Improving Security of THz Communication with Intelligent Reflecting Surface

Intelligent Reflecting Surface (IRS)-assisted beamforming is an emerging paradigm proposed to enhanced signal coverage and multiplexing in terahertz (THz) communications with low hardware cost and reduced power consumption. This paper investigates the beamformer and IRS design to enhance security of downlink THz communication in multi-input single-output (MISO) wiretap channel, where an Alice-Bob pair wishes to communicate secretly with the assist of IRS, in the presence of Eve. Our goal is to maximize the secrecy rate by designing the active beamformer at the base station (BS) and the passive reflecting phase shifters (PSs) at the IRS, which turns out to be an intractable mixed integer non-convex optimization problem. To tackle it, we respectively propose a low-computational- complexity successive design and a high-secure- performance joint design, which both attain high- quality suboptimal solutions. Specifically, in the successive design, we first design the PSs under an introduced principle, followed by that the beamformer is optimally derived, both in closed form. In the joint design, an alternating manner is proposed to optimize the PSs and the beamformer one by one with each other being fixed in each iteration until both reach the convergence. Numerical results show that the proposed designs achieve better secrecy performance than existing IRS-assisted design and traditional non-IRS- assisted secure design.