DM-RS — Demodulation reference signal — inside the allocation

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Reference signals

Where it sits

Sits atLevel 5 of the hierarchy · Reference signals · explained
Learn firstResource element · Antenna port — 5 concepts in the full chain, see the paths
UnlocksCRS · Transport block size · PDSCH
Primary clauseTS 38.211 §7.4.1.1
Used innr-pdsch 19 · ref-signals 6 · nr-frame-structure 2
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

Demodulation reference signals are known symbols placed inside the allocation so the receiver can estimate the channel it must invert.

Two configuration types (TS 38.211 clause 7.4.1.1): type 1 uses every other subcarrier with 2 CDM groups and up to 8 ports; type 2 uses adjacent pairs with 3 CDM groups and up to 12. Denser in frequency against more ports.

Position depends on the mapping type: type A counts from the start of the slot with the first DM-RS at symbol 2 or 3; type B counts from the start of the allocation, at symbol 0. dmrs-AdditionalPosition adds up to three more.

Additional positions are a Doppler setting wearing a different name — one reference symbol per slot suffices only if the channel is the same at the end of the slot as at the beginning.

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 DM-RS (level 5) you first need 5 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
Immediately before DM-RS: Resource element, Antenna port.
Keep going — where this sits on the route
◀SS/PBCH blockstep 48 · level 5
Step 49 of 118level 5 · Reference signals
CSI-RSstep 50 · level 5▶
Level 5 → 6 · this unlocks
CRS · Transport block size level 11 · PDSCH level 12
↳ all 11 concepts at level 6 · level 5 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.
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