[Paper Review] Reflections in the Sky: Joint Trajectory and Passive Beamforming Design for Secure UAV Networks with Reconfigurable Intelligent Surface

Hui Long, Ming Chen+5 more|arXiv (Cornell University)|May 21, 2020

Advanced Wireless Communication TechnologiesEngineering28 references80 citations

TL;DR

This work studies secure energy-efficient UAV-enabled communications with an RIS, jointly optimizing UAV trajectory, RIS phase shifts, user association, and transmit power via an alternating, SCA-based algorithm. It reports notable gains over traditional AF relays.

ABSTRACT

This paper investigates the problem of secure energy efficiency maximization for a reconfigurable intelligent surface (RIS) assisted uplink wireless communication system, where an unmanned aerial vehicle (UAV) equipped with an RIS works as a mobile relay between the base station (BS) and a group of users. We focus on maximizing the secure energy efficiency of the system via jointly optimizing the UAV's trajectory, the RIS's phase shift, users' association and transmit power. To tackle this problem, we divide the original problem into three sub-problems, and propose an efficient iterative algorithm. In particular, the successive convex approximation method (SCA) is applied to solve the nonconvex UAV trajectory, the RIS's phase shift, and transmit power optimization sub-problems. We further provide two schemes to simplify the solution of phase and trajectory sub-problem. Simulation results demonstrate that the proposed algorithm converges fast, and the proposed design can enhance the secure energy efficiency by up to 38\% gains, as compared to the traditional schemes without any RIS.

Motivation & Objective

• Motivate secure communications in UAV-assisted networks with RIS to overcome obstacles and eavesdropping.
• Maximize the minimum (fair) secrecy rate while optimizing energy efficiency.
• Develop an efficient algorithm to jointly optimize UAV trajectory, RIS phase shifts, user association, and transmit power.
• Provide insights into the benefits of RIS in UAV-enabled secure networks compared to traditional relays.

Proposed method

• Model an uplink RIS-assisted system with a UAV-equipped RIS relaying between users and a base station.
• Formulate a nonconvex max-min secrecy-energy-efficiency problem and decompose into three subproblems (association, power, phase/trajectory).
• Apply successive convex approximation to nonconvex subproblems and use an alternating optimization framework.
• Derive a closed-form phase alignment for RIS elements to maximize received energy and propose two trajectory-phase optimization schemes.
• Provide an iterative Algorithm 2 with convergence guarantees and analyze computational complexity.

Experimental results

Research questions

• RQ1 How can UAV trajectory, RIS phase shifts, user association, and transmit power be jointly optimized to maximize secrecy energy efficiency?
• RQ2 What are the benefits of adding RIS in UAV-assisted secure networks versus traditional AF relays?
• RQ3 How do trajectory and phase optimization interact under practical constraints like UAV speed and energy consumption?

Key findings

• The proposed algorithm can enhance secrecy energy efficiency by up to 38% compared to schemes without RIS.
• RIS-assisted UAV can outperform traditional AF relays in secrecy energy efficiency, with gains increasing with the number of RIS elements.
• Optimal UAV trajectories form a closed loop visiting users while avoiding proximity to the eavesdropper to maintain secrecy.
• RIS provides noticeable energy-efficiency gains across various heights and pathloss conditions, with higher RIS element counts yielding larger benefits.
• Scheme I (joint phase and trajectory optimization) offers better performance than Scheme II (alternative simplifications).
• Numerical results illustrate fast convergence and clear advantages of RIS in obstructed outdoor environments.