# Security Performance Analysis of Full-Duplex UAV Assisted Relay System Based on SWIPT Technology
## 2 System Models and Transmission Protocols
### Transmission Protocols  
In the UAV eavesdropping scenario, a secure transmission protocol for the communication system is proposed. This protocol is based on SWIPT technology (the SWIPT technique follows the nonlinear RF to DC conversion model in practice [21], but a nonlinear model in this model would make the system performance analysis too complicated, so in this paper the SWIPT technique is treated according to the linear DC conversion model) and adopts PSR policy and AF protocol to realize the secure transmission of full-duplex relay communication system for UAV. Assuming that the total time slot length of the entire a full-duplex relay transmission is T, dividing T into two distinct phases, with both the first and second phase transmission times being \(\frac{\gamma}{2}\) (as shown in Table 1), \(\beta\) is the power splitting ratio \((0\leq\beta\leq 1)\), which denotes the proportion of the transmitting power that is used for energy harvesting, and \((1-\beta)\) denotes the proportion used to process the received signal. The first stage consists of signal transmission and energy harvesting processes:  
The received signal \(y_{u}\) of the UAV relay node U is:  
\[y_{u}=\sqrt{(1-\beta)P_{u}L_{au}}\cdot h_{au}\cdot x_{au}+\sqrt{(1-\beta)P_{u }}\cdot f_{u}\cdot x_{ue}+\sqrt{(1-\beta)}\cdot n_{u}+n_{p} \tag{5}\]  
where \(x_{au}\) is the useful signal sent by node U to node A, \(x_{ue}\) denotes the interference signal sent by node U to node E and also the self-interference signal received by node U (This \begin{table}
\begin{tabular}{l l l} \hline \hline
**Transmission Process** & **Phase I: 0\(-\frac{\gamma}{2}\)** & **Phase II: \(\frac{\gamma}{2}\)\(-\)\(T\)** \\ \hline \multirow{3}{*}{signal transmission} & A \(\rightarrow\) U (Effective signal transmission) & U \(\rightarrow\) B (relay amplification and forwarding) \\  & U \(\rightarrow\) E (Sending Interference Noise) & B \(\rightarrow\) E (Sending Interference Noise) \\  & U \(\rightarrow\) U (Self-interference) & B \(\rightarrow\) B (Self-interference) \\ \hline \multirow{2}{*}{energy harvesting} & A \(\rightarrow\) U (SWIPT) & \multirow{2}{*}{not have} \\  & U \(\rightarrow\) U (SWIPT) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PS-based secure transmission protocol. interference is assumed to be Gaussian white noise), \(E\Big{\{}\|x_{au}\|^{2}\Big{\}}=E\Big{\{}\|x_{ue}\|^{2}\Big{\}}=1\), \(n_{p}\) is the signal processing noise under the power splitting component with power \(N_{p}\) and \(n_{u}\) is the channel impairments caused by the thermal noise at the UAV relay node U, which was simulated as additive complex Gaussian white noise, \(n_{u}\sim CN(0,N_{0})\). \(P_{a}\) is the transmit power of the source node A in the first stage, \(P_{u}\) is the interference noise power sent by the drone relay node U to the drone eavesdropping node E in the first stage, therefore, the total transmit power in the first stage is \(P=P_{a}+P_{u}\). \(\lambda\) is the power allocation factor (\(0<\lambda\leq 1\)), \(P_{a}=\lambda P\), \(P_{u}=(1-\lambda)P\). When \(\lambda=1\), the UAV relay node U does not send jamming signals to the eavesdropping node E in the first stage, and only works as a receiver. The drone eavesdropping node E intercepts the signal sent by node A and also receives the jamming signal sent by the drone relay node U. Therefore, the received signal from node E is:  
\[y_{E}=\sqrt{P_{a}L_{ae}}.h_{ae}.x_{au}+\sqrt{P_{u}L_{ue}}.h_{ue}.x_{ue}+n_{e} \tag{6}\]  
where \(n_{e}\) is the additive complex Gaussian white noise received by node E, \(n_{e}\sim CN(0,N_{0})\). The energy collected by the UAV relay node U comes from the signal sent by the source node A and the jamming signal sent by itself to the eavesdropping node E. The energy gained by node U is:  
\[E=\eta\beta(P_{a}L_{au}S_{au}+P_{u}S_{u}+N_{0}).\frac{T}{2} \tag{7}\]  
where \(\eta\) is the power conversion efficiency factor.