5G NR Downlink Reference Signals: DM-RS, CSI-RS, TRS and PT-RS Without the Headache

The one-line version

LTE had one reference signal that did everything (the CRS) and was on all the time. NR split that job into four signals, each transmitted only when needed and each answering one question:

If you keep the question in mind, the placement and density of each signal become obvious.

The mental model: the measuring tape vs the weather station

• DM-RS is a measuring tape you send with the parcel. It goes through exactly the same precoder/beam as the data, so what the UE measures is the effective channel it needs to undo. It is useless for anyone else and for any other transmission — which is precisely why it is cheap: no data, no DM-RS.
• CSI-RS is the weather station. It is not precoded like the data (or is precoded per beam), so it tells the UE about the raw channel or a set of candidate beams, which the UE reports back so the gNB can choose the next precoder/beam.
• TRS is the clock on the wall — a stable, known signal the UE uses to keep its timing and frequency locked.
• PT-RS is a tiny level-check in the corner of the parcel: not for the channel, but for the phase wobble of the oscillators.

DM-RS for PBCH

Covered in detail in the PBCH article; the essentials:

• Every 4th subcarrier in the three PBCH symbols of the SSB (symbols 1, 2, 3), shifted by v = PCI mod 4 to avoid collisions between neighbours.
• Gold sequence seeded by PCI, the half-frame bit and the 2 or 3 LSBs of the SSB index — so decoding the DM-RS reveals the beam index.
• 144 of the 576 PBCH REs (25 %). Dense on purpose: the UE has no other channel knowledge at this point.

DM-RS for PDCCH

• Present in every REG of a PDCCH candidate: subcarriers 1, 5, 9 of each RB in each CORESET symbol (3 of 12 REs, 25 %).
• If the CORESET's precoderGranularity is allContiguousRBs, the DM-RS is present in every REG of the CORESET (not just the candidate), so the UE can estimate across the whole CORESET bandwidth.
• Gold sequence seeded by pdcch-DMRS-ScramblingID (or the PCI for CORESET#0 and when not configured). Using a configurable ID lets different CORESETs / TRPs use different sequences.
• Same precoder is assumed across a REG bundle, which is why bundle size matters for estimation quality.

DM-RS for PDSCH — the one you need to understand well

This is the most configurable reference signal in NR, because PDSCH varies so much: 1 to 8 layers, 2 to 14 symbols, static or fast-moving UEs. Every configuration knob answers one of three questions.

Question 1: How many antenna ports do I need? → Type 1 vs Type 2

DM-RS ports are multiplexed by CDM groups (a set of REs shared by ports that are separated by orthogonal cover codes).

Rule of thumb: Type 1 for quality, Type 2 for quantity. Type 1 is the default and what DCI 1_0 always assumes.

Question 2: Where in time does the first DM-RS go? → Mapping type A vs B

• Mapping type A (slot-based): the first DM-RS symbol is at symbol 2 or 3 of the slot (chosen by dmrs-TypeA-Position in the MIB), regardless of where the PDSCH starts. Designed for full-slot allocations that follow a PDCCH.
• Mapping type B (mini-slot): the first DM-RS symbol is the first symbol of the PDSCH allocation ("front-loaded"). Designed for short, low-latency transmissions anywhere in the slot: the UE can start channel estimation immediately.

The DM-RS is front-loaded in both cases in the sense that it comes as early as possible — the UE needs the estimate before it can demodulate the data symbols. LTE's CRS was spread through the subframe; NR wants low latency, so the estimate must be available early.

Question 3: How fast is the channel changing? → Additional positions and double symbols

• dmrs-AdditionalPosition ∈ {0, 1, 2, 3} adds up to three more DM-RS symbols spread across the allocation (positions from TS 38.211 Table 7.4.1.1.2-3/4, depending on allocation length). A static UE needs only the front-loaded symbol; a high-speed UE needs several so the UE can track the changing channel across the slot.
• maxLength = 2 allows a double-symbol DM-RS (two adjacent symbols) — this is what enables the 8 / 12 port counts, and also improves estimation SNR.

The sequence and the DCI

• Sequence: a Gold sequence seeded by one of two configurable scrambling IDs (scramblingID0/1), selected by the n_SCID bit in DCI 1_1 (DM-RS sequence initialisation). Two IDs let neighbouring cells or TRPs use different sequences for the same UE.
• The antenna ports field in DCI 1_1 tells the UE which DM-RS ports (hence which layers) are its own, and how many CDM groups are "without data" — i.e. reserved for other UEs' DM-RS in MU-MIMO. The UE rate-matches its data around those groups even though it does not use them. This is the mechanism that lets two UEs share the same PRBs.

Why NR does it this way (vs LTE)

LTE's CRS was cell-specific, unprecoded and always on. That has three costs NR wanted to eliminate: constant power and interference even in empty cells, no way to beamform the reference signal with the data, and dense overhead regardless of need. NR's DM-RS is UE-specific and precoded exactly like the data, so the gNB may use any precoder (non-codebook, reciprocity-based, anything) and the UE never needs to know it. This "transparent precoding" is the reason NR can beamform every channel.

