Reference Signals — DMRS, CSI-RS, SRS & PT-RS

#1Why Reference Signals Exist

In wireless communication, the transmitted signal is distorted by the channel — multipath fading, Doppler shift, phase noise, and interference. The receiver must estimate and compensate for these effects to recover the data.

  Transmitted Signal                    Received Signal
  ─────────────────                    ────────────────

  [Data Symbols]                      [Distorted Data]
       │                                    │
       │         Channel h(t,f)             │
       └────────────────────────────────────┘
                │ multipath
                │ Doppler
                │ phase noise
                │ interference

  Problem: Receiver doesn't know h(t,f)
  Solution: Transmit known reference signals → estimate h(t,f) → equalize data

The Four 5G NR Reference Signals

#2DMRS — Demodulation Reference Signal

DMRS is the most critical reference signal. It provides the channel estimate needed to demodulate the data (PDSCH or PUSCH) in the same resource block.

Why DMRS Exists

Without DMRS, the receiver cannot know the channel response at the exact time-frequency location of the data. DMRS is co-located with the data it helps demodulate — same PRB, same slot.

  DMRS in a PRB (simplified view):

  Frequency ▲
            │
   SC 11    │ ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┐
   SC 10    │ │ D │ D │ D │ D │ D │ D │ D │ D │ D │ D │ D │ D │
    ...     │ ├───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┤
   SC  3    │ │ D │ R │ D │ D │ R │ D │ D │ R │ D │ D │ R │ D │  ← DM-RS (Type 1)
   SC  2    │ │ D │ R │ D │ D │ R │ D │ D │ R │ D │ D │ R │ D │
   SC  1    │ ├───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┤
   SC  0    │ │ C │ R │ C │ D │ D │ D │ D │ D │ D │ D │ D │ D │
            └─┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴─► Time
              Sym0 Sym1 Sym2 Sym3 Sym4 Sym5 Sym6 Sym7 Sym8 ... Sym13

  D = PDSCH data, R = DM-RS, C = PDCCH

DMRS Types

  DMRS Type 1 (comb-2):              DMRS Type 2 (comb-3):

  SC:  0  1  2  3  4  5  6  7  8  9 10 11     SC:  0  1  2  3  4  5  6  7  8  9 10 11
  Sym: R  R  .  .  R  R  .  .  R  R  .  .      Sym: R  R  .  .  .  R  R  .  .  .  R  R
       .  .  R  R  .  .  R  R  .  .  R  R           .  .  .  R  R  .  .  .  R  R  .  .

DMRS Configuration

Front-loaded DMRS: The first DMRS is placed early in the slot (symbol 2 or 3) to enable fast channel estimation and low-latency decoding.

How DMRS Works

1. Transmitter inserts known DMRS symbols at predefined positions.
2. Receiver extracts DMRS from received signal.
3. Channel estimation: h = received_DMRS / known_DMRS.
4. Interpolation: Estimate h for all data REs between DMRS positions.
5. Equalization: Data symbols divided by estimated channel.

#3CSI-RS — Channel State Information Reference Signal

CSI-RS is transmitted by the gNB to allow UEs to measure and report channel state information — CQI, PMI, and RI.

Why CSI-RS Exists

DMRS is only transmitted when data is scheduled. But the gNB needs channel measurements even when no data is being sent — to decide which UE to schedule, what MCS to use, and which beam to steer.

  CSI-RS vs DMRS:

  DMRS:  Only in scheduled PRBs, only when data is sent
  CSI-RS: Can span the entire carrier bandwidth, periodic or aperiodic

  Carrier Bandwidth (e.g., 100 MHz, 273 PRBs)
  ┌─────────────────────────────────────────────────────────────┐
  │  [PRB 0] [PRB 1] ... [PRB 272]                             │
  │   DMRS    DMRS        DMRS      ← only in scheduled PRBs    │
  │                                                             │
  │  CSI-RS across full bandwidth (periodic)                    │
  │  ● ─ ● ─ ● ─ ● ─ ● ─ ● ─ ● ─ ● ─ ... ─ ● ─ ● ─ ●        │
  └─────────────────────────────────────────────────────────────┘

CSI-RS Use Cases

CSI Reporting

#4SRS — Sounding Reference Signal

SRS is transmitted by the UE on the uplink to allow the gNB to estimate the uplink channel.

