RIS

A Reconfigurable Intelligent Surface is essentially a smart mirror for radio waves. It's a flat panel covered in many small, individually tunable reflecting elements; by adjusting the phase each element imposes, the surface can steer an incoming signal in a chosen direction rather than scattering it randomly. The headline appeal is that the elements are mostly passive — there's no power amplifier or RF chain behind each one — so in principle an RIS can shape the radio environment far more cheaply and with far less power than an active relay.

The use case people reach for is the blocked link. mmWave especially struggles when a building or a body sits between the base station and the user; an RIS mounted on a nearby wall can bounce the beam around the obstacle to reach an otherwise-dead spot. It's a genuinely active research area for 5G-Advanced and 6G, but worth a dose of realism: controlling the surface needs accurate channel knowledge, the gains depend heavily on geometry, and a purely passive reflector still obeys physics on how much it can recover. Promising, not yet a deployed staple.