Scrambling — Whitening, and making a mis-addressed decode fail

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

Bits into a codeword

Where it sits

Sits atLevel 8 of the hierarchy · Bits into a codeword · explained
Learn firstTransport block · RNTI — 8 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 38.211 §5.2.1, §7.3.1.1
Used innr-pdsch 6 · index 1 · 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

Scrambling adds a pseudo-random sequence to every bit before modulation. It is not encryption — the sequence is public and the RNTI is sent in the clear.

Initialised with $c_{\text{init}} = n_{\text{RNTI}} \cdot 2^{15} + q \cdot 2^{14} + n_{\text{ID}}$ (TS 38.211 clause 7.3.1.1) — who, which codeword, which cell.

It does two engineering jobs. It whitens interference, so two cells sending the same pattern interfere like noise rather than coherently. And it makes a mis-addressed decode fail loudly: descrambling with the wrong RNTI yields a random sequence that fails the CRC.

Actual encryption lives in PDCP, several layers up.

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 Scrambling (level 8) you first need 8 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
6 CRC
7 RNTI
Immediately before Scrambling: Transport block, RNTI.
Keep going — where this sits on the route
◀LDPCstep 86 · level 8
Step 87 of 118level 8 · Bits into a codeword
PMIstep 88 · level 8▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ all 8 concepts at level 9 · level 8 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 · Scrambling Top · Concept map · Hub