Multi-Subarray FD-RIS Enhanced Multi-user Wireless Networks: With Joint Distance-Angle Beamforming

TL;DR

The paper addresses how to boost spectral efficiency in multi-user wireless networks by enabling joint distance-angle beamforming via a multi-subarray FD-RIS. It develops a signal-processing model, formulates a nonconvex weighted sum-rate optimization, and proposes a low-complexity alternating algorithm that combines MMSE transformation, Lagrange multipliers, Riemannian CG, and GCMMA to jointly optimize active beamforming, time delays, and modulation frequencies. Numerical results demonstrate significant performance gains over conventional RIS, particularly when users share angular directions, and confirm convergence and computational practicality. The work advances FD-RIS capabilities for robust, two-dimensional spatial control and harmonic management in next-generation networks.

Abstract

The concept of the frequency diverse reconfigurable intelligent surface (FD-RIS) technology has been introduced, which can enable simultaneous implementation of distance-angle beamforming in far-field communication scenarios. In order to improve the managing ability on undesired harmonic signals and the diversity of frequency offsets, this paper presents a novel multi-subarray FD-RIS framework. In this framework, the RIS is evenly divided into multiple subarrays, each employing a distinct time-modulation frequency to enable the diversity of frequency offsets. Additionally, to suppress the undesired harmonic signals, a new time-modulation technique is employed to periodically adjust the phase-shift of each element. Based on the proposed multi-subarray FD-RIS, the signal processing model is first analytically derived. To evaluate the effectiveness of the proposed multi-subarray FD-RIS, we integrate it into a multi-user communication scenario and formulate an optimization problem that aims to maximize the weighted sum rate of all users. This is achieved by jointly optimizing the active beamforming, time delays, and modulation frequencies. Subsequently, a novel iterative algorithm is proposed to effectively solve this problem with low computing complexity. To evaluate the beamforming capability of the proposed multi-subarray FD-RIS,three communication scenarios with varying spatial correlations among users are considered. Simulation results demonstrate that the proposed multi-subarray FD-RIS can significantly enhance and sustain the performance of communication networks by leveraging unique distance-angle beamforming, even when users share the same angular position where traditional RIS experiences severe degradation.

Figures (10)

• Figure 1: Optimal normalized beampattern considering $(\hat{d}, \theta, \varphi)=(70m, 50^\circ, 40^\circ)$, $f_l=0.2+20(l-1)/(L-1)$ MHz, $l\in\mathcal{L}$: (a) Conventional RIS; (b) FD-RIS.
• Figure 2: The multi-user wireless network supported by the multi-subarray FD-RIS.
• Figure 3: Schematic illustration of how the conservative factor $\rho$ adjusts the conservativeness of the MMA convex approximation.
• Figure 4: Three communication scenarios with various spatial correlations among users.
• Figure 5: The convergence performance of the proposed algorithm taking account of different $L$, $N_\mathrm{t}$ and $P_\mathrm{tmax}$.

Theorems & Definitions (1)

• proof