PT-RS — Phase tracking reference signal

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Reference signals

Where it sits

Sits atLevel 5 of the hierarchy · Reference signals · explained
Learn firstResource element · Modulation — 5 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 38.211 §7.4.1.2
Used inref-signals 6 · nr-pdsch 3 · nr-frame-structure 2 · index 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. A thin column of known symbols, sparse in frequency and dense in time. TS 38.211 clause 7.4.1.2, reception in TS 38.214 clause 5.1.6.3.

The problem it solves. An oscillator's phase noise rotates the entire constellation by a slowly-varying angle — common phase error. DM-RS measures the channel once at the start of the allocation (PDSCH §7.1) and cannot see a rotation that develops afterwards.

Why it is dense in time and sparse in frequency, which is the opposite of DM-RS: phase noise is common to all subcarriers and changes fast. One subcarrier every few resource blocks is enough to measure it; but it must be measured in nearly every symbol.

Why it is configured rather than always present. Its presence and density depend on the scheduled MCS and bandwidth. At QPSK it is not needed — a few degrees of rotation moves nothing across a decision boundary. At 256QAM it is essential, which is the quantified claim in ref-modulation §4.

Why FR2 needs it more. Phase noise worsens with carrier frequency. This is one of the concrete reasons the frame-structure note gives for wider subcarrier spacing at high bands, and PT-RS is the other half of the answer.

Read on

This concept was first written up in ref-signals, 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 PT-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 Modulation
Immediately before PT-RS: Resource element, Modulation.
Keep going — where this sits on the route
◀CSI-RSstep 50 · level 5
Step 51 of 118level 5 · Reference signals
Layerstep 52 · level 5▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ 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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