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：智能无线电环境通过允许对信道进行控制来增强无线通信。它们已通过具有可重构电磁特性的表面（称为可重构智能表面）以及灵活天线实现，可分别视为智能无线电环境在电磁域和空间域的具体实现。然而，这些技术依赖于电子可重构或可移动组件，引入了可能阻碍商业化的实施挑战。为克服这些挑战，我们提出通过可移动信号的概念在频域这一新维度实现智能无线电环境，其中信号频谱可沿频率轴动态移动。我们首先在视距条件下分析多输入单输出系统中的可移动信号，证明其能获得比量化等增益传输更高的平均接收功率。随后研究非视距条件下的可移动信号，发现通过利用由均匀间距固定电磁特性单元构成的表面（称为固定智能表面）的反射，该技术仍保持有效性。分析结果表明，采用可移动信号的固定智能表面辅助系统，其接收功率最高可达采用固定频率信号的可重构智能表面辅助系统的四倍。