method of bidirectional channel reciprocity. In order to easily visualize the turbulent channel situation under different background noises, the turbulent channel at a scintillation factor of \(\sigma^{2}=0.0596\) was taken as an example. Figure 6 illustrates the time-domain performance of the signal passing through the turbulent channel with a scintillation coefficient of \(\sigma^{2}=0.0596\) when various background disturbances are introduced to the turbulent channel. As shown in Figure 6a-d, the time domain curve became distorted when noise was added. For one thing, the numerical values of MSE took 0.529, 0.530, 0.540, and 0.562. As the amount of noise added rose, the signal distortion was more severe and the MSE value was higher.