## 2 Airborne Air-to-Air Channel Laser Communication Link Analysis and Modeling
During the simulation process, typical parameters were employed for the developed equipment, along with meteorological parameters from a specific season in the Chengdu region, ensuring low optical beam pointing errors. The specific parameters are as follows:  
1. The parameters of the laser communication device used during the simulation are constrained to the following: Operating wavelength, \(\lambda=1.55\) um; initial Gaussian pulse half-width, \(T_{0}=400\) ps, with the transmitting and receiving antenna apertures being the same and \(D_{t}=D_{r}=100\) mm; exiting pupil divergence angle, \(\theta_{t}=20\) umd; entering pupil receiving field of view, \(\theta_{r}=2\) mrad; beam alignment error angle, \(\theta_{ATP}=2\) umd; transmit power, \(P_{t}=1000\) mW; receiver sensitivity, \(P_{r}=-35\) dBm; and transmission distance, \(L=200\) km, \(N_{optics}=500,000\);
2. The impact of scattering effects was modeled using the Mie scattering model;
3. The input parameters during the laminar transport simulation based on the photon/turbulence bubble model are as follows: standard deviation of atmospheric refractive index undulation, \(\sigma_{n}=1\times 10^{-8}\); turbulence outer scale, \(L_{0}=10\) m; and turbulence inner scale, \(l_{0}=0.1\) mm;
4. The atmospheric visibility of the airborne air-to-air channel was simulated by taking a set value of \(V=50\) km for all simulations, except for the unidirectional item simulation, which used different meteorological conditions. Laser Pulse Time-Response Characteristics Are Simulated and Analyzed under Different Influencing Factors and Change Conditions  
The simulation will be divided into four parts, varying the turbulence intensity, atmospheric visibility, initial pulse half-width, and transmission distance for the simulation. The simulation flow is shown in Figure 4.  
In Figure 4, n indicates that the current simulated photon is the nth photon and \(N_{optics}\) is the total number of simulated photons. The specific parameters of this simulation are shown in Table 1. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Operating wavelength (\(\lambda\)) & 1.55 μm \\ Initial gaussian pulse half-width (\(T_{0}\)) & 400 ps \\ Transmitting and Receiving antenna apertures (\(D_{t},D_{r}\)) & 100 mm \\ Exiting pupil divergence angle (\(\theta_{t}\)) & 20 μmd \\ Entering pupil receiving field of view (\(\theta_{r}\)) & 2 mrad \\ Beam alignment error angle (\(\theta_{ATP}\)) & 2 μmd \\ Transmit power (\(P_{t}\)) & 1000 mW \\ Receiver sensitivity (\(P_{r}\)) & \(-35\) dBm \\ Transmission distance (\(L\)) & 200 km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameter table. Simulation of Time-Response Characteristics of Laser Signals Transmitted through Airborne Air-to-Air Channels with Different Atmospheric Turbulence Intensities  
In this subsection, the MCRT simulation of laser emissions after transmission over a multipath airspace channel with and without turbulence is carried out. The simulation results of the time-domain waveforms of the laser pulses after they are effectively received  
Figure 4: Photonic simulation flowchart. by the communication system (over a transmission distance of 200 km) are shown in Figure 5a,b shows the laser pulse spreading and the half-width ratios of the laser pulses for the two transmission models considering different transmission distances. Mean and variance in laser pulse time-domain broadening for different transmission distances are shown in Table 2.  
Considering only the scattering effect without considering the effect of turbulence, the time-domain pulse broadening (pulse half-width) in ps after laser transmission is 22 (422) and the ratio of the laser pulse time-domain broadening to the pulse half-width is 5.2%. When considering the effects of scattering and turbulence at the same time, the laser time-domain pulse broadening (pulse half-width) is 74 (474) and the ratio of the laser pulse time-domain broadening to the pulse half-width is 15.8%.