## 1 Introduction  
The millimeter-wave band generally covers 30 to 300 GHz. Since the introduction of fourth-generation (4G) mobile communication, frequencies below 30 GHz have been occupied by military and civilian applications, leaving few frequency resources available. To meet the heightened demand for mobile users and data services driven by the rapid development of mobile Internet and the Internet of Things, the millimeter-wave band has been applied in 5G mobile communications [1, 2, 3]. Compared to ultrashort waves and microwaves, millimeter waves [4, 5] offer faster data transmission speeds, larger transmission bandwidths, and delays on the order of a millisecond, all of which have become advantages and technical bases for 5G millimeter waves. However, the characteristics of these high-frequency millimeter waves also introduce significant challenges for 5G technologies. Wireless communication signals, especially mobile signals, are essential for cities to maintain normal and efficient operations. Urban microcellular environments are the basic application scenarios for transmitting and receiving wireless communication signals. In the millimeter-wave band, the distribution density and roughness of buildings in urban microcellular environments have a significant impact on the distribution of wireless communication signals in cities. The propagation characteristics and models of wireless channels are prerequisites for technology evaluation, equipment development, and the system design and network deployment of mobile communication. The propagation mechanism of the millimeter-wave band differs from that of the lower frequency band. As the millimeter-wave wavelength becomes shorter, the diffraction ability of the radio wave is reduced, and it is vulnerable to extreme weather conditions such as interference from the atmosphere, rainfall, snowfall, and fog, as well as other obstacles. However, the scattering effect is more pronounced in the millimeter wave band (as seen in [6]). Therefore, conducting research on channel modeling methods based on scattering effects plays an important role in the development of 5G technology. In general, there exist two methods for channel modeling. One is the statistical model [7; 8; 9] based on experimental data, which offers the advantages of easy and fast calculation but is hindered by poor prediction accuracy. Experiments must be conducted for various scenarios, and a large amount of experimental data must be acquired at an early stage. The other method is the deterministic model based on theory, which offers the advantages of high accuracy across a wide variety of application scenarios but is hindered by long calculation times and poor efficiency. Considering the very complex and diverse application scenarios of 5G, it takes considerable human and material resources to carry out complete statistical modeling. Therefore, conducting research on deterministic radio wave propagation methods in the millimeter wave is the key to performing multi-scene 5G channel modeling. At present, the main deterministic models are the PE and RT [10; 11; 12]. RT can clearly portray the propagation trajectory of electric waves and provide a powerful and intuitive tool for the study of propagation characteristics. The PE can portray the spatial distribution of the field of electromagnetic wave propagation, which facilitates the reasonable planning of system design and network layout according to the field distribution. Both methods offer distinct advantages in the study of electric wave propagation and have received considerable attention. There have been a number of studies on the use of the three-dimensional PE [13; 14; 15; 16] to calculate the propagation characteristics of electromagnetic waves through buildings. The characteristics of the field distribution through a single building were analyzed by [PERSON] using a scalar three-dimensional PE model in 1992 [17]. In 1996, [PERSON] and [PERSON] introduced the finite difference (FD) method to the solution of the three-dimensional PE.