# Security Performance Analysis of Full-Duplex UAV Assisted Relay System Based on SWIPT Technology
## 2 System Models and Transmission Protocols
### Transmission Protocols
In the second phase, the power of the node U to send a signal to the destination node B is derived from the energy captured in the first phase, and the sending power is denoted as:  
\[P_{ub}=\frac{E}{T/2}=\eta\beta(P_{a}L_{au}S_{au}+P_{u}S_{u}+N_{0}) \tag{8}\]  
The signal sent by node U to destination node B is denoted as:  
\[x_{ub}=G\cdot y_{u} \tag{9}\]  
where \(G\) is the amplification factor, is:  
\[G=\sqrt{\frac{P_{ub}}{(1-\beta)\cdot(P_{a}L_{au}S_{au}+P_{u}S_{u}+N_{0})+N_{p}}} \tag{10}\]  
The second stage of the signal transmission process is analyzed as follows:  
1. The destination node B receives the signal sent by the UAV relay node U and, at the same time, sends interference to the UAV eavesdropping node E. The node B is also affected by the self-interference signal: \[ \begin{array}{l}y_{b}=\sqrt{L_{ub}}\cdot h_{ub}\cdot x_{ub}+\sqrt{P_{b}} \cdot f_{b}\cdot x_{be}+n_{b}\\ =G\sqrt{(1-\beta)P_{a}L_{au}L_{ub}}\cdot h_{au}\cdot h_{ub}\cdot x_{au}+G\sqrt {(1-\beta)P_{u}L_{ub}}\cdot h_{ub}\cdot f_{u}\cdot x_{ue}\\ \hskip 56.905512 pt+G\sqrt{(1-\beta)L_{ub}}\cdot h_{ub}\cdot n_{u}+G\sqrt{L_{ub} }\cdot h_{ub}\cdot n_{p}+\sqrt{P_{b}}\cdot f_{b}\cdot x_{be}\\ +n_{b}\end{array} \] (11) where \(n_{b}\) is the additive complex Gaussian white noise at node B, \(n_{b}\sim CN(0,N_{0})\). 2. The eavesdropping node E intercepts the signal \(x_{ub}\) amplified and forwarded by node U, and also receives the jamming signal sent by node B. In this case, the signal received by node E is:\[ \begin{array}{c}y_{e}=\sqrt{L_{ue}}\cdot h_{ue}\cdot x_{ub}+\sqrt{P_{b}L_{be}} \cdot h_{be}\cdot x_{be}+n_{e}\\ \quad\quad=G\sqrt{(1-\beta)P_{a}L_{au}L_{ue}}\cdot h_{au}\cdot h_{ue}\cdot x_{ aa}+G\sqrt{(1-\beta)P_{u}L_{ue}}\cdot h_{ue}\cdot f_{u}\cdot x_{uv}\\ \quad\quad+G\sqrt{(1-\beta)L_{ue}}\cdot h_{ub}\cdot n_{u}+G\sqrt{L_{ue}}\cdot h _{ue}\cdot n_{p}+\sqrt{P_{b}L_{be}}\cdot h_{be}\\ \quad\quad\cdot x_{be}+n_{e}\end{array}  \tag{12}\]  
According to the above-proposed secure transmission protocol, the signal-to-interference plus noise ratio (SINR) of the destination node B and the drone eavesdropping node E can be obtained, and the SINR at the destination node B is:  
\[\gamma_{AB}=\frac{\eta\beta(1-\beta)P_{a}L_{au}L_{ub}S_{au}S_{ub}}{\eta\beta(1 -\beta)P_{u}L_{ub}S_{ub}S_{u}+\eta\beta(1-\beta+\xi)L_{ub}S_{ub}N_{0}+(P_{b}S _{b}+N_{0})(1-\beta+\epsilon)} \tag{13}\]  
where \(\xi=\frac{N_{\rm P}}{N_{0}}\) and \(\epsilon=\frac{N_{\rm P}}{P_{L}L_{us}S_{u}+P_{b}S_{u}+N_{0}}\). The SINR received by the drone eavesdropping node E contains the first and second phases, which are respectively \(\gamma_{E}^{(1)}\) and \(\gamma_{E}^{(2)}\):  
\[\gamma_{E}^{(1)}=\frac{P_{a}L_{ue}S_{ae}}{P_{u}L_{ue}S_{ue}+N_{0}} \tag{14}\]  
\[\gamma_{E}^{(2)}=\frac{\eta\beta(1-\beta)P_{a}L_{au}L_{ub}S_{au}S_{ub}}{\eta \beta(1-\beta)P_{u}L_{ue}S_{ue}S_{u}+\eta\beta(1-\beta+\xi)L_{ue}S_{ue}N_{0}+ (P_{b}L_{be}S_{be}+N_{0})(1-\beta+\epsilon)} \tag{15}\]  
Since there is information eavesdropping in both the first and second phases, considering the worst-case scenario, node E applies the Maximum Ratio Combining (MRC) strategy to the received two-phase signals to process the intercepted confidential messages [15], and therefore, the SINR received by node E is \(\gamma_{E}=\gamma_{E}^{(1)}+\gamma_{E}^{(2)}\). Where \(\gamma_{E}^{(1)}\) and \(\gamma_{E}^{(2)}\) are given by Equations (14) and (15).