## 1 Introduction  
At present, using sound waves is the only method for transmitting data over long distances in seawater. Underwater acoustic communication has become an indispensable part of data transmission technology for exploring, developing, and protecting the ocean. The complexity of UAC systems is mainly manifested in the time-varying and space-varying channels. The signal-to-noise ratio (SNR) as a measurement parameter of system performance design is also an important indicator for evaluating the quality of underwater acoustic communication. The spatiotemporal variation range of the SNR can be used to describe the spatiotemporal fluctuation characteristics of underwater acoustic communication signals. Due to the complexity and time variation of the marine environment, the SNR of communication signals varies widely in time and space. Therefore, the degeneration in the UAC system performance would be caused by two reasons: the difficulties in optimizing the synchronization signal detection threshold and determining the location of the UAC system equipment. However, from the perspective of the sound field, it is possible to describe the spatiotemporal variation from the signal field and noise field, and then analyze the fluctuation characteristics of the underwater acoustic communication channel. Environmental factors, such as ocean currents, tides, and internal waves, with large spatial-temporal fluctuations have not been considered. Since the 1950s, researchers have gradually paid attention to signal interference in shallow-water sound fields, and they mainly analyzed the spatial-temporal characteristics of sound fields based on ray acoustic theory and normal wave theory [1; 2; 3]. Ray acoustics researchers focused on the Loe mirror effect in optics, which assumes that the interfering sound rays are approximately parallel, and they derived a partially analytical solution and discovered vertical distribution characteristics of the sound field [4]. In the 1980s, in view of the normal wave theory and far-field assumption, scholars analyzed the interference sound field and put forward the conception of waveguide invariants. However, previous theoretical analyses of the interference phenomenon, which are based on restricted scenarios and simplified assumptions, were detrimental to the establishment of a universal analysis model. This study mainly focused on the influence of short-range shallow sea interference on signal fluctuations. Compared with the normal wave method, the ray acoustic method is superior given its clear concept and simple calculation; therefore, the ray acoustic method was adopted to analyze the spatial distribution of the sound field. From the perspective of the sound field, the influencing factors of the spatial variation range of the SNR are not only the spatial-temporal distribution characteristics of the signal field caused by the signal interference and interface fluctuations but also the spatial-temporal distribution characteristics of the system noise field. The operating performance of a UAC system is significantly affected by the noise of the system and the marine environment. The marine environmental noise was first measured in two studies [5; 6]. After analyzing a large amount of measured data, it was found that marine environmental noise is mainly composed of wind-induced noise, ship noise, and biological noise; reference [7] gives a marine environmental noise spectrum and notes that the low-frequency noise components mainly come from the machinery of ships, while high-frequency noise is mainly wind-induced wave noise. Due to the continuous development of signal acquisition technology and the increase in marine research investment, research on marine environmental noise has become a popular topic of discussion [8; 9; 10; 11; 12; 13; 14; 15]. On the basis of different sound field propagation theories, researchers proposed various marine environmental noise models. The classic models include the C/S model, K/I fast-field model, and P/K model [16; 17; 18].