Secrecy Energy Efficiency Maximization for UAV-Enabled Mobile Relaying

Abstract

This paper investigates the secrecy energy efficiency (SEE) maximization problem for unmanned aerial vehicle enabled mobile relaying system, where a high-mobility UAV is exploited to assist delivering the confidential information from a ground source to a legitimate ground destination with the direct link blocked, in the presence of a potential eavesdropper. We aim to maximize SEE of the UAV by jointly optimizing the communication scheduling, power allocation, and UAV trajectory over a finite time horizon. The formulated problem is a mixed-integer non-convex optimization problem that is challenging to be solved optimally. To make the problem tractable, we decompose the problem into three subproblems, and propose an efficient iterative algorithm that alternately optimizes the subproblems. In addition, two special cases are considered as benchmarks in this paper. Simulation results show that the proposed design significantly improves the SEE of the UAV, as compared to the benchmarks. Index TermsUAV communication, physical layer security, mobile relaying, secrecy energy efficiency, trajectory design. Recently, due to the high mobility, on-demand deployment/placement and line-of-sight (LoS) link, unmanned aerial vehicle (UAV) has attracted significant research interests in wireless communications [1], such as for traffic offloading, aerial BSs, mobile relaying [2]-[6], information broadcasting and data collection [7]-[10]. Compared to the traditional terrestrial communications, UAV-enabled communications have more flexible mobility and potentially reduced cost. For one thing, UAV-enabled communication systems is especially suitable to be applied for on-demand coverage or unexpected events due to the swift and flexible deployment of UAV. For another, there is more likely to have line-of-sight (LoS) link between UAV-ground link, which can significantly improve link capacity. In addition, UAV-enabled communications provide a new degree of freedom for performance enhancement by trajectory design. Generally speaking, UAV-enabled communications can best suit the communication requirement by trajectory optimization, where the UAV is subject to practical mobility constraints, such as initial/final locations, maximum speed, and maximum acceleration. However, the limited on-board energy of a UAV is one of the biggest challenges in UAV-enabled communications since a UAV needs much additional propulsion energy to maintain aloft. As a result, the authors in [11] obtained the analytical UAVs energy consumption model for fixed-wing UAVAs, which was expressed as a function with respect to UAVs speed and acceleration. Based on this, the work [6] studied the spectrum and energy efficiency maximization issues in a UAV-enable communication system, in which the UAV trajectory and transmit power are jointly optimized. In particular, the UAVs trajectory is needed to be optimized to achieve a high-rate communication with the ground nodes, while the energy consumption of the UAV is expected to be lower as much as possible [12]. One particula...

Joint Trajectory and Resource Allocation Design for Energy-Efficient Secure UAV Communication Systems

“…In the considered system, the air-to-ground channel is dominated by LoS links with a reasonable flight height and all size [28], [29]. To simplify the design in the sequel, we assume that the channel is modeled by pure LoS links as commonly adopted in the literature, e.g., [14], [15], [21], [22]. As the UAV communication channel is dominated by the LoS links 4 , the CSI between each node and each UAV can be determined by its location [21], [22], [23], [25], [26].…”

“…Optimizing jammer UAV's trajectory is an interesting but challenging work and will be considered in our future study. 3 We note that since the channel between the UAV and the ground terminals are LoS dominated [22], [23], [26], the UAVs would fly at the lowest allowable flight altitude to obtain a higher channel gain for maximizing the system energy efficiency. Thus, we consider a fixed UAVs' flight altitude of H = 100 m. 4 Based on field measurements [28], [29], the air-to-ground links between the UAVs and the ground terminals are LoS channels in rural areas when the flight altitude of a UAV is 100 meters and the length of side of the service area is 500 meters.…”

“…The formulated problem in (22) is non-convex. In general, a brute force approach may be required to obtain a globally optimal solution which is computationally intractable even for a moderate size of system.…”

“…For instance, the authors in [21] proposed a joint power allocation and trajectory design to maximize the secrecy rate in both uplink and downlink systems. In [22], secure energy efficiency maximization for UAV-based relaying systems was studied. However, both works only considered the case of single-user and the proposed designs in [21], [22] are not applicable to the case of multiple users.…”

“…In [22], secure energy efficiency maximization for UAV-based relaying systems was studied. However, both works only considered the case of single-user and the proposed designs in [21], [22] are not applicable to the case of multiple users. Besides, the availability of the eavesdropper location was assumed in [21], [22], which is generally over optimistic.…”