Foundation: a phased array without the radio
Recall how a phased array steers a beam: it feeds each antenna element the same signal with a controlled phase shift, so the wavefronts add up in one chosen direction. RIS borrows exactly this idea but for reflection. Each surface element re-radiates the wave that hits it with a tunable phase; choose the phases so the reflected wavefronts combine toward the user, and a dumb wall becomes a steerable relay — with no power amplifier and no baseband, just controllable phase.
RIS is beamforming by controlled phase, the same principle behind massive-MIMO beam steering already in 5G. If you understand why per-element phase shifts point a beam, you understand the core of RIS. The novelty is doing it on a passive reflector, not the antenna.
ranbits — Foundations (5G MIMO basics) 2025Why 6G is interested
6G's interest in the upper mid-band and sub-THz creates a coverage problem: high-frequency links break on blockage. RIS offers a low-power way to create a controllable secondary path around obstacles, extending coverage without a full relay or base station. That is its main 6G value proposition.
ericsson.com 2026-01 secondaryRIS was the subject of 3GPP study in Release 19. Whether and how it becomes a normative 6G feature — control signalling, channel estimation through a passive surface, deployment and ownership models — remains open. Present RIS as promising and unsettled, never as a delivered 6G capability.
ericsson.com 2026-01 secondaryThe honest caveats
RIS demos dazzle, but the hard problems are practical: estimating the channel through a passive surface (it has no receiver to measure with), signalling phase updates fast enough as users move, powering and siting thousands of panels, and proving the cost beats simply adding a small cell. These are why RIS sits in the candidate column, not the requirement column.
| Technology | Power consumption | Channel estimation | Deployment cost | Array gain | 6G status |
|---|---|---|---|---|---|
| Small cell / relay (active) | Full TX + processing | Direct — has receiver | High (power, backhaul) | Full TX power | foundation |
| Passive RIS | Near-zero (control only) | Indirect — no receiver (hard) | Low (panel + control link) | ∝ N² (N = element count) | candidate Rel-19 studied |
| Active RIS (semi-passive) | Low amplifiers per element | Better — has some receive chain | Medium | Higher than passive | candidate early research |
| Problem | Why it is hard | Study status |
|---|---|---|
| Channel estimation | No receiver on the surface — must estimate cascaded BS→RIS→UE channel from UE measurements only | candidate open |
| Phase control latency | Phases must update faster than channel coherence time; control link adds delay | candidate open |
| Interference management | Uncontrolled reflections can create interference for other users | candidate open |
| Deployment economics | Must beat cost of adding a small cell; unclear in dense urban scenarios | candidate open |
Where this connects
RIS exists mainly to rescue links in 6G's higher bands, it is beamforming built on the physical layer, and it leans on ML to estimate and control passive-surface channels.