This paper investigates the potential of reconfigurable intelligent surface (RIS) for passive information transfer in a RIS-aided multiple-input multiple-output (MIMO) system. We propose a novel simultaneous active and passive information transfer (SAPIT) scheme. In SAPIT, the transmitter (Tx) and the RIS deliver information simultaneously, where the RIS information is carried through the RIS phase shifts embedded in reflected signals. We introduce the coded modulation technique at the Tx and the RIS. The main challenge of the SAPIT scheme is to simultaneously detect the Tx signals and the RIS phase coefficients at the receiver. To address this challenge, we introduce appropriate auxiliary variables to convert the original signal model into two linear models with respect to the Tx signals and one entry-by-entry bilinear model with respect to the RIS phase coefficients. With this auxiliary signal model, we develop a message-passing-based receiver algorithm. Furthermore, we analyze the fundamental performance limit of the proposed SAPIT-MIMO transceiver. Notably, we establish the state evolution to predict the receiver performance in a large-size system. We further analyze the achievable rates of the Tx and the RIS, which provides insight into the code design for sum-rate maximization. Numerical results validate our analysis and show that the SAPIT scheme outperforms the passive beamforming counterpart in achievable sum rate of the Tx and the RIS.

翻译：本文研究了可重构智能表面在RIS辅助多输入多输出系统中实现被动信息传递的潜力。我们提出了一种新颖的同时主动与被动信息传递方案。在该方案中，发射机和RIS同时传递信息，其中RIS信息通过嵌入反射信号中的RIS相位偏移进行承载。我们在发射机和RIS端引入了编码调制技术。SAPIT方案的主要挑战在于接收端需要同时检测发射机信号和RIS相位系数。为应对这一挑战，我们引入适当的辅助变量，将原始信号模型转化为关于发射机信号的两个线性模型，以及关于RIS相位系数的逐项双线性模型。基于该辅助信号模型，我们开发了一种基于消息传递的接收机算法。此外，我们分析了所提出SAPIT-MIMO收发机的基本性能极限。值得注意的是，我们建立了状态演化过程以预测大规模系统中的接收机性能，并进一步分析了发射机和RIS的可达速率，这为最大化总速率的编码设计提供了洞见。数值结果验证了我们的分析，并表明SAPIT方案在发射机和RIS的可达总速率方面优于被动波束赋形方案。