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

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. Simulation results demonstrate that the proposed multi-subarray FD-RIS can significantly enhance the performance of far-field communication networks by leveraging unique distance-angle beamforming. Furthermore, to achieve same performance gains, the FD-RIS-assisted system can substantially reduce the required number of RIS elements, number of antennas, and power budget, than the conventional RIS-assisted schemes. The proposed algorithm also demonstrates a notably superiority in performance and computational complexity compared with the baseline algorithms such as semi-definite relaxation (SDR) and zero-forcing (ZF).