This paper explores the use of reconfigurable intelligent surfaces (RIS) in mitigating cross-system interference in spectrum sharing and secure wireless applications. Unlike conventional RIS that can only adjust the phase of the incoming signal and essentially reflect all impinging energy, or active RIS, which also amplify the reflected signal at the cost of significantly higher complexity, noise, and power consumption, an absorptive RIS (ARIS) is considered. An ARIS can in principle modify both the phase and modulus of the impinging signal by absorbing a portion of the signal energy, providing a compromise between its conventional and active counterparts in terms of complexity, power consumption, and degrees of freedom (DoFs). We first use a toy example to illustrate the benefit of ARIS, and then we consider three applications: (1) Spectral coexistence of radar and communication systems, where a convex optimization problem is formulated to minimize the Frobenius norm of the channel matrix from the communication base station to the radar receiver; (2) Spectrum sharing in device-to-device (D2D) communications, where a max-min scheme that maximizes the worst-case signal-to-interference-plus-noise ratio (SINR) among the D2D links is developed and then solved via fractional programming; (3) The physical layer security of a downlink communication system, where the secrecy rate is maximized and the resulting nonconvex problem is solved by a fractional programming algorithm together with a sequential convex relaxation procedure. Numerical results are then presented to show the significant benefit of ARIS in these applications.

翻译：本文探讨了可重构智能表面（RIS）在频谱共享与安全无线应用中缓解跨系统干扰的潜力。与仅能调节入射信号相位且基本反射所有入射能量的传统RIS、以及虽能放大反射信号但显著增加复杂度、噪声和功耗的有源RIS不同，本文考虑采用可重构吸收智能表面（ARIS）。ARIS可通过吸收部分信号能量来同时调整入射信号的相位和幅值，在复杂度、功耗与自由度（DoFs）之间实现折中。我们首先通过一个简单示例阐明ARIS的优势，进而研究三类应用场景：（1）雷达与通信系统的频谱共存——通过构建凸优化问题以最小化通信基站至雷达接收机信道矩阵的Frobenius范数；（2）设备到设备（D2D）通信中的频谱共享——提出最大化D2D链路中最差信干噪比（SINR）的最大最小化方案，并采用分数规划求解；（3）下行通信系统的物理层安全——以最大化保密速率为目标，通过分数规划算法结合序贯凸松弛过程求解非凸问题。最后通过数值结果验证ARIS在上述应用中的显著优势。