Research on Secure and Covert Transmission for UAV-Assisted Visible Light Communication Systems
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摘要: 无人机(UAV, unmanned aerial vehicle)因自身的机动性和按需覆盖能力,可作为空中基站实现空地之间的可见光通信(VLC, visible light communication)。然而,空地之间的通信链路暴露在开放环境中,这使得VLC容易受到数据窃听和恶意检测。针对此问题,从物理层安全和隐蔽通信的角度提出了面向UAV的VLC系统中的安全隐蔽传输策略。该策略在考虑隐蔽通信要求、照明目标要求、UAV发射功率和悬停高度限制的基础上,对UAV发射功率和部署高度进行了联合优化以最大化系统的保密容量。由于所构建的优化问题高度非凸,因此设计了基于粒子群优化的双层优化算法对该问题进行求解。仿真结果表明,所提算法能够较快收敛,并且相较于基准方案能够提高系统的保密性能。Abstract:
Objective Owing to mobility and on-demand coverage capabilities, unmanned aerial vehicles (UAVs) can serve as aerial base stations to enable visible light communication (VLC). However, air-ground communication links are exposed to open environments, making VLC vulnerable to data eavesdropping and malicious detection. To address this issue, this paper proposes a secure and covert transmission strategy for a UAV-assisted VLC system from the perspectives of physical layer security and covert communication. The proposed strategy jointly optimizes the transmit power and hovering altitude of a UAV to maximize the system’s secrecy capacity, subject to the requirements of covert communication and illumination targets, as well as the operational constraints on UAV transmit power and hovering altitude. Methods This paper investigates secure and covert communication for UAV-assisted VLC. First, a UAV-assisted VLC system model is proposed, in which a mobile UAV equipped with light-emitting diodes is employed to establish a VLC link with a ground user in the presence of an eavesdropper and a warden. Subsequently, an optimization problem is formulated to maximize the secrecy capacity of the system by jointly optimizing the transmit power and hovering altitude of the UAV. To solve the formulated problem, we propose a two-layer optimization (TLOP) algorithm to decompose the transformed problem into two subproblems, including an inner-layer subproblem for transmit power optimization and an outer-layer subproblem for the design of UAV hovering altitude. On this basis, a closed-form expression for the optimal transmit power is derived for the inner-layer problem, while a particle swarm optimization (PSO) algorithm is developed to solve the outer-layer problem. Results and Discussions In the simulations, the proposed optimization scheme is compared with two baselines. First, the convergence of the proposed TLOP algorithm is verified ( Fig. 3 ). The results demonstrate that the algorithm achieves rapid convergence within a limited number of iterations. Second, the optimal hovering altitude of the UAV with respect to the UAV’s horizontal coordinates under spatial distribution is illustrated (Fig. 4 ). The results indicate that as the UAV gradually approaches the ground legitimate user, its optimal hovering altitude exhibits a downward trend. Then, the secrecy capacity with respect to the UAV’s horizontal coordinates is presented (Fig. 5 ). It can be clearly observed that the secrecy capacity shows an upward trend as the UAV approaches the ground legitimate user. This is because as the UAV gets closer to the user, the constraints imposed by secure and covert communication are gradually relaxed. Furthermore, as $ \epsilon $ increases, the secrecy capacity of all schemes exhibits an increasing trend (Fig. 6 ). This is because the relaxation of covertness requirements enables the UAV to flexibly adjust its hovering altitude and transmit power to increase the secrecy capacity of the system. In addition, the secrecy capacity shows a declining trend as the number of symbols increases (Fig. 7 ). The reason can be attributed to the fact that the increase in the number of symbols provides the warden with more signal samples available for detection. Finally, the secrecy capacity of all schemes decreases as the uncertainty region radius of malicious users increases (Fig. 8 ). This trend is explained by the increased uncertainty requires the UAV to address potential threats over a wider area, which forces the UAV to adopt a more conservative strategy. In conclusion, simulation results confirm that the proposed scheme can improve the secrecy capacity of the UAV-assisted VLC system.Conclusions This paper investigates secure and covert communication in a UAV-assisted VLC system. Our goal is to maximize the secrecy capacity of the system by jointly optimizing the transmit power and hovering altitude of a UAV. Given that the formulated problem is a highly non-convex problem, a TLOP algorithm based on PSO is designed to solve it. Simulation results demonstrate that the proposed algorithm achieves fast convergence and improves the system’s secrecy performance compared to baselines. -
1 基于PSO的TLOP算法求解$ {\mathcal{P}}_{0} $
初始化:在$ [0,L] $范围内随机初始$ S $个UAV悬停高度
$ {H}_{u}{}^{(1)},{H}_{u}{}^{(2)},\cdots,{H}_{u}{}^{(S)} $。(1) 循环 (2) 给定$ {H}_{u}{}^{(s)} $,对$ {h}_{b}{}^{2}\sigma _{e}^{2}-h_{e}^{*}{}^{2}\sigma _{b}^{2}\gtrless 0 $进行判决; (3) 基于(33)求解以获得最优解$ p_{u}^{opt} $; (4) 给定$ p_{u}^{opt} $,基于(36)获得$ {H}_{u}{}^{(s)} $对应的适应度; (5) 根据适应度更新最优位置,基于(39)和(40)更新每个粒子
的速度和位置;(6) 重复步骤(2)-步骤(5),直到收敛。 -
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