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

翻译：射频信号的传播损耗是无线信息与能量同传系统中的重要问题。此外，由于无线信道的广播特性，确保信息安全至关重要。为应对这些挑战，本文在多输入多输出正交频分复用无线信息与能量同传系统中探索了主动智能反射表面的潜力。主动智能反射表面比被动智能反射表面提供更优的波束成形增益，从而减轻“双衰落”效应。此外，主动智能反射表面引入的噪声可作为人工噪声干扰窃听者。本文构建了关于预编码矩阵、功率分配比和智能反射表面矩阵的保密和速率最大化问题。由于该问题具有高度非凸性，我们提出一种基于块坐标下降的算法来求取次优解。此外，我们开发了一种基于迫零预编码方案的启发式算法以降低计算复杂度。仿真结果表明，主动智能反射表面系统相比被动智能反射表面系统及无智能反射表面系统能实现更高的保密和速率，尤其在发射功率较低或直射链路受阻时更为显著。同时，增加主动智能反射表面的功率预算可显著提升保密和速率。