Active reconfigurable intelligent surfaces (RISs) have recently been proposed to compensate for the severe multiplicative fading effect of conventional passive RIS-aided systems. Each reflecting element of active RISs is assisted by an amplifier such that the incident signal can be reflected and amplified instead of only being reflected as in passive RIS-aided systems. This work addresses the practical challenge that, on the one hand, in active RIS-aided systems the perfect individual CSI of the RIS-aided channels cannot be acquired due to the lack of signal processing power at the active RISs, but, on the other hand, this CSI is required to calculate the expected system data rate and RIS transmit power needed for transceiver design. To address this issue, we first derive closed-form expressions for the average achievable rate and the average RIS transmit power based on partial CSI of the RIS-aided channels. Then, we formulate an average achievable rate maximization problem for jointly optimizing the active beamforming at both the base station (BS) and the RIS. This problem is then tackled using the majorization--minimization (MM) algorithm framework, and, for each iteration, semi-closed-form solutions for the BS and RIS beamforming are derived based on the Karush-Kuhn-Tucker (KKT) conditions. To ensure the quality of service (QoS) of each user, we further formulate a rate outage constrained beamforming problem, which is solved using the Bernstein-Type inequality (BTI) and semidefinite relaxation (SDR) techniques. Numerical results show that the proposed algorithms can efficiently overcome the challenges imposed by imperfect CSI in active RIS-aided wireless systems.
翻译:最近有人提议积极重新校正智能表面(RIS),以弥补常规被动的RIS辅助系统产生的严重多复制性消退效应。每个反映活跃的RIS的元素都得到放大器的辅助,使事件信号能够反映和放大,而不是仅仅反映在被动的RIS辅助系统中。这项工作解决了实际挑战,一方面,在活跃的RIS辅助系统中,由于活跃的RIS缺乏信号处理能力,因此无法获得RIS辅助渠道的完美单个CSI,但另一方面,需要该CSI来计算预期的系统数据率和RIS传输感应设计所需的能量。为了解决这个问题,我们首先为平均可实现率和平均RIS传输能力制作封闭式表达方式,在活跃的RIS辅助系统中,我们制定了一个平均可实现率最大化的问题,以便联合优化在运行的RISS系统(B)和RIS的动态测试能力,然后用基S-RIS的系统质量数据交换率来计算。