Secrecy Sum-Rate Maximization for Active IRS-Assisted MIMO-OFDM SWIPT System

Abstract

The propagation loss of RF signals is a significant issue in simultaneous\nwireless information and power transfer (SWIPT) systems. Additionally, ensuring\ninformation security is crucial due to the broadcasting nature of wireless\nchannels. To address these challenges, we exploit the potential of active\nintelligent reflecting surface (IRS) in a multiple-input and multiple-output\n(MIMO) orthogonal frequency division multiplexing (OFDM) SWIPT system. The\nactive IRS provides better beamforming gain than the passive IRS, reducing the\n"double-fading" effect. Moreover, the noise introduced at the active IRS can be\nused as artificial noise (AN) to jam eavesdroppers. This paper formulates a\nsecrecy sum-rate maximization problem related to precoding matrices, power\nsplitting (PS) ratios, and the IRS matrix. Since the problem is highly\nnon-convex, we propose a block coordinate descent (BCD)-based algorithm to find\na sub-optimal solution. Moreover, we develop a heuristic algorithm based on the\nzero-forcing precoding scheme to reduce computational complexity. Simulation\nresults show that the active IRS achieves a higher secrecy sum rate than the\npassive and non-IRS systems, especially when the transmit power is low or the\ndirect link is blocked. Moreover, increasing the power budget at the active IRS\ncan significantly improve the secrecy sum rate.\n