## 2 System Model
### Communication Model
#### 2.2.2 Cooperative Communication Model  
When the \(EU_{m}\) directly transmits its data to the BS, performance may suffer due to the poor link quality. Aiming for improving the connective experience of the \(EU_{m}\), we design a BSWW-OSSS. Based on the proposed scheme, the original time period \(T\) allocated for the \(EU_{m}\) may be divided into two time slots, namely \(t_{1}=\tau_{mn}\) and \(t_{2}=1-\tau_{mn}\), as shown in Figure 2. Hence, based on the cooperative model, the communications may include two phases, as seen in Figure 2.  
* **PHASE 1**: During the first time slot \(t_{1}\), the \(EU_{m}\) transmits data to \(DT_{n}\) and BS. The achievable rate of the direct transmission from the \(EU_{m}\) to the BS may be expressed as: \[R_{EU_{m}\to BS}=\tau_{mn}\log_{2}\left(1+\frac{h_{bm}P_{m}}{N_{0}}\right)\] (3) Furthermore, the transmit rate \(R_{EU_{m}\to DT_{n}}\) achieved by the transmission from the \(EU_{m}\) to the \(DT_{n}\) may be expressed as: \[R_{EU_{m}\to DT_{n}}=\tau_{mn}\log_{2}\left(1+\frac{h_{mn}P_{m}}{N_{0}}\right)\] (4) where \(h_{mn}\) represents the channel gain of the link from the \(EU_{m}\) to the \(DT_{n}\). \begin{table}
\begin{tabular}{c c} \hline \hline
**Notation** & **Physical Meaning** \\ \hline \(P_{m}\) & \(EU_{m}\)’s transmit power \\ \(P_{n}\) & \(DT_{n}\)’s transmit power \\ \(P_{MAX}\) & EUs’ and DTs’ maximum transmit power \\ \(h_{mm}\) & channel gain between \(EU_{m}\) and \(DT_{n}\) \\ \(h_{bm}\) & channel gain between \(EU_{m}\) and BS \\ \(h_{nm}\) & channel gain between \(DT_{n}\) and \(DR_{n}\) \\ \(h_{bm}\) & channel gain between \(DT_{n}\) and BS \\ \(N_{0}\) & power of AWGN \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Table of notation. Figure 2: Time frame structure for cooperation. * **PHASE 2**: During the second time slot \(t_{2}\), the \(DT_{n}\) simultaneously transmits \(EU_{m}\)'s and itself data with the aid of NOMA technology. The \(DT_{n}\) transmits its own data to \(DR_{n}\) with the power of \(P_{mn}^{D}\) where \(P_{mn}^{D}=\alpha_{mn}P_{n}\), while forwards the \(EU_{m}\)'s data with the power of \(P_{mn}^{B}\) where \(P_{mn}^{B}=(1-\alpha_{mn})\ P_{n}\). Moreover, \(\alpha_{mn}\) is power allocation coefficient. Equation (5) describes the data sent by the \(DT_{n}\) when the NOMA technology is employed. \[y=\sqrt[2]{P_{mn}^{B}}S_{mn}^{1}+\sqrt[2]{P_{mn}^{D}}S_{mn}^{2}\] (5) where \(S_{mn}^{1}\) is the \(EU_{m}\)'s data which has to be forwarded by the \(DT_{n}\), while \(S_{mn}^{2}\) denotes the \(DT_{n}\)'s data. The total power consumed by the \(DT_{n}\) during the phase 2 is denoted by \(P_{n}\), which is equal to \(P_{MAX}\). The SIC technology is invoked at both the BS and the \(DR_{n}\) for the sake of separating \(EU_{m}\)'s and \(DT_{n}\)'s data. During phase 2, the Signal to Interference Plus Noise Ratio (SINR) of the \(EU_{m}\)'s data received by the BS may be given by: \[SINR=\frac{h_{bn}\left(1-\alpha_{mn}\right)P_{n}}{ah_{bn}\alpha_{mn}P_{n}+N_{0}}\] (6) where \(h_{bn}\) represents the channel gain of the link from the \(DT_{n}\) to the BS. Moreover, \(a\) represents the relationship between the power \(P_{mn}^{D}\) and \(P_{mn}^{B}\). According to the NOMA and the SIC technologies, if \(DT_{n}\) forwards \(EU_{m}\)'s data at a higher transmit power, namely \(P_{mn}^{B}>P_{mn}^{D}\), \(a\) is equal to 1. Otherwise, the value of \(a\) is 0, as shown in Equation (7). \[a=\left\{ \begin{array}{ll}1&P_{mn}^{B}>P_{mn}^{D}\\ 0&P_{mn}^{B}\leq P_{mn}^{D}\end{array} \right.\] (7) When the BS receives data from \(DT_{n}\), SIC technology is exploited to separate the \(EU_{m}\)'s and \(DT_{n}\)'s data. Then, both the previous data from the \(EU_{m}\) in phase 1 and the later data from the \(DT_{n}\) in phase 2 are combined by the BS with the aid of Maximum Ratio Combining (MRC) [40; 41].