## 4 The Physical Layer Security Performance of the Relay Transmission Model
### The Influence of the Relay Power for P(Cs > 0)
#### 4.1.2 Experimental Results with Various Relay Power  
In the experiment, S, R, D and E are located at fixed two-dimensional coordinates (0; 0), (1.8 m; 0), (3.6 m; 0) and (0; 3.6 m), respectively. In both D and E node, BER of both main channel and wiretap  
Figure 5: The system model. Figure 6: Simulation results: \(P(C_{S}>0)\) versus \(P_{r}/P_{S}\) under different behavior. channel are obtained by combining the complex signal from S and R using maximum-ratio-combining (MRC), which a weighted superposition of the source and relay signals arriving at the destination are collected with the maximum available diversity order. We calculate \(P(C_{S}>0)\) by SNR according to (11), (12) and (13). The relay power is changed about 3 dBm every step and the source power is fixed at \(-39\) dBm. In malicious behavior case, pseudo random noise is forwarded by the relay, in which there are 5972 error bits in all 12,144 bits. The experimental data of cooperative relay and malicious relay are shown in Tables 2 and 3 respectively, where \(P_{D}\) is the BER at the destination and \(P_{E}\) is the BER at the eavesdropper. In Table 2, \(\gamma_{M}=\gamma_{SRD}+\gamma_{SD}\) is the SNR of the main channel, and \(\gamma_{W}=\gamma_{SRE}+\gamma_{SE}\) is the SNR of the eavesdropper channel. The last row of Table 2 is the calculated probability of \(C_{S}>0\) (denoted by \(P(C_{S}>0)\). In Table 3, \(\gamma_{M}=\gamma_{SD}/(1+\gamma_{RD})\) is the SNR of the main channel, and \(\gamma_{W}=\gamma_{SE}(1+\gamma_{RE})\) is the SNR of the eavesdropper channel. The graphic experimental results are shown in Figure 7.  
In our experimental setup, the results from Figure 7 indicate that the optimal relay power value is larger than the sources power (\(P_{r}/P_{s}=5\) dB), which means the relay need more power to achieve the optimal security performance in real non ideal DF environment. Therefore, generally speaking, in practice we can assume, the source to destination(S-D) channel is not ideal, the more relay power will be needed to achieve the same results as that in the ideal S-D channel. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Transmit** & \(\mathbf{-51}\) & \(\mathbf{-49}\) & \(\mathbf{-46.5}\) & \(\mathbf{-44}\) & \(\mathbf{-41.5}\) & \(\mathbf{-39}\) & \(\mathbf{-36}\) & \(\mathbf{-34}\) & \(\mathbf{-31}\) & \(\mathbf{-28.5}\) \\ \hline \(P_{D}\) & 0.3383 & 0.2472 & 0.1873 & 0.113 & 0.061 & 0.0196 & 0.0026 & 0.0002 & 0.0001 & 0.0000 \\ \(\gamma_{M}\) & 0.0974 & 0.2614 & 0.4415 & 0.825 & 1.3386 & 2.3814 & 4.3727 & 7.1942 & 8.1577 & \(\infty\) \\ \(P_{E}\) & 0.3472 & 0.2639 & 0.2096 & 0.152 & 0.1118 & 0.0692 & 0.0226 & 0.0054 & 0.0005 & 0.0000 \\ \(\gamma_{W}\) & 0.0864 & 0.2231 & 0.3652 & 0.593 & 0.8293 & 1.2289 & 2.2451 & 3.6327 & 6.1224 & \(\infty\) \\ \(P(C_{S}>0)\) & 0.5299 & 0.5394 & 0.5473 & 0.582 & 0.6175 & 0.6596 & 0.6608 & 0.6645 & 0.5713 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental results with different transmit power under cooperative relay. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Transmit** & \(\mathbf{-51}\) & \(\mathbf{-49}\) & \(\mathbf{-46.5}\) & \(\mathbf{-44}\) & \(\mathbf{-41.5}\) & \(\mathbf{-39}\) & \(\mathbf{-36}\) & \(\mathbf{-34}\) & \(\mathbf{-31}\) & \(\mathbf{-28.5}\) \\ \hline \(P_{D}\) & 0.3670 & 0.3810 & 0.4017 & 0.4198 & 0.4358 & 0.4459 & 0.4635 & 0.4811 & 0.4878 & 0.4918 \\ \(\gamma_{M}\) & 0.0647 & 0.0513 & 0.0343 & 0.0229 & 0.0146 & 0.0104 & 0.0047 & 0.0013 & 0.0005 & 0.0002 \\ \(P_{E}\) & 0.3609 & 0.3725 & 0.3902 & 0.4104 & 0.4251 & 0.4359 & 0.4530 & 0.4685 & 0.4847 & 0.4918 \\ \(\gamma_{W}\) & 0.0710 & 0.0592 & 0.0416 & 0.0287 & 0.0200 & 0.0146 & 0.0078 & 0.0035 & 0.0010 & 0.0002 \\ \(P(C_{S}>0)\) & 0.4767 & 0.4624 & 0.4519 & 0.4438 & 0.4231 & 0.4155 & 0.3761 & 0.2652 & 0.3333 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental results with different transmit power under malicious relay. Figure 7: Experimental results: \(P(C_{S}>0)\) versus \(P_{r}/P_{s}\) under different behavior.