Joint UAV Deployment and Power Allocation for Secure Space-Air-Ground Communications

Owing to their intrinsic advantages of seamless coverage and of high data rate, space-air-ground communications networks (SAGCN) are recognized as one of the emerging technologies for the future wireless communications systems. However, the broadcasting nature of wireless communications inevitably imposes security issues on SAGCN. In this paper, we consider the uplink of the full-duplex unmanned aerial vehicle (UAV)-aided three-layer SAGCN, comprising of ground Internet of Remote Things (IoRT) terminals, an unmanned aerial vehicle, and a low-earth orbit (LEO) satellite, where eavesdroppers are intercepting the information transmitted. In order to ensure a secure uplink transmission, a joint UAV deployment and power allocation scheme is conceived for maximizing the secrecy rate of the SAGCN, subject to the following constraints: i) UAV's power, ii) the UAV deployment area, and iii) the secrecy rate, which are imposed on the different layers. More explicitly, once we formulate a joint optimization problem to maximize the secrecy rate, we decouple the variables and decompose the original problem into multiple subproblems in a tractable manner. Then, we simplify the subproblems with the aid of slack variables and solve them relying on the successive convex approximation method. Following this, initialization schemes are designed to exploit the one-direction greedy method for diverse environment settings, for speeding up the convergence of the iterative algorithm proposed. Finally, simulation results reveal that the convergence can be achieved within a small number of iterations by the proposed initialization scheme, while the algorithm conceived is capable of attaining a substantial improvement of the secrecy rate for the SAGCN.