CRC — Cyclic redundancy check — the verdict, not the correction

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Bits into a codeword

Where it sits

Sits atLevel 6 of the hierarchy · Bits into a codeword · explained
Learn firstTransport block — 6 concepts in the full chain, see the paths
UnlocksPolar coding · Code block · RNTI · HARQ
Primary clauseTS 38.212 §5.1, §7.2.1
Used innr-pdsch 21 · ref-channels 1 · ref-modulation 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 cyclic redundancy check is the verdict, not the correction. It decides whether a decoded block is delivered upward or a retransmission is requested.

Two lengths, chosen by size (TS 38.212 clause 7.2.1): 24 parity bits with $g_{\text{CRC24A}}$ when the payload exceeds 3824 bits, 16 bits with $g_{\text{CRC16}}$ otherwise. On a 100 000-bit block 24 bits is 0.024 %; on a 200-bit block it is 12 %, so the standard buys back four points where the percentage hurts.

A second CRC appears per code block when a block is segmented, which is what makes partial retransmission possible.

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 CRC (level 6) you first need 6 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 Physical layer
5 Transport block
Immediately before CRC: Transport block.
Keep going — where this sits on the route
◀Zero-power CSI-RSstep 64 · level 6
Step 65 of 118level 6 · Bits into a codeword
Precodingstep 66 · level 6▶
Level 6 → 7 · this unlocks
RNTI · Code block · Polar coding · HARQ level 10
↳ all 13 concepts at level 7 · level 6 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 · CRC Top · Concept map · Hub