## 2 Airborne Air-to-Air Channel Laser Communication Link Analysis and Modeling
Clouds consist of tiny water droplets or ice crystals suspended in the air and their formation is mainly caused by the upward movement of moist air. The particle size, droplet content, and distribution of water droplets in various kinds of clouds and in clouds of varying thicknesses are not the same, which leads to differences in parameters such as the optical thickness of their scattering. Clouds vary greatly in height and thickness, with high-altitude clouds often being distributed in the 4.5 km-10 km region. The time response characteristics of the laser pulse airborne channel transmission, obtained from the simulation of this transmission through clouds with different optical thicknesses, is an important input condition for the ISI simulation of the detection and reception phase. Under meteorological cloud conditions, the airspace channel has random change characteristics and the clouds in the airborne air-to-air channel also present non-uniform distribution characteristics. The analysis of different parts of the cloud object is also relevant to determine the scattering and absorption characteristics of the different cloud types in order to more accurately describe the pulse-broadening characteristics of the laser beam transmitted through the channel and to provide a basis for the design of equalization programs. In this paper, an atmospheric scattering model and an atmospheric turbulence model are also established based on the airspace channel transmission time-response characteristics. Then, based on the atmospheric scattering model and atmospheric turbulence model, a Monte Carlo simulation model of the influence of the airborne air-to-air channel on the time-domain broadening characteristics of a Gaussian laser pulse is established. Combined with the design parameters of a typical airborne laser communication system, the Monte Carlo ray tracing (MCRT) method is used to simulate and analyze the time-response characteristics of the laser pulse through multiple cloud scattering channels. The simulation modeling of laser multipath transmission through an airborne air-to-air channel focuses on the following two aspects [10]: first, the Mie scattering effect generated by the interaction between the laser beam and the molecules and particles dispersed in the channel, and second, the change in the beam transmission direction due to the change in the atmospheric refractive index affecting the pulse time-response characteristics. The simulation model is shown in Figure 2.  
Airborne air-to-air channel laser transmission simulation modeling can be divided into the laser emission state setup, the laser cloud scattering transmission process (i.e., the generation of the free scattering range; the selection of scattering element parameters, Mie scattering effect, and scattering angle transformation; and the iterative simulation of continuous Mie scattering processes), the laser turbulence layer transmission process (i.e., laser and turbulence bulb interactions and iterative simulation of continuous turbulence effect process), and the identification of the effective pupil of the laser, in addition to other simulation and computation processes [11; 12]. Taking the initial transmission coordinates as the origin, with the upward direction as the positive \(x\)-axis and the leftward direction as the positive \(y\)-axis, the transmission  
Figure 2: Simulation flow and model of laser transmission in the air-surface multipath channel. direction corresponds to the positive \(z\)-axis. During laser transmission in the airborne air-to-air channel, the interaction between photons and the molecules and particles dispersed in the airborne transmission channel generates a number of consecutive Mie scattering effects, so that the transmission direction vector before and after each Mie scattering is \(\left(u_{x,n},u_{y,n},u_{z,n}\right)\) and \(\left(u_{x,n+1},u_{y,n+1},u_{z,n+1}\right)\), respectively. The photon scattering angle is \(\theta_{scan}\) in rad.