RIS, Active RIS or RDARS: A Comparative Insight Through the Lens of Energy Efficiency

TL;DR

This paper compares RIS, active RIS, and RDARS in terms of energy efficiency and coverage in sub-6GHz and mmWave systems, highlighting the conditions under which each approach is most effective.

Contribution

It provides a comprehensive comparison of RIS, active RIS, and RDARS, analyzing their energy efficiency and coverage based on placement and element count in different frequency bands.

Findings

RDARS is highly energy-efficient in sub-6GHz coverage.

Active RIS is more energy-efficient in mmWave systems.

RIS is more energy-efficient with fewer elements and near UEs.

Abstract

Multiplicative fading is a major limitation of reconfigurable intelligent surfaces (RIS), restricting their effective coverage in both existing sub-6GHz systems and future mmWave networks. Although active RIS architectures mitigate this issue, they require high power consumption and introduce practical challenges due to the need for integrated amplifiers. Recently, reconfigurable distributed antenna and reflecting surfaces (RDARS) have been proposed to alleviate multiplicative fading through connected modes. In this work, we compare RIS, active RIS, and RDARS in terms of coverage and energy efficiency (EE) in both sub-6GHz and mmWave bands, and we investigate the impact of placement and the number of elements of reconfigurable surface (RS) on EE and coverage. The simulation results show that RDARS offers a highly energy-efficient alternative of enhancing coverage in sub-6GHz systems, while active RIS is significantly more energy-efficient in mmWave systems. Additionally, for a lower number of RS elements and for near UEs, RIS remains considerably more energy-efficient than both active RIS and RDARS.