Free tool · 3GPP TS 38.211 §7.4.1.1

DM-RS patterns, pixel-exact

Pick configuration type, duration and additional positions — the PRB grid redraws with every DM-RS resource element in its true place, colored by CDM group, with port mapping and overhead live.

DM-RS configuration

PDSCH mapping type A · dmrs-TypeA-Position = pos2 · single-symbol · TS 38.211 Table 7.4.1.1.2-3

One PRB × one slot — subcarrier 11 on top

Under the hood

Why the pattern looks like that

Type 1 is a comb-2: k = 4n + 2k′ + Δ. CDM group 0 (Δ=0) owns the even subcarriers, group 1 (Δ=1) the odd ones — 6 REs per group per PRB, supporting 4 ports single-symbol (1000–1003) or 8 double-symbol. Dense in frequency ⇒ robust channel estimates for high delay spread.

Type 2 packs pairs: k = 6n + k′ + Δ with Δ ∈ {0,2,4} — three CDM groups of 2+2 adjacent REs, 4 REs per group per PRB, up to 6 ports single-symbol (1000–1005) or 12 double. Less overhead per port, better for MU-MIMO, at the price of sparser frequency sampling.

Additional positions repeat the front-loaded symbol later in the slot — pos1/2/3 add time samples so the estimator can track the channel at speed. At 500 Hz Doppler the channel decorrelates within a slot; without additional DM-RS the last symbols would be demodulated against a stale estimate.

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FAQ

Common questions

What is DMRS in 5G NR?
The demodulation reference signal is the pilot the receiver uses to estimate the radio channel for coherently demodulating PDSCH/PUSCH. Unlike LTE’s always-on cell-specific reference signals, NR DM-RS is UE-specific, beamformed and transmitted only inside scheduled allocations.
Type 1 vs Type 2 DMRS — when is each used?
Type 1 (comb-2, 6 REs per CDM group per PRB) gives denser frequency sampling — the default and the only type allowed for initial access. Type 2 (three 2+2 blocks, 4 REs per group) supports more orthogonal ports (up to 12) with less overhead per port, suiting MU-MIMO at moderate delay spread.
What does dmrs-AdditionalPosition do?
It adds 1–3 extra DM-RS symbols after the front-loaded one (the exact symbols come from TS 38.211 Table 7.4.1.1.2-3 based on duration). More positions track faster channels — high-speed users need pos2/pos3 — at the cost of PDSCH REs.
What is a CDM group?
Ports sharing the same REs, separated by orthogonal cover codes in frequency (FD-OCC) and optionally time (TD-OCC). Type 1 has 2 CDM groups, type 2 has 3 — the visualizer colors each group so you can see which REs each port set occupies.
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