CSI-RS — the flexible measurement signal

Purpose. A known, unprecoded (or per-beam precoded) signal on 1 to 32 antenna ports, so the UE can measure the channel and report CSI: CQI, PMI, RI, LI (for link adaptation and precoder choice), CRI / L1-RSRP (for beam management), and CSI-RSRP/RSRQ/SINR for L3 mobility. It also serves as the base for TRS (below) and, in zero-power form, for interference measurement and rate matching.

Configuration (TS 38.211 §7.4.1.5). A CSI-RS resource is defined by:

• Number of ports: 1, 2, 4, 8, 12, 16, 24 or 32 — Table 7.4.1.5.3-1 lists 18 "rows", each a template for how the ports' REs are placed within one RB and one slot.
• CDM type: noCDM, FD-CDM2, CDM4 (FD2-TD2), CDM8 (FD2-TD4) — how many ports share each RE group.
• Density: 1 (one RE per port per RB), 0.5 (every other RB — even or odd), or 3 (three REs per RB, only for the single-port row used by TRS).
• Frequency range: start RB and number of RBs.
• Time: which symbol(s) in the slot, and — for periodic / semi-persistent resources — a periodicity (4 to 640 slots) and slot offset.

Three transmission types:

Zero-power CSI-RS (ZP-CSI-RS) is a resource where the gNB transmits nothing. The UE uses the empty REs to measure interference (CSI-IM), and the PDSCH is rate matched around them — which is also how a gNB keeps its own PDSCH away from a neighbour's CSI-RS.

CSI-RS in beam management. A resource set can be marked repetition = on (same beam on every resource: lets the UE sweep its receive beams) or off (different beam per resource: lets the UE compare transmit beams and report the best CRI + L1-RSRP). The SSB does coarse beam selection; CSI-RS does the fine refinement.

TRS — the tracking reference signal

Purpose. NR removed the CRS that LTE UEs used to keep time and frequency synchronised while connected. The SSB is too sparse (tens of ms apart) for fine tracking. So NR defines a special CSI-RS configuration for time/frequency tracking, delay-spread and Doppler estimation — the source of QCL Type A properties.

Configuration (TS 38.214 §5.1.6.1.1). An NZP-CSI-RS resource set with trs-Info = true:

• 1 port, density 3 (three REs per RB, spaced 4 subcarriers apart) — dense in frequency for accurate delay-spread and timing estimation.
• Two symbols per slot, 4 symbols apart (e.g. symbols 4 & 8, 5 & 9 or 6 & 10), in two consecutive slots (FR1) — the 4-symbol spacing gives an unambiguous frequency-offset estimate; the slot-to-slot spacing gives fine Doppler.
• Bandwidth: the whole BWP, or 52 RBs if the BWP is wider.
• Periodicity: 10, 20, 40 or 80 ms.

A UE must be configured with a TRS before it can rely on QCL Type A for PDSCH/PDCCH — in practice every connected UE has one.

PT-RS — phase tracking

The problem. At mmWave, the oscillators in both gNB and UE have significant phase noise: the phase of the received constellation rotates within a slot, differently from symbol to symbol. DM-RS is estimated once (or a few times) per slot, so it cannot follow this rotation. For QPSK you might not care; for 64QAM/256QAM a few degrees of rotation causes errors.

The solution (TS 38.211 §7.4.1.2). A sparse reference signal on one subcarrier every K RBs, transmitted every L symbols from the first DM-RS symbol onward, precoded identically to one DM-RS port so the UE can compare it against that port's estimate and track the common phase error.

• Time density L_PT-RS ∈ {1, 2, 4} — chosen by the scheduled MCS against three RRC thresholds (ptrs-MCS1/2/3): higher MCS → denser in time.
• Frequency density K_PT-RS ∈ {2, 4} — chosen by the scheduled bandwidth against two thresholds (N_RB0, N_RB1): wider allocation → sparser in frequency (one sample per 4 RBs is enough).
• Not present at all if the MCS is below ptrs-MCS1 or the bandwidth is below N_RB0, or if not configured — so in FR1 with modest MCS you typically never see it.
• Associated with the lowest-indexed scheduled DM-RS port; downlink uses at most one PT-RS port.

Why not just denser DM-RS? DM-RS estimates the full channel per subcarrier — expensive. Phase noise is a common rotation across all subcarriers, so a single subcarrier per few RBs, sampled every symbol, is enough to track it. PT-RS is the cheapest possible way to measure that one number per symbol.

Putting the four together for one PDSCH in FR2

1. The UE has been tracking time/frequency on the TRS (periodic, 1 port, density 3).
2. It measured beams on CSI-RS and reported the best CRI; the gNB set a TCI state pointing to that CSI-RS.
3. DCI 1_1 schedules a 2-layer, 256QAM PDSCH with mapping type A: DM-RS Type 1, double-symbol, one additional position because the UE is moving.
4. Because MCS is high and the allocation is wide, PT-RS with L = 1, K = 4 rides along on one subcarrier per 4 RBs, every symbol.
5. The UE: locks timing (TRS) → points the Rx beam (QCL-D from the CSI-RS) → estimates the effective channel (DM-RS) → corrects per-symbol phase (PT-RS) → demodulates.

Common confusions

Spec pointers

Recap

Interview questions

The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.