# Channel Correlation-Based Adaptive Power Transmission for Free-Space Optical Communications
## 3 Simulations and Results  
The proposed technique was evaluated through a simulation. The different link distances and turbulence intensities were used to reduce the chance of the experimental results. The parameters of the LD used for simulation are shown in Table 1. The additive white gaussian noise (AWGN) was considered when detecting the optical signal using PD, and other parameters will be discussed in future research. Figure 3 illustrates the time domain fluctuations of the signal when passing through the uplink and downlink turbulence channel with different turbulence intensities for a 30 km FSO link. Figure 3a shows the time domain fluctuations of the uplink and downlink transmissions with \(\sigma_{I}^{2}\) of 0.37 and 0.15, respectively. Figure 3b shows the time domain fluctuations of the uplink and downlink transmissions with \(\sigma_{I}^{2}\) of 0.5 and 0.2, respectively. Figure 3c shows the time domain fluctuations of the uplink and downlink transmissions with \(\sigma_{I}^{2}\) of 0.65 and 0.28, respectively. When establishing the uplink and downlink turbulence channels, the single random normal distribution function is employed to generate the normal-distribution random sequence to ensure the reliability of the simulation results. This is due to the relatively high correlation that occurs between uplink and downlink turbulence channels at short-distance transmissions [20]. And as a result, there is a difference in the fluctuation strength propagated through the same region of the atmosphere for the established uplink and downlink turbulence channels in the case of alignment. The signal fluctuation during uplink transmission is noticeably higher than those during downlink transmission, which is due to the difference in link geometry [21]. The optical signal suffers from atmospheric turbulence at the initial stage during uplink transmission, which results in the distortion, scattering, and attenuation of the beam. During a downlink transmission, the optical signal is transmitted from a high altitude downwards. Although the optical signal is also affected by atmospheric turbulence at the end of the transmission route, since it is transported in a relatively stable atmosphere for most of the time, it is probably relatively slightly affected by turbulence. Therefore, it is known that \(\sigma_{R}^{2}\), \(\mu\), and \(\sigma\) of the uplink are larger than those of the downlink according to Equations (1) and (4). Thus, the uplink exponential parameter \(y_{u}=\sigma_{u}\cdot U+\mu_{u}\) and downlink exponential parameter \(y_{d}=\sigma_{d}\cdot U+\mu_{d}\) are generated using a normal-distribution random sequence \(U\) with \(\sigma\) and \(\mu\). Then, the exponential transformation is conducted on the uplink and downlink exponential parameters, and the exponentially transformed data are resampled to generate the intensity fluctuated signal \(F_{u}\) and \(F_{d}\) for uplink and downlink turbulence channels, respectively. The sampling frequency was set to 20 MHz because the data rate was configured as 10 Mbps. The BER was reduced due to the turbulence scintillation effect for both uplink and downlink transmissions. But there was a correlation between different uplink and downlink turbulence channels. Therefore, a channel correlation-based adaptive power transmission technique is proposed to mitigate the turbulence scintillation effect. Table 2 demonstrates the channel correlation coefficients between the uplink and downlink for different link distances and turbulence intensities. The channel correlation coefficient is variable due to the change in the channel environment, and even if the signal is transmitted in the same channel, the channel correlation coefficient is different. This is due to the random characteristics of the background and PD noise during signal transmission, which leads to the difference in the received signal fluctuation intensity. It is easily observed that the correlation coefficient between the uplink and downlink decreases with the increase in \(\sigma_{I}^{2}\).