## 4 Simulation Results  
### Simulation Configuration  
In order to evaluate the achievable performance of the proposed RSWW-OSSS, we consider a cellular network with radius 500 m where a BS is located in the center of the cell. Furthermore, EUs are randomly located on the edge of the cellular network. D2D transmission pairs are randomly distributed across the entire network. The maximum power of both the each EU and each D2D pairs are assumed to be 0.1 W [13]. Rate requirement coefficient \(\xi\) is set to be 2.5. The simulation parameters are summarized in Table 4 [13,39]. We consider the pricing and spectrum leasing scheme (PSLS) which was proposed in [13] as the cooperative benchmarker of our scheme. The D2D transmission pair in the PSLS shares the spectrum resources with the cellular users by using the overlay approach. More explicitly, based on the PSLS, the DT provides cooperative assistance for the EU in order to obtain a D2D transmission opportunity and to improve the QoS of the uplink transmission from the EU to the BS. After successfully forwarding EU's data, the DT in the PSLS conveys its own data during the successive sub-timeslot [13]. It is noted that power allocation was not considered in the PSLS [13]. Figure 3 compares the attainable rates of both the uplink transmissions from EUs to BS and the D2D transmissions versus the different number of D2D transmission pairs. Given the number of D2D transmission pairs, the proposed RSWW-OSSS is capable of achieving higher rates for both the EUs and the D2D transmission pairs than the PSLS which was proposed in [13], as seen in Figure 3. More explicit, when the number of D2D pairs is 20, the achievable rate of EUs in our RSWW-OSSS is about 12% more than that in PSLS and the achievable rate of D2D pairs in our RSWW-OSSS is about 28% more than that in PSLS. When the network hosts more D2D transmission pairs, the attainable rates of EUs are significantly improved by our RSWW-OSSS, due to the increased number of potential D2D transmission pairs which can provide cooperative assistance for the EUs, as seen in Figure 3. When the number of D2D transmission pair is increased from 4 to 20, more D2D transmission pairs may share the cellular spectrum for D2D communications. Therefore, the achievable rate of D2D communications is increased when the D2D network becomes larger, as shown in Figure 3. However, when the number of D2D transmission pairs is more than 30, the achievable rates of D2D communications almost reach the saturation point, because all the EUs are matched with appropriate D2D transmission pairs. Figure 4 illustrates the transmit power consumed by the DTs for forwarding EUs' data versus the different number of D2D transmission pairs. As shown in Figure 4, compared to the PSLS, our RSWW-OSSS is capable of decreasing the power for forwarding EU's data with the aid of the proposed two-stage Stackelberg game and the proposed matching algorithm. When the D2D network becomes larger, cooperative matchings may be generated due to the increased number of D2D transmission pairs which can be the potential cooperative partner of EUs. Hence, the transmit power for forwarding EUs' data is increased when the network hosts more D2D transmission pairs. However, when the number of D2D transmission pairs is more than 30, all the EUs may be matched \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline _Cell radius_ & 500 m \\ _Noise power_\((N_{0})\) & \(-114\) dBm \\ _D2D link distance_ & 50 m \\ _Each EU Tx power_ & 100 mW \\ _Each D2D pair Tx power_ & 100 mW \\ _Pathloss constant_\((L)\) & \(10^{-2}\) \\ _Pathloss exponent_\((\eta)\) & 4 \\ _Rate requirement coefficient_\((\xi)\) & 2.5 \\ _Number of EUs_\((M)\) & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Simulation Configure Parameters. with an appropriate D2D transmission pair. Hence, the number of cooperative matchings may not increase when more than 30 D2D transmission pairs intend to share cellular spectrum with the EUs.