Some relevant parts:

"After years of designing superefficient transmitters and receivers ...[we].. are approaching the practical limits of transmitter and receiver efficiency... To get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself.

Thin two-dimensional metamaterials, known as metasurfaces, can ... tune ... wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface.

Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

That’s important, because as we move to higher frequencies, the propagation characteristics become more “hostile” to the signal.

higher-frequency signals are absorbed, reflected, or scattered

The first public demonstrations of the technology occurred in late 2018, by NTT Docomo in Japan and Metawave, of Carlsbad, Calif.

RIS functions like a very sophisticated mirror, whose orientation and curvature can be adjusted in order to focus and redirect a signal in a specific direction. But rather than physically moving or reshaping the mirror, you electronically alter its surface so that it changes key properties of the incoming electromagnetic wave, such as the phase.

The materials are fabricated using ordinary metals and electrical insulators, or dielectrics.

An RIS node is made up of hundreds or thousands of metamaterial elements called unit cells. Each cell consists of metallic and dielectric layers along with one or more switches or other tunable components.

To control the direction of the larger wave reflecting off the entire RIS, you synchronize all the unit cells to create patterns of constructive and destructive interference in the larger reflected waves "