Abstract
In this paper, we propose intelligent reflecting surfaces (IRS) assisted secure wireless communications with multi-input and multi-output antennas (IRS-MIMOME). The considered scenario is an access point (AP) equipped with multiple antennas communicates with a multi-antenna enabled legitimate user in the downlink at the present of an eavesdropper configured with multiple antennas. Particularly, the joint optimization of the transmit covariance matrix at the AP and the reflecting coefficients at the IRS to maximize the secrecy rate for the IRS-MIMOME system is investigated, with two different assumptions on the phase shifting capabilities at the IRS, i.e., the IRS has the continuous reflecting coefficients and the IRS has the discrete reflecting coefficients. For the former case, due to the non-convexity of the formulated problem, an alternating optimization (AO)-based algorithm is proposed, i.e., for given the reflecting coefficients at the IRS, the successive convex approximation (SCA)-based algorithm is used to solve the transmit covariance matrix optimization, while given the transmit covariance matrix at the AP, alternative optimization is used again in individually optimizing of each reflecting coefficient at the IRS with other fixed reflecting coefficients. For the individual reflecting coefficient optimization, the closed-form or an interval of the optimal solution is provided. Then, the proposed algorithm is extended to the discrete reflecting coefficient model at the IRS. Finally, some numerical simulations have been done to demonstrate that the proposed algorithm outperforms other benchmark schemes.
Highlights
Due to the broadcast nature of the wireless media, wireless communications are vulnerable to eavesdropping
In order to provide the wireless communications with sound and solid security, physical layer security based technologies, such as the artificial noise (AN), cooperative jamming (CJ) and friendly jamming, have been studied for the recent years [1], [2]. These techniques only focus on the signal processing at the transceiver to adaptive the changes of the wireless environments, but cannot eliminate the negative effects caused by the uncontrollable electromagnetic wave propagation environments [3], [4]
We discuss the joint optimization of the transmit covariance matrix at the access point (AP) and the reflecting coefficients at the intelligent reflecting surfaces (IRS) to maximize the secrecy rate for the IRS-MIMOME system, with two different assumptions on the phase shifting capabilities at the IRS, i.e., the IRS has the continuous reflecting coefficients and the IRS has the discrete reflecting coefficients
Summary
Due to the broadcast nature of the wireless media, wireless communications are vulnerable to eavesdropping. For the IRS assisted secure wireless communications, in [19], the authors studied the problem in jointly optimizing the beamforming at the transmitter and the IRS phase shifts to maximize the system secrecy rate, based on the block coordinate descent (BCD) and the MM techniques, two suboptimal algorithms were proposed to solve the resulted non-convex optimization problem for small- and large-scale IRS, respectively. In [24] and [25], for the IRS assisted MISO secure communications with AN transmission at the transmitter, an alternate optimization algorithm to jointly optimize active beamforming, AN interference vector and reflection beamforming with the goal of maximizing system secrecy rate was presented The difference between these two papers is that, [24] focused on the scenario with a single legitimate user and multiple eavesdroppers, while [25] considered the scenario with multiple legitimate users but single eavesdropper. The notation diagp‚q represents a diagonal matrix where the diagonal elements are from a vector, and r‚srepresents maxp0, ‚q
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