# Channel Correlation-Based Adaptive Power Transmission for Free-Space Optical Communications
## 3 Simulations and Results
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Wavelength & 1550 nm \\ Output power & 20 dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the LD. Figures 4 and 5 demonstrate the comparison of the BER performance of the proposed technique with FTD and ATD techniques under different turbulence intensities for 30 km and 40 km FSO links, respectively. This paper focuses on the BER performance of the proposed technique for distance transmissions of 30 km and 40 km. The BER performance of the proposed technique at shorter transmission distances will be discussed in a subsequent study. Figures 4a and 5a illustrate the comparison of the BER performance of the three techniques for uplink transmission. Figures 4b and 5b present the comparison of the BER performance of the three techniques for downlink transmission. It is obviously seen that the BER performance of the proposed technique is significantly better than the FTD technique, and it is close to the ATD technique under different link distances and turbulence intensities. The FTD technique uses the same threshold to determine the received signal, and it is unable to more accurately determine the received signal with unstable power due to changes in channel conditions. The BER floor remains flat even if the SNR is high. Therefore, the proposed adaptive power transmission technique is investigated, and it is easier to decide the signal at the receiving end by the FTD technique. The reason is that the detected optical power at the receiving end is improved by adapting the power of the transmitted signal. As shown in Figure 3, although a correlation is observed between the corresponding uplink and downlink turbulence channels, there is a difference in the time domain fluctuations during uplink and downlink signal transmissions. And the proposed technique is not able to cope well with the severe signal strength degradation within the channel at certain moments due to the limitation of the transmitted power. However, the ATD technique is able to adapt better to different channel conditions by dynamically adjusting the threshold at the receiver  
Figure 3: Time domain fluctuations of the signal when passing through the uplink and downlink turbulence channels with different turbulence intensities for a 30 km FSO link. (**a**) Uplink and downlink with \(\sigma_{l}^{2}\) of 0.37 and 0.15, respectively; (**b**) uplink and downlink with \(\sigma_{l}^{2}\) of 0.5 and 0.2, respectively; (**c**) uplink and downlink with \(\sigma_{l}^{2}\) of 0.65 and 0.28, respectively. side via CSI. Therefore, the BER of the proposed technique is lower than the BER of the ATD technique. But the CSI used in the ATD technique is the comprehensive description of the whole channel state, including the information of signal scattering, environmental fading, distance decay, etc., which makes it difficult to obtain the accurate CSI in a real system. In contrast, the signal fluctuation strength is a relatively simple measurement that directly reflects the power level of the received signal. The proposed technique combines signal fluctuation strength and channel correlation coefficient to improve the transmitted power, which reduces the system complexity and improves the system energy efficiency. In addition, the BER performance of the uplink transmission is more similar to the BER of the ATD technique compared to the downlink transmission. This is because the uplink transmission is usually subject to greater signal fluctuations caused by atmospheric turbulence. The BER performance of the uplink transmission is more easily improved by adjusting the power of the transmission signal. Therefore, the proposed adaptive power transmission technique effectively compensates for the turbulence scintillation effect, and it needs to be further studied in the future.