Modern Base Station Architecture: Enabling Passive Beamforming with Beyond Diagonal RISs

TL;DR

This paper proposes a novel base station architecture integrating beyond diagonal RISs for passive beamforming in mmWave systems, offering a cost-effective, energy-efficient alternative with performance comparable to active arrays.

Contribution

It introduces a new BS architecture with BD-RIS, utilizing Takagi's decomposition for passive beamforming, and demonstrates its robustness and superiority over traditional RIS configurations.

Findings

BD-RIS enables passive beamforming with stable performance.

BD-RIS outperforms traditional diagonal RIS in large-scale configurations.

Simulation results show BD-RIS matches active array performance.

Abstract

Beamforming plays a crucial role in millimeter wave (mmWave) communication systems to mitigate the severe attenuation inherent to this spectrum. However, the use of large active antenna arrays in conventional architectures often results in high implementation costs and excessive power consumption, limiting their practicality. As an alternative, deploying large arrays at transceivers using passive devices, such as reconfigurable intelligent surfaces (RISs), offers a more cost-effective and energy-efficient solution. In this paper, we investigate a promising base station (BS) architecture that integrates a beyond diagonal RIS (BD-RIS) within the BS to enable passive beamforming. By utilizing Takagi's decomposition and leveraging the effective beamforming vector, the RIS profile can be designed to enable passive beamforming directed toward the target. Through the beamforming analysis, we reveal that BD-RIS provides robust beamforming performance across various system configurations, whereas the traditional diagonal RIS (D-RIS) exhibits instability with increasing RIS size and decreasing BS-RIS separation-two critical factors in optimizing RIS-assisted systems. Comprehensive computer simulation results across various aspects validate the superiority of the proposed BS-integrated BD-RIS over conventional D-RIS architectures, showcasing performance comparable to active analog beamforming antenna arrays.