MCS — Modulation and coding scheme — five bits, four tables

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Bits into a codeword

Where it sits

Sits atLevel 10 of the hierarchy · Bits into a codeword · explained
Learn firstModulation · Rate matching — 11 concepts in the full chain, see the paths
UnlocksTransport block size · CQI
Primary clauseTS 38.214 §5.1.3.1
Used innr-pdsch 32 · ref-core 1 · ref-modulation 1 · ref-signals 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 modulation and coding scheme is a five-bit index selecting a modulation order and a target code rate together. One field, two decisions.

Four tables (TS 38.214 clause 5.1.3.1): table 1 to 64QAM, table 2 to 256QAM, table 3 the low-spectral-efficiency one for URLLC, table 4 to 1024QAM. The same index means different things in different tables, so the table in force must be known before the index can be read.

The tables overlap deliberately — table 4's index 22 and table 2's index 27 are the same point. 1024QAM did not extend the range at the bottom; it spent codepoints at the top and gave up resolution elsewhere.

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 MCS (level 10) you first need 11 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 Modulation · Physical layer
5 Transport block
6 CRC
7 Code block
8 LDPC
9 Rate matching
Immediately before MCS: Modulation, Rate matching.
Keep going — where this sits on the route
◀5GSMstep 98 · level 9
Step 99 of 118level 10 · Bits into a codeword
HARQstep 100 · level 10▶
Level 10 → 11 · this unlocks
CQI · Transport block size
↳ all 6 concepts at level 11 · level 10 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 · MCS Top · Concept map · Hub