RI — Rank indicator — how many layers the link can carry

Space — layers, ports, reporting

Where it sits

What it is

What it is. The number of layers the device believes the channel can support. TS 38.214 clause 5.2.2.

Why rank is reported separately from quality. They answer different questions and can move in opposite directions. A device very close to the base station with a line-of-sight path has an excellent channel — high CQI — but a poorly conditioned one: all paths look alike, so a second stream would not be separable, and rank is low. A device in a scattering-rich indoor environment may have a worse average signal but a much better-conditioned channel, supporting four layers.

Rank adaptation is the scheduler acting on that. More layers divide the same power among more streams, so each is weaker; the right rank is the one maximising total rate, not the maximum available. This is the mechanism that decides whether PDSCH §11.3's eight-layer configuration is even reachable — and it is why the peak-rate formula's $v_{\text{Layers}}$ is a capability, not what happens.

Rank is also why two codewords exist. At five layers and above the streams differ enough in quality that one MCS cannot serve them all (PDSCH §6.3).

Read on

This concept was first written up in ref-mimo, which reads the whole group as one argument.