## 2 System Model and Parameters Analysis  
This section discusses the maritime channel model and network setting for the downlink NOMA offshore communication system. At the end of this chapter, we consider the objective function of the resource optimization problem. With reference to the offshore communication environment of the _South China Sea_, this paper constructs a Longley-Rice-based channel model that obeys a Rice distribution. The reference value for the Longley-Rice transmission loss is expressed as follows:  
\[L_{cr}(dB)=L_{bf}+A_{cr}\;\text{dB}, \tag{1}\]  
where the formula for fundamental transmission loss \(L_{bf}\) in free space is:  
\[L_{bf}(\text{dB})=32.45+20\log_{10}f+20\log_{10}d\;\text{dB}, \tag{2}\]  
where the distance \(d\) is in km and the radio frequency \(f\) is in MHz. It is worth noting that the Longley-Rice model is suitable for signal propagation with carrier frequencies between 20 MHz and 20 GHz. The reference attenuation \(A_{cr}\) relative to free space depends on the communication distance:  
\[A_{cr}=\left\{ \begin{array}{cc}max(0,A_{e}+k_{1}d+k_{2}\log_{10}d),(dLoS_{ min})\\ A_{d}+m_{d}d,(d_{LoS}\leq d\leq d_{x})\\ A_{s}+m_{s}d,(d_{x}\leq d)\end{array} \right.\quad\text{dB}. \tag{3}\]We conclude from Equation (3) that the Longley-Rice transmission loss model is divided into three cases according to the size of the communication range: the LoS model (\(dLoS_{min}\)), the diffraction model (\(d_{LoS}\leq d\leq d_{x}\)), and the scattering model (\(d_{x}\leq d\)) [8]. In Equation (3), \(d_{x}\) indicates the boundary point where the diffraction loss and the scattering loss are equal. \(A_{e}\), \(A_{d}\), and \(A_{s}\) (unit: dB) represent the propagation loss of LoS, diffraction, and scattering in free space, respectively. Moreover, \(k_{1}\) and \(k_{2}\) represent the propagation loss coefficients. \(m_{d}\) and \(m_{s}\) are the diffraction and scattering loss coefficients, respectively. Due to the limited coverage of a 5G BS, our communication range between coastal BS and ships is within the horizon. Therefore, our offshore channel needs to only consider the LoS model. It is worth noting that the applicable transmission range of the Longley-Rice model specifies a lower bound \(d_{min}\) (generally set to 1 km). Enlightened by the guidebook [29], our channel model can be regarded as the fundamental transmission loss in free space (Equation (2)) within 1 km. The calculative process of the reference attenuation \(A_{cr}\) is very complicated. In that case, we have to consider the following essential parameters: climate factor (1-7), terrain irregularity parameter \(\Delta h\) (Table 1), the effective height of the transmitter/receiver antennas \(H_{T}/H_{R}\), the system deployment parameters (_very careful_, _careful_, _random_), the conductivity and permittivity of the seawater (Table 2), and the surface refractivity (Table 3). The mature Longley-Rice model provides seven climate backgrounds: (1) _equatorial_; (2) _continental subtropical_; (3) _maritime tropical_; (4) _desert_; (5) _continental temperate_; (6) _maritime temperate_ (_over land_); and (7) _maritime temperate_ (_over sea_). These seven kinds of climates cover the communication background of the global offshore ports, meaning that our model is the most widely used. The irregular terrain parameter \(\Delta h\) indicates the degree of sea wave undulation. Table 1 presents the parameter values given by the Longley-Rice model for different types of irregular terrain. In addition, the system deployment parameter reflects the qualitative description of the site selection scenario for each terminal, which will impact the signal strength around the terminal and the effective height of the antenna. According to the simulated geographic environment of the _South China Sea_, we will specifically discuss the assignment of the above parameters in Section 7. For the detailed calculation process of their participation in \(A_{cr}\), please refer to [8] and [29].