## 3 Setup of the Atmospheric Turbulence Emulation Device  
In order to study the correlation of bidirectional atmospheric turbulence channels under different atmospheric turbulence intensities, a hot air convective atmospheric turbulence emulation device was used as a controllable atmospheric turbulence channel [20]. This thesis adopts the hot air convection type atmospheric turbulence emulation technology, and we used the theory of thermal convection turbulence between parallel plates to design the structure [20]. The atmospheric turbulence emulation device is based on the theoretical basis of flow similarity to complete the emulation of the optical characteristics of atmospheric turbulence [21]. When the flow has similar geometric boundary conditions  
Figure 1: Schematic diagram of correlation measurement of bidirectional optical instantaneous fading.  
and the same Reynolds number, even if the size or velocity is different or even the fluid itself is different, they will have similar dynamics [20,21].  
The device mainly includes an atmospheric turbulence control system, cooling system, heating system, temperature measurement system, pool body, and window, as shown in Figure 2. The pool body is composed of high-temperature, heat-resistant, and heat-insulating plates, which are mainly used to reduce the heat exchange between the inside of the pool body and the outside world. The bottom of the pool body is a heating panel, which is evenly heated after being energized and can reach a high enough temperature to generate turbulence of different intensities. The top of the pool body is a cooling panel. The circulating flow of tap water (also available for cooling, using cooling water) keeps the cooling panel at a constant room temperature (or low temperature) to achieve different temperature differences between the upper and lower parallel plates. The temperature measurement system is composed of a temperature detector inside the tank, which can collect and record the temperature information of each part of the device in real time. The automatic control system adjusts the heating system in real time according to user preset information and temperature collection information to form a closed-loop control process.  
When the atmospheric turbulence emulation device is working, the lower plate is heated and the upper plate is cooled, so that the temperature difference between the two plates is gradually generated to form convection. When the temperature difference (that is, the Rayleigh number) exceeds a certain value, the flow forms turbulence, which is characterized by the physical quantity Fried constant, which is the atmospheric coherence length \(r_{0}\). When the intensity of atmospheric turbulence increases, the atmospheric coherence length decreases. The relationship between the temperature difference of the upper and lower plates of the atmospheric turbulence emulation device and the correlation length of the atmosphere can be expressed as  
\[r_{0}=48\times\left(\Delta T\right)^{-0.81} \tag{3}\]  
Therefore, the corresponding turbulence intensity can be obtained by adjusting the temperature difference between the two plates of the box. Figure 3 shows the atmospheric coherence length at a temperature ranges from 5 \({}^{\circ}\)C to 250 \({}^{\circ}\)C measured by the Hartmann method. It can be seen that the range is 0-12 cm and decreases with the increase in the  
Figure 2: Schematic diagram of the overall structure of the hot air convection type atmospheric turbulence emulation device.  
temperature difference. In this paper, the temperatures of 80, 150, and 220 \({}^{\circ}\)C (corresponding to \(r_{0}\) of 1.38, 0.83, 0.57 cm, respectively) between the panels was used as the propagation channel.