CSI — Channel state information — what the device reports back
Space — layers, ports, reporting
Where it sits
What it is
What it is. The whole apparatus by which a device measures the downlink and reports it. TS 38.214 clause 5.2 — around 130 pages, the largest single subject in that specification.
Its structure is two configurations joined by a trigger:
Resource settings (clause 5.2.1.2) — what to measure: which CSI-RS resources for the channel, which CSI-IM or NZP resources for the interference.
Reporting settings (clause 5.2.1.1) — what to say about it: which quantities
(reportQuantity — CQI, PMI, RI, L1-RSRP and their combinations), how often, on which uplink
channel, and with what codebook.
Triggering (clause 5.2.1.5) — when: periodic, semi-persistent, or aperiodic on a DCI trigger. The same three-way choice, for the same battery reason, as the SRS resource types in SRS §4.1.
CSI processing criteria (clause 5.2.1.6) — how many reports a device can compute at once. A CSI computation is real work; the standard bounds how much can be demanded in parallel.
The report itself is a small set of numbers, and the three that matter for PDSCH are RI, PMI and CQI — how many streams, how to aim them, and how fast they can go. They are computed together and are not independent: CQI is conditioned on the reported RI and PMI, because the achievable rate depends on how the transmission will be formed.
Read on
This concept was first written up in ref-mimo, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: