# A Cooperative Transmission Scheme in Radio Frequency Energy-Harvesting WBANs
## 5 Simulation Results  
To evaluate the proposed scheme, a series of simulations were conducted in Matlab, and the results are presented in this section. The simulation considered a saturated WBAN that consists of one coordinator and several RF energy harvesting-based sensor nodes. The simulation parameters are listed in Table 2. The nodes were placed on the human torso and formed a 60 cm \(\times\) 40 cm plane. A coordinate system was established where a long side and an adjacent short side were selected as the x-axis and y-axis, respectively. The coordinator was placed at the coordinate origin (0, 0). The length of each superframe was set to 0.1 s. When a potential relay node forwarded packets, the received power was set to be the same as the transmit power. The proposed scheme was compared with the single-hop scheme without cooperation in considered scenarios. In the first scenario, ten sensors are deployed in two columns; each column contains five nodes uniformly deployed within a distance of 40 cm. The abscissa of one column is fixed at 50 cm, and the abscissa of the other column is within the range of 10 cm to 50 cm. The length of the packets uploaded by the sensors is set to be 50 bytes at first, with the energy conversion efficiency set to be 0.7 or 0.9. The packet reception rate is mainly evaluated, which can be formulated as  
\[Pr=N_{r}/N_{s}, \tag{14}\]  
where \(N_{r}\) represents the number of non-duplicate packets received by the coordinator, and \(N_{s}\) represents the number of packets uploaded by the sensor. Figure 3 illustrates the packet reception rates in the first scenario. With the distance of the sensors increasing, the packet reception rates demonstrate a downward trend. However, our proposed scheme performs better than the single-hop transmission scheme without cooperation. The sensors closer to the coordinator can use part of their energy for data forwarding to improve network performances. Sensors deployed with abscissa between 40 cm and 50 cm cannot harvest enough energy to ensure satisfactory packet reception rates. No potential relay exists when all sensors are deployed far from the coordinator, and the network performance of the two schemes is similar. When the distance of one column is within range from 10 cm to 35 cm, sensors in this column can harvest enough energy and act as potential relays. These sensors can forward packets uploaded by sensors far from the coordinator, which can improve the network performance through cooperation among nodes. Compared with the single-hop transmission scheme, our scheme increases the packet reception rate by 4.9% to 7.8% when the abscissa of one column is between 10 cm and 30 cm. Subsequently, we evaluated the performance improvement of the proposed scheme under different load conditions. One column has a fixed abscissa of 50 cm, while the abscissa of the other column ranges from 10 cm to 35 cm. The packet size generated by the \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Description** & **Value** \\ \hline \(P_{c}\) & transmission power of the coordinator & 1 mW \\ \(R\) & transmission rate & 100 kbps \\ \(PL(d_{0})\) & path loss at the referent distance \(d_{0}\) & 55 \\ \(n\) & path loss exponent & 2.4 \\ \(d_{0}\) & the reference distance & 1 m \\ \(\sigma^{2}\) & noise power & \(-\)104 dB \\ \(\delta\) & energy conversion efficiency & 0.9, 0.7 \\ \(\alpha\) & power split ratio & 0.6 \\ \(L_{i}\) & packet length & 20\(\sim\)100 bytes \\ \hline \end{tabular}
\end{table} 
Table 2: Simulation parameter. sensors is set from 20 bytes to 80 bytes, and the energy conversion efficiency is set to 0.7. The main metric evaluated is the improved packet arrival rate, which can be expressed as  
\[Pi=(N_{r}^{{}^{\prime}})/N_{s}, \tag{15}\]  
where \(N_{r}^{{}^{\prime}}\) represents the number of packets uploaded by ordinary sensors that were not received by the coordinator but were successfully relayed to the coordinator through the relay nodes. Figure 4 shows the improved packet arrival rate under the proposed scheme.