## 1 Introduction
However, it is more difficult to apply this method in real-world scenarios with multiple buildings [18]. In 1999, [PERSON] et al. proposed the topographic boundary conditions of the three-dimensional PE and calculated the PE using the FD method [19]. In 1999, [PERSON] proposed a strict three-dimensional vector PE model based on the FD method to solve the problems in the calculation of bistatic RCS for electric large-scale targets and urban wave propagation [20]. In 2003, [PERSON] elaborated the split-step Fourier transform (SSFT)-based algorithm for solving simple urban radio wave propagation characteristics using the three-dimensional PE in a single lossy medium for the first time [21]. [PERSON] et al. used the scalar three-dimensional PE method to calculate the wave propagation characteristics of single and multirow buildings based on the SSFT on flat terrain in 2006 [22]. In 2017, [PERSON] et al. considered the bidirectional propagation of the three-dimensional vector PE on flat terrain with peaks and irregular terrain comprising hills [23]. In 2018, [PERSON] et al. applied a 3 DPE method to calculate the propagation factor over the flat terrain [24]. In 2019, [PERSON] et al. used a 3 DPE method to calculate the field strength due to the forest on a lossy ground [25]. In 2022, [PERSON] et al. proposed a scheme of ground clutter simulation based on the 3 DPE model in simple urban environments [14]. However, only simple urban environments and terrain are computed using three-dimensional PE. A number of studies have calculated the scattering effects of buildings. In 1999, [PERSON] proposed an effective roughness model to show that scattering effects are important for both received power and channel dispersion in various situations [26]. In 2001, [PERSON] [27] proposed a method based on the Lambertian model to simulate the scattering of buildings in urban wireless propagation environments. In 2007, [PERSON] et al. [28] conducted experiments which were compared with RT, specifically, a fusion of the Lambertian or directive models. The results showed that the simulations agree well with the measurements when a suitable directive model is used. In 2011, [PERSON] et al. [29] examined the prediction accuracy of an advanced deterministic model for the depolarization of the channel and frequency selectivity in an indoor wireless propagation channel. Their results showed that RT, which considers the directive pattern model, significantly improved the prediction accuracy of path loss and cross-polarization discrimination, as well as the prediction of delay spread. In 2012, [PERSON] et al. [30] compared the RT method including the directive scattering pattern model with experimental data and showed that upon including diffuse scattering, it was able to reproduce the angular spread of dense multipaths with an error of approximately 4\({}^{\circ}\) to 20\({}^{\circ}\). Later, [PERSON] et al. [31] added a Lambertian model to the RT method, which was compared with the post-processed data, and their results showed that diffuse scattering is a key component of the received power. In 2014, [PERSON] et al. [32] developed a polarized diffuse scattering model based on the effective roughness model. In 2015, [PERSON] et al. [33] proposed that choosing the appropriate parameter values for the effective roughness model could improve the accuracy of evaluating the power delay distribution at 60 GHz. In 2017, [PERSON] et al. [34] added a Lambertian model to the RT method to calculate the total electric field for horizontal and vertical polarizations. In 2021, [PERSON] et al. [35] accounted for the directive diffuse scattering model. In 2022, [PERSON] et al. [36] proposed an empirical diffuse scattering in RT. In 2023, [PERSON] et al. [37] set diffuse scattering to a value. Therefore, the scattering effect is crucial. However, the traditional RT emits a lot of rays.