Smart radio environments (SREs) enhance wireless communications by allowing control over the channel. They have been enabled through surfaces with reconfigurable electromagnetic (EM) properties, known as reconfigurable intelligent surfaces (RISs), and through flexible antennas, which can be viewed as realizations of SREs in the EM domain and space domain, respectively. However, these technologies rely on electronically reconfigurable or movable components, introducing implementation challenges that could hinder commercialization. To overcome these challenges, we propose a new domain to enable SREs, the frequency domain, through the concept of movable signals, where the signal spectrum can be dynamically moved along the frequency axis. We first analyze movable signals in multiple-input single-output (MISO) systems under line-of-sight (LoS) conditions, showing that they can achieve higher average received power than quantized equal gain transmission (EGT). We then study movable signals under non-line-of-sight (NLoS) conditions, showing that they remain effective by leveraging reflections from surfaces made of uniformly spaced elements with fixed EM properties, denoted as fixed intelligent surfaces (FISs). Analytical results reveal that a FIS-aided system using movable signals can achieve up to four times the received power of a RIS-aided system using fixed-frequency signals.

翻译：智能无线电环境（SREs）通过允许对信道进行控制来增强无线通信。目前，SREs可通过具有可重构电磁特性的表面（即可重构智能表面，RISs）和柔性天线实现，这两种技术可分别视为SREs在电磁域和空间域的实现。然而，这些技术依赖电子可重构或可移动组件，引入可能阻碍商业化的实现挑战。为克服这些挑战，我们提出一个全新域——频域，通过可移动信号概念实现SREs，即信号频谱可沿频率轴动态移动。我们首先分析视距（LoS）条件下多输入单输出（MISO）系统中的可移动信号，证明其可实现比量化等增益传输（EGT）更高的平均接收功率。随后研究非视距（NLoS）条件下的可移动信号，表明此类信号可通过利用由均匀间隔且电磁属性固定的元件构成的表面（记为固定智能表面，FISs）的反射保持有效性。分析结果表明，采用可移动信号的FIS辅助系统可实现比采用固定频率信号的RIS辅助系统高达四倍的接收功率。