## 2 System Model
### Channel  
We assume that a wireless channel can be established between a transmitter and a receiver, and the signal is transmitted over this channel. During the signal transmission, there are multiple copies of the transmitted EM wave, and they simulataneously excite the receiver antenna. The TWDP model is general and it assumes two dominant line-of-sight (LOS) components and diffuse components. The two specular components have amplitudes that are constant, and their phases are distributed uniformly over the interval from 0 to \(2\pi\). We denote the amplitudes of these specular components by \(V_{1}\) and \(V_{2}\), while their phases are denoted by \(\Psi_{1}\) and \(\Psi_{2}\). The scattering component statistics correspond to a Rayleigh distribution, i.e., it consists of two Gaussian-distributed in-phase and quadrature components having zero mean and same standard deviations of \(\sigma_{F}\). These random in-phase and quadrature components are denoted by \(x_{F}\) and \(y_{F}\), respectively. The resulting complex signal envelope at the receiver is [7]  
\[V_{r}=V_{1}\,\mathrm{e}^{i\Psi_{1}}+V_{2}\,\mathrm{e}^{i\Psi_{2}}+x_{F}+jy_{F}. \tag{1}\]  
The complex fading can be presented in terms of the envelope \(r\) and argument \(\theta\) as  
\[V_{r}=r\,\mathrm{e}^{j\theta}. \tag{2}\]  
The resulting envelope is given by  
\[r=\sqrt{(V_{1}\cos\Psi_{1}+V_{2}\cos\Psi_{2}+x_{F})^{2}+(V_{1}\sin\Psi_{1}+V_{2 }\sin\Psi_{2}+y_{F})^{2}}. \tag{3}\]  
The PDF of the fading envelope can be expressed in different forms. An approximation is given by ([7], Equation (4)). However, some shortcomings of this approximation were emphasized in [8]. By using an analogous with the situation when there are a useful signal, co-channel interference and Gaussian noise, an infinite series form of PDF was presented in [11], where the authors introduced the novel way for parametrization. Namely, this fading model can be described using two parameters denoted by \(K\) and \(\Gamma\). The value of parameter \(K\) represents the power of the specular components-to-power of the diffuse component, while parameter \(\Gamma\) is the ratio between the amplitudes of specular components. These two parameters are defined as [11]  
\[K=(V_{1}^{2}+V_{2}^{2})/(2\sigma_{F}^{2}),\quad\Gamma=V_{2}/V_{1}. \tag{4}\]  
By varying the value of the basic parameter denoted by \(K\) in our work, it is possible to make specular components dominant compared with diffuse components, and vice versa. With increasing the value of parameter \(K\), the specular components become dominant in comparison to the diffuse component. For example, in the terrestrial mobile links, parameter \(K\) has values that typicaly range from 0 dB to 15 dB [7; 8; 9; 10; 11; 12]. In addition, by varying the value of parameter \(\Gamma\), it is possible to scale the values of amplitudes of two specular components. The value of parameter \(\Gamma\) belongs to the interval between 0 and 1. When \(\Gamma=0\), it follows that the one specular component is equal to zero, while when \(\Gamma=1\), it follows that the amplitudes of specular components are equal. Instead of parameter \(\Gamma\), the authors of [7; 8; 9; 10] used parameter \(\Delta\) defined as \(\Delta=2V_{1}V_{2}/(V_{1}^{2}+V_{2}^{2})\).