Time-Modulated Intelligent Reflecting Surface for Waveform Security

We consider an OFDM transmitter aided by an intelligent reflecting surface (IRS) and propose a novel approach to enhance waveform security by employing time modulation (TM) on the IRS side. By controlling the periodic TM pattern of the IRS elements, the system is designed to preserve communication information towards an authorized recipient and scramble the information in all other directions. Compared with applying TM at the transmitter, the substantial beam-forming gain from the IRS compensates for the power loss due to the deactivation of radiating elements during the use of TM, and also the scrambling is realized in both azimuth and elevation directions. We introduce two modes of TM pattern control, namely, the linear and planar modes. While the linear mode is simpler to implement compared to the planar mode, it does introduce sidelobes, over which the transmitted information is not adequately scrambled. We demonstrate how the sidelobes of the linear mode can be effectively suppressed by leveraging the high diversity inherent in that mode.