Active RIS vs. Passive RIS: Which Will Prevail in 6G?

By Zijian Zhang, Linglong Dai, Xibi Chen, Changhao Liu, Fan Yang, Robert Schober, H. Vincent Poor. IEEE Transactions on Communications, 2021

1,138 citations recorded by the source corpus.

As a revolutionary paradigm for controlling wireless channels, reconfigurable intelligent surfaces (RISs) have emerged as a candidate technology for future 6G networks. However, due to the “multiplicative fading” effect, the existing passive RISs only achieve limited capacity gains in many scenarios with strong direct links. In this paper, the concept of active RISs is proposed to overcome this fundamental limitation. Unlike passive RISs that reflect signals without amplification, active RISs can amplify the reflected signals via amplifiers integrated into their elements. To characterize the signal amplification and incorporate the noise introduced by the active components, we develop and verify the signal model of active RISs through the experimental measurements based on a fabricated active RIS element. Based on the verified signal model, we further analyze the asymptotic performance of active RISs to reveal the substantial capacity gain they provide for wireless communications. Finally, we formulate the sum-rate maximization problem for an active RIS aided multi-user multiple-input single-output (MU-MISO) system and a joint transmit beamforming and reflect precoding scheme is proposed to solve this problem. Simulation results show that, in a typical wireless system, passive RISs can realize only a limited sum-rate gain of 22%, while active RISs can achieve a significant sum-rate gain of 130%, thus overcoming the “multiplicative fading” effect.

Subfields: Aerospace Engineering, Electrical and Electronic Engineering

View Paper: https://www.semanticscholar.org/paper/06db70510148991c98fbc4bb71613ecd4d1a2b16

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Citations in the corpus

An Overview of Signal Processing Techniques for RIS/IRS-Aided Wireless Systems (2021, neutral): “After integrating active reflection-type amplifiers [258], in addition to adjusting the phase shifts, active RISs can simultaneously amplify the magnitude of the reflected signals.”
Reconfigurable Intelligent Surfaces for Wireless Communications: Overview of Hardware Designs, Channel Models, and Estimation Techniques (2022, background): “analyses [35]–[37], physical layer security [38]–[40], nonorthogonal multiple access schemes (NOMA) [41]–[43], noncoherent modulation [44], [45], vehicular networks [46], [47], hybrid passive and sensing designs [48]–[50], active and joint reflective-transmissive designs [51], [52], deep learning so”
A Joint Precoding Framework for Wideband Reconfigurable Intelligent Surface-Aided Cell-Free Network (2020, background): “Besides, some new hardware architecture such as active RIS [46] can be considered to further enhance the capacity.”
Active RIS Versus Passive RIS: Which is Superior With the Same Power Budget? (2021, methodology): “To tackle this challenge, the active RIS equipped with the power ampliﬁcation capability has been proposed and investigated in [7]–[13].”