Two key impediments to reconfigurable intelligent surface (RIS)-aided vehicular communications are, respectively, the double fading experienced by the signal on RIS-aided cascaded links and the high-mobility-induced intractability of acquiring channel state information (CSI). To overcome these challenges, a novel kind of RIS is presented in this paper, namely active reconfigurable intelligent omni-surface (RIOS), each element of which is supported by active loads, that concurrently transmits and reflects the incident signal amplified rather than just reflecting it as compared to the case of a passive reflecting-only RIS. We consider the use of an active RIOS to a vehicular communication system for mitigating double fading effect. Specifically, the active RIOS is mounted on the vehicle window to enhance transmission for users in the vehicle and for adjacent vehicles. We aim to jointly optimize the transmit precoding matrix at the base station (BS) and RIOS coefficient matrices to minimize the BS’s transmit power relying exclusively upon the imperfect knowledge of the large-scale CSI. To significantly relax the frequency of channel information updates, initially an efficient transmission protocol is put forward to reap the high active RIOS beamforming gain with low channel training overhead by appropriately tailoring the time-scale of CSI acquisition. Then, two algorithms, namely an alternating optimization (AO)-based algorithm and a constrained stochastic successive convex approximation (CSSCA)-based algorithm, are developed to tackle with the investigated resource allocation problem, whose pros and cons are elaborated, respectively. Simulation results substantiate the significant performance improvement of active RIOS as well as determine the validity and robustness of our proposed algorithms over various benchmark schemes.

中文翻译：

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用于车辆通信中主动可重构智能全表面的鲁棒波束成形


可重构智能表面（RIS）辅助车辆通信的两个主要障碍分别是RIS辅助级联链路上信号所经历的双衰落以及高移动性导致的获取信道状态信息（CSI）的困难性。为了克服这些挑战，本文提出了一种新型RIS，即有源可重构智能全向表面（RIOS），其每个元件均由有源负载支持，可以同时传输和反射放大的入射信号，而不仅仅是反射与仅被动反射 RIS 的情况相比。我们考虑在车辆通信系统中使用有源 RIOS 来减轻双衰落效应。具体来说，有源RIOS安装在车窗上，以增强车内用户和邻近车辆的传输。我们的目标是联合优化基站（BS）处的发射预编码矩阵和RIOS系数矩阵，以最小化仅依赖于大规模CSI的不完善知识的BS的发射功率。为了显着放宽信道信息更新的频率，最初提出了一种有效的传输协议，通过适当调整 CSI 获取的时间尺度，以低信道训练开销获得高主动 RIOS 波束成形增益。然后，开发了两种算法，即基于交替优化（AO）的算法和基于约束随机逐次凸逼近（CSSCA）的算法来解决所研究的资源分配问题，并分别阐述了它们的优缺点。 仿真结果证实了主动 RIOS 的显着性能改进，并确定了我们提出的算法相对于各种基准方案的有效性和鲁棒性。