# Security Performance Analysis of Full-Duplex UAV Assisted Relay System Based on SWIPT Technology
## 2 System Models and Transmission Protocols  
### System Model  
This paper proposes a secure transmission model for a UAV full-duplex relay communication system based on an aerial eavesdropping scenario, which is shown in Figure 1, where all the nodes are fixed points and the source node is set as A, the destination node is set as B, the UAV trustworthy relay node is set as U, and the UAV eavesdropping node is set as E. Passive eavesdropping is adopted. Assuming that the source node A and the UAV eavesdropping node E are single antenna nodes and the UAV relay node U and the destination node D are multi-antenna nodes, the node U utilizes its own battery energy storage to control hovering and sends noise to the eavesdropping node. The UAV relay node U adopts the Amplify-and-Forward (AF) protocol and the power splitting-based relaying (PSR) strategy, and the full-duplex UAV relay communication process is divided into two phases. In the first phase, the UAV relay node U adopts the full-duplex mode of operation to receive signals from the source node A and send interference to the UAV eavesdropping node E while using the SWIPT technique to collect energy; in the second phase,the destination node B adopts the full-duplex mode and sends noise interference to node E while receiving the signal from node U. A three-dimensional Cartesian coordinate system is established for the system model, then the coordinates of the four nodes A, B, U and E are: \(W_{A}=(x_{A},y_{A},0)\), \(W_{B}=(x_{B},y_{B},0)\), \(W_{U}=(x_{U},y_{U},H_{1})\), \(W_{E}=(x_{E},y_{E},H_{2})\), Where \(H_{1}\) is the height of the UAV relay node U and \(H_{2}\) is the height of the UAV eavesdropping node E.  
The communication links of the UAV relay system consider both large-scale fading and small-scale fading, and because the air-ground links are in the line-of-sight propagation mode, the Rician model is used to describe the small-scale fading for all air-ground links. Considering channel reciprocity, the normalized channel power gain is expressed as: \(S_{ij}\triangleq\left|h_{ij}\right|^{2},ij\in\left\{au,ub,ue,be\right\}\). Both node U and node B operate in full duplex mode, considering that the existing self-interference cancellation techniques do not completely eliminate the self-interference signals; therefore, it is assumed that the self-interference channel fading of node U and node B are \(f_{u}\) and \(f_{B}\), which are the residual self-interference cancellation channels after the actual isolation and cancellation techniques, and the normalized channel power gains are \(S_{u}\triangleq\left|f_{u}\right|^{2}\) and \(S_{B}\triangleq\left|f_{B}\right|^{2}\), respectively. In this paper, the antenna isolation method in passive interference suppression is used. This method reduces the power of the self-interference signal by increasing the distance between the transmitting antenna and the receiving antenna, which makes the self-interference cancellation performance better. At this time, the self-interference gains can be as low as 0.235 [19]. Considering the line-of-sight link signal transmission condition, the probability density function (PDF) and cumulative distribution function (CDF) of Rician fading are expressed as:  
\[f_{ij}(x)=\left(K_{ij}+1\right)\cdot exp\left(-\left(K_{ij}+1\right)x-K_{ij} \right)\cdot I_{0}\left(2\sqrt{K_{ij}\left(K_{ij}+1\right)x}\right) \tag{1}\]  
\[F_{ij}(x)=1-Q\left(\sqrt{2K_{ij}},\sqrt{2\left(1+K_{ij}\right)x}\right) \tag{2}\]  
Figure 1: Full-duplex UAV relay communication system based on airborne UAV eavesdropping scenario. where \(ij\in\{au,ub,ue,be\}\), \(x\) is an arbitrary non-negative number, \(I_{0}(\cdot)\) denotes the first type of zero\(-\)order corrected Bessel function and \(Q(\cdot,\cdot)\) is the first type of Marcu\(-\)Q function.