5G NR Reference Signals: DMRS, PT-RS, CSI-RS, and SRS Explained

This article discusses 5G NR reference signals, including Demodulation Reference Signal (DMRS), Phase Tracking Reference Signal (PT-RS), Channel State Information Reference Signal (CSI-RS), and Sounding Reference Signal (SRS). These signals play crucial roles in both downlink and uplink communication. We’ll cover their functions, contents, and mapping. A table comparing 4G LTE SRS and 5G NR SRS is also provided.

5G NR DMRS

• DMRS stands for Demodