Enabling Smart Radio Environments in the Frequency Domain With Movable Signals

TL;DR

The paper presents movable signals as a novel frequency-domain mechanism to enable smart radio environments, shifting the transmitted signal's center frequency to optimize the channel without reconfigurable hardware. In LoS, a fixed precoder with optimized frequency can achieve the maximum power $P_R^{\star}=P_TN$, with optimal wavelength $\lambda^*(\theta)=d_A|\sin\theta|$ and frequency $f^*(\theta)=f_A/|\sin\theta|$, yielding coverage that grows with the available spectral width $W$. In NLoS, movable signals combined with a fixed intelligent surface (FIS) can reach up to $P_R^{\star}=4P_TN^2$ by aligning the direct and reflected paths through frequency adjustment, while offering a much simpler hardware profile than RIS. A practical transmission protocol enables spectrum-driven optimization without explicit channel knowledge, and simulations show notable gains over quantized-frequency and RIS baselines, with performance improving as $W$ increases. The results suggest frequency-domain SREs are a viable, hardware-light path to enhanced wireless performance, motivating future work on multi-user scenarios, offline parameter design, and joint-domain optimization.

Abstract

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.

Figures (15)

• Figure 1: Two-ray radio environment.
• Figure 2:
• Figure 3: LoS communication between a multi-antenna transmitter and a single-antenna receiver.
• Figure 4: Optimal frequency $f^\star$ normalized by $f_A=c/d_A$.
• Figure 5: Multi-antenna transmitter using movable signals with frequency range width $W=1.8$, having $\theta^+=34^\circ$ according to \ref{['eq:t+los']}. (a) The region within coverage is highlighted in yellow. (b) The radiation pattern is shown for different frequencies $f$.