Link adaptation — Choosing a rate the link will actually carry

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

Procedures over time

Sits atLevel 12 of the hierarchy · Procedures over time · explained
Learn firstHARQ · CQI — 19 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 38.214 §5.1.3, §5.2.2.1
Used innr-pdsch 4 · ref-system 3
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is. Choosing $Q_m$, $R$ and rank so the transmission is as fast as the channel will bear. PDSCH §10.3 shows the gap: 3GPP defines the CQI tables and the MCS tables, and never defines the mapping between them.

Inner loop — take the reported CQI, convert it to an MCS using the table correspondence of Table 10, schedule.

Outer loop — maintain an offset applied to that conversion, decreased on every NACK and increased on every ACK, so the measured block error rate converges on the 10 % the CQI definition assumed (PDSCH §10.1).

Why the outer loop must exist. The report is stale, it describes a reference resource rather than the actual allocation, and it is the device's opinion of its own receiver. The outer loop corrects all three without needing to know which caused the error.

This is standard engineering practice rather than a 3GPP requirement, and the usual citation is given in PDSCH §15.2. Leaving the control law unspecified is what lets implementations differ.

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

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

To understand Link adaptation (level 12) you first need 19 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 HARQ · MCS
11 CQI
Immediately before Link adaptation: CQI, HARQ.
Keep going — where this sits on the route
◀PDSCHstep 111 · level 12
Step 112 of 118level 12 · Procedures over time
URLLC and eMBBstep 113 · level 12▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ all 1 concept at level 13 · level 12 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 · Link adaptation Top · Concept map · Hub