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Reconfigurable intelligent surfaces (RISs) have emerged as a cost- andenergy-efficient technology that can customize and program the physicalpropagation environment by reflecting radio waves in preferred directions.However, the purely passive reflection of RISs not only limits the end-to-endchannel beamforming gains, but also hinders the acquisition of accurate channelstate information for the phase control at RISs. In this paper, we provide anoverview of a hybrid relay-reflecting intelligent surface (HR-RIS)architecture, in which only a few elements are active and connected to poweramplifiers and radio frequency chains. The introduction of a small number ofactive elements enables a remarkable system performance improvement which canalso compensate for losses due to hardware impairments such as the deploymentof limited-resolution phase shifters. Particularly, the active processingfacilitates efficient channel estimation and localization at HR-RISs. Wepresent two practical architectures for HR-RISs, namely, fixed and dynamicHR-RISs, and discuss their applications to beamforming, channel estimation, andlocalization. The benefits, key challenges, and future research directions forHR-RIS-aided communications are also highlighted. Numerical results for anexemplary deployment scenario show that HR-RISs with only four active elementscan attain up to 42.8 percent and 41.8 percent improvement in spectralefficiency and energy efficiency, respectively, compared with conventionalRISs.

Hybrid Relay-Reflecting Intelligent Surface-Aided Wireless Communications: Opportunities, Challenges, and Future Perspectives