CQI — Channel quality indicator — a statement about a hypothetical PDSCH

5G Systems Notes · Concept map · CQI Route · Hierarchy · Index · All concepts · Hub

Space — layers, ports, reporting

Where it sits

Sits atLevel 11 of the hierarchy · Space — layers, ports, reporting · explained
Learn firstCSI · MCS — 17 concepts in the full chain, see the paths
UnlocksLink adaptation · URLLC and eMBB
Primary clauseTS 38.214 §5.2.2.1
Used innr-pdsch 22 · index 2 · ref-mimo 1 · ref-session 1 · ref-system 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

The channel quality indicator is not a signal-to-noise ratio. TS 38.214 clause 5.2.2.1 defines it as a statement about a hypothetical PDSCH.

The device reports "the highest CQI index whose PDSCH transport block could be received with a transport block error probability not exceeding 0.1" — or 0.00001 with CQI table 3.

The 0.1 is the most consequential constant in the system. The link is deliberately run at a point where one transmission in ten fails, because a link that never failed would be running far below what the channel can carry. That is why HARQ exists — it is not a safety net, it is a load-bearing part of the rate selection.

Read on

The full treatment is in nr-pdsch — this page is the definition and the map reference; that note is where the subject is actually developed.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand CQI (level 11) you first need 17 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 Subcarrier
1 OFDM
2 OFDM symbol
3 Resource element
4 Antenna port · Modulation · Physical layer
5 CSI-RS · Layer · Transport block
6 CRC · Precoding
7 CSI · Code block
8 LDPC
9 Rate matching
10 MCS
Immediately before CQI: CSI, MCS.
Keep going — where this sits on the route
◀Transport block sizestep 106 · level 11
Step 107 of 118level 11 · Space
5QIstep 108 · level 11▶
Level 11 → 12 · this unlocks
URLLC and eMBB · Link adaptation
↳ all 7 concepts at level 12 · level 11 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
5G Systems Notes · Concept map · CQI Top · Concept map · Hub