Abstract
In this paper, we introduce an intelligent reflecting surface (IRS) to provide a programmable wireless environment for physical layer security. By adjusting the reflecting coefficients, the IRS can change the attenuation and scattering of the incident electromagnetic wave so that it can propagate in a desired way toward the intended receiver. Specifically, we consider a downlink multiple-input single-output (MISO) broadcast system where the base station (BS) transmits independent data streams to multiple legitimate receivers and keeps them secret from multiple eavesdroppers. By jointly optimizing the beamformers at the BS and reflecting coefficients at the IRS, we formulate a minimum-secrecy-rate maximization problem under various practical constraints on the reflecting coefficients. The constraints capture the scenarios of both continuous and discrete reflecting coefficients of the reflecting elements. Due to the non-convexity of the formulated problem, we propose an efficient algorithm based on the alternating optimization and the path-following algorithm to solve it in an iterative manner. Besides, we show that the proposed algorithm can converge to a local (global) optimum. Furthermore, we develop two suboptimal algorithms with some forms of closed-form solutions to reduce the computational complexity. Finally, the simulation results validate the advantages of the introduced IRS and the effectiveness of the proposed algorithms
Highlights
A variety of wireless technologies have been proposed to enhance the spectrum- and energy-efficiency due to the tremendous growth in the number of communication devices, such as multiple-input multiple-output (MIMO) [1], cooperative communications [2], cognitive radio (CR) [3] and so on
It is intractable to guarantee the secret communications with the use of beamforming only at the transceivers
SIMULATION RESULTS we present simulation results to validate the advantages of using the intelligent reflecting surface (IRS) to improve the secret communication of the downlink multiple-input single-output (MISO) broadcast system
Summary
A variety of wireless technologies have been proposed to enhance the spectrum- and energy-efficiency due to the tremendous growth in the number of communication devices, such as multiple-input multiple-output (MIMO) [1], cooperative communications [2], cognitive radio (CR) [3] and so on. By smartly adjusting the reflecting coefficients with a preprogrammed controller, the IRS can change the attenuation and scattering of the incident electromagnetic wave so that it can propagate in the desired way before reaching the intended receiver, which is called as programmable and controllable wireless environment This inspires us to design the communication systems by jointly considering the signal. MIMO beamforming techniques were further introduced to improve the secrecy capacity (improving SNR of legitimate receivers and suppressing SNR of eavesdroppers) [29]–[32] Both power minimization and secrecy rate maximization were studied in [30] in a single user/eavesdropper MIMO systems considering both perfect and imperfect CSI. For the first case with one legitimate receiver and one eavesdropper, we develop an alternating optimization method to solve the formulated problem in an iterative manner, but in each iteration we provide the closed-form solutions, which leads the algorithm to be low complexity.
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