## 1 Introduction  
Wireless laser communication has the advantages of high capacity, low latency, and high-throughput transmission. Its use in interplanetary channel application scenarios has been validated and has entered the engineering implementation stage [1]. However, the use of airborne laser communication in airborne air-to-air channels involving turbulence has yet to be successfully applied due to the existence of many technical and engineering problems. Among these problems, the degradation of communication link quality or even the link's interruption due to inter-symbol interference caused by the multipath effect is one of the most important problems affecting this technology's reliability and availability [2, 3]; it is also one of the key problems to be solved in airborne laser communication research. Equalization is one of the techniques heavily used in the field of communication and is proven to be effective in mitigating or suppressing the effects of inter-symbol interference [4, 5]. The goal of equalization is to make the communication system achieve as close to interference-free transmission conditions as possible using an equalizer (e.g., by restoring as much of the original transmitted signal waveform as possible after the detection of reception/before sampling judgment). Equalization is realized by eliminating as much inter-symbol interference as possible in the impulse response of the whole communication system, including the equalizer. Channel technology is one of the most fundamental and crucial aspects in the field of wireless optical communication [6]. Due to the typical time-varying and stochastic characteristics of the free-space channel, adaptive adjustments to the parameters of the airborne laser communication link equalizers are necessary to accommodate the dynamic nature of the channel. Conducting research on the multipath transmission characteristics of on-board free-space laser channels is a prerequisite for studying equalization techniques in laser communication links. It forms the foundation for addressing issues related to inter-symbol interference, aiming to enhance communication quality and improve the reliability and availability of communication links. The pulse broadening of laser beams after transmission through turbulent channels has been identified as the foremost factor contributing to inter-symbol interference in receiver systems. This paper employs the Monte Carlo ray tracing (MCRT) method to model and simulate the interactions of lasers with varying molecular and particle constituents within the turbulent atmospheres of free-space channels. The results of these simulations will yield the impulse response of the transmission channel, which is essential for supporting the design of equalizers [7]. The objective of equalization is to realize interference-free transmission conditions in communication systems equipped with equalizers to the greatest extent possible. This involves restoring the original transmission signal waveform post-reception and pre-sampling/decision. The equalizer's function is to ensure that the entire communication system, including the equalizer itself, satisfies the conditions for minimal inter-symbol interference. The channel's impulse response obtained from simulations will guide and influence the overall design aspects of the equalizer, such as its structure, algorithm, convergence performance, and steady-state residual indicators. Due to the extensive nature of equalizer design considerations, these aspects will be discussed in a separate article. In the present paper, a numerical MCRT simulation of airborne air-to-air channel laser multipath transmission and an analysis of inter-symbol interference is performed.