  Reconfigurable intelligent surfaces (RISs) have emerged as a cost- and
energy-efficient technology that can customize and program the physical
propagation environment by reflecting radio waves in preferred directions.
However, the purely passive reflection of RISs not only limits the end-to-end
channel beamforming gains, but also hinders the acquisition of accurate channel
state information for the phase control at RISs. In this paper, we provide an
overview of a hybrid relay-reflecting intelligent surface (HR-RIS)
architecture, in which only a few elements are active and connected to power
amplifiers and radio frequency chains. The introduction of a small number of
active elements enables a remarkable system performance improvement which can
also compensate for losses due to hardware impairments such as the deployment
of limited-resolution phase shifters. Particularly, the active processing
facilitates efficient channel estimation and localization at HR-RISs. We
present two practical architectures for HR-RISs, namely, fixed and dynamic
HR-RISs, and discuss their applications to beamforming, channel estimation, and
localization. The benefits, key challenges, and future research directions for
HR-RIS-aided communications are also highlighted. Numerical results for an
exemplary deployment scenario show that HR-RISs with only four active elements
can attain up to 42.8 percent and 41.8 percent improvement in spectral
efficiency and energy efficiency, respectively, compared with conventional
RISs.