Why SRS Exists

For uplink scheduling, the gNB needs to know the uplink channel quality and spatial characteristics. SRS provides this information.

  Uplink Channel Estimation Flow:

  UE transmits SRS ──► gNB receives SRS ──► gNB estimates UL channel
                                                     │
                                                     ▼
                                          ┌─────────────────────┐
                                          │ • Select UL MCS     │
                                          │ • Allocate UL PRBs  │
                                          │ • Configure UL beam │
                                          │ • Enable UL MIMO    │
                                          └─────────────────────┘

SRS Configuration

SRS vs CSI-RS

#5PT-RS — Phase Tracking Reference Signal

PT-RS is a specialized reference signal used to track and compensate for phase noise introduced by local oscillators, especially at high frequencies (mmWave).

Why PT-RS Exists

At high carrier frequencies (e.g., 28 GHz, 39 GHz), local oscillators exhibit significant phase noise — rapid, random fluctuations in the phase of the carrier. This:

• Rotates received constellation points randomly
• Degrades higher-order modulation (256-QAM, 1024-QAM) severely
• Varies faster than the channel can be estimated by DMRS

  Phase Noise Effect on Constellation:

  Without phase noise:           With phase noise:

  Q │   •   •                    Q │    •  •
    │                           ──┼─── •  • ──
  ──┼──── I                       │  •   •
    │   •   •                    ──┼──── •  • ──► I

  Clean constellation              Phase noise blurs constellation

DMRS is typically transmitted once or twice per slot — too infrequent to track rapid phase noise variations. PT-RS fills this gap with dense, time-domain sampling of the phase.

PT-RS Configuration

When Is PT-RS Needed?

PT-RS vs DMRS

#6Reference Signal Relationships

The four reference signals work together in a coordinated system:

  Reference Signal Flow (Downlink):

  gNB                                              UE
   │                                                │
   │  ──── CSI-RS (periodic) ──────────────────►    │
   │                                                │  Measure SINR
   │  ◄──── CQI/PMI/RI report ──────────────────── │
   │                                                │
   │  ──── PDSCH + DMRS ────────────────────────►   │
   │       (scheduled based on CQI)                 │  Estimate channel
   │                                                │  Demodulate data
   │  ──── PT-RS (if μ≥2) ──────────────────────►   │
   │                                                │  Track phase noise

  Reference Signal Flow (Uplink):

  gNB                                              UE
   │                                                │
   │  ◄──── SRS (periodic) ───────────────────────  │
   │                                                │
   │  Estimate UL channel                           │
   │  Select UL MCS                                 │
   │                                                │
   │  ──── UL grant (PDCCH) ────────────────────►   │
   │                                                │
   │  ◄──── PUSCH + DMRS ─────────────────────────  │
   │  ◄──── PT-RS (if μ≥2) ──────────────────────── │

Summary: Who Does What?

#7Channel Estimation Accuracy

The quality of channel estimation depends on:

  Channel Estimation Error vs DMRS Density:

  Estimation Error
       │
  High │ ●
       │   ●
       │     ●
       │       ●
       │         ●
  Low  │           ●─────────────
       └──────────────────────────► DMRS Density
          Low    Medium    High

  More DMRS → lower estimation error, but less room for data
  Trade-off: balance estimation accuracy vs spectral efficiency

#8Practical Considerations

Common Pitfalls

Reference Signal Overhead

5G NR vs LTE Reference Signals

#9Further Reading

3GPP Specifications

• TS 38.211 — NR Physical Channels and Modulation (defines all RS patterns)
• TS 38.214 — NR Physical Layer Procedures for Data (DMRS, CSI-RS config)
• TS 38.215 — NR Physical Layer Measurements (CSI-RS procedures)

Research Papers

• "5G NR Reference Signals" — IEEE Communications Surveys & Tutorials
• "Channel Estimation in 5G New Radio" — Ericsson Technology Review
• "Phase Noise in mmWave 5G Systems" — IEEE Transactions on Communications

Podcast

Beamforming Deep Dive — Telecom Leaders Podcast

Companion Resources

• OFDM.md — OFDM, numerology, and resource grid
• Modulation.md — Modulation schemes from analog to 5G/6G QAM
• Beamforming.md — Antenna arrays, beamforming, spatial processing
• SpatialMultiplexing.md — MIMO layers, precoding, spatial multiplexing