# Resource Allocation of UAV-Assisted IoT Node Secure Communication System
###### Abstract  
To balance the information security and energy harvest for massive internet-of-things (IoT) devices, an unmanned aerial vehicle (UAV)-assisted secure communication model is proposed in this paper. We extend the secure transmission model with physical layer security (PLS) to simultaneous wireless information and power transfer (SWIPT) technology and optimize the UAV trajectory, transmission power, and power splitting ratio (PSR). The nonconvex object function is decomposed into three subproblems. Then a robust iterative suboptimal algorithm based on the block coordinate descent (BCD) method is proposed to solve the subproblems. Numerical simulation results are provided to show the effectiveness of the proposed method. These results clearly illustrate that our resource allocation schemes surpass baseline schemes in terms of both transmit power and ratio of harvesting energy, while maintaining an approximately instantaneous secrecy rate.  
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Nowadays, many papers start to investigate the combination of the UAV-assisted communication, SWIPT and PLS. In [9], two collaborating UAVs with SWIPT technology are considered, one UAV transmits confidential information to the destination node, and the other sends jamming signals to prevent the other nodes from eavesdropping. By jointly optimizing the transmission power and trajectory of UAVs, and power splitting ratio (PSR), researchers formulate an average secrecy rate maximization problem. However, it is complex to safely transmit information to a node by two UAVs, and the system only cares about the confidentiality of one node by two UAVs. In order to avoid the inevitable eavesdroppers, in [10], the authors study the problem of secrecy rate maximum by jointly optimizing the trajectory and transmit power of a UAV with an energy receiver and information receiver, respectively, which is difficult to realize due to the limited size of IoT devices.  
Though a lot of papers have studied UAV-assisted wireless energy transmission [9, 11] and secure wireless communication [12] with different perspectives, UAV-assisted energy harvesting or secure wireless communication is studied independently. Therefore, a UAV-assisted secure communication system for IoT nodes is considered in this paper. There are two kinds of IoT nodes: one is the destination node, which can receive the information and energy by SWIPT, and the other is the energy node, which can only receive the energy. The information transmission security of the destination node is ensured by joint trajectory and resource allocation with a PLS technique. We extend the secure transmission model in [12] to SWIPT technology and optimize the PSR at the nodes. In order to enhance the information security [13], some restrictions are added on the information threshold and energy harvesting threshold of nodes. The challenge of the proposed model is to maximize the information security transmission capacity while balancing the energy storage capacity by optimizing the UAV trajectory, transmission power, and PSR. The object function is nonconvex due to the coupling of multiple variables. Consequently, the objective function is decomposed into three subproblems and solved by an iterative algorithm based on block coordinate descent (BCD) and successive convex approximation (SCA).  
In this article, we will introduce the UAV-assisted system model and problem formulation in Section 2. The proposed optimization algorithm, including optimizing transmit power, trajectory, PSR, etc., is presented in Section 3. Then numerical simulation results are provided in Section 4 to show the effectiveness of our proposed method. Finally, we conclude this paper in Section 5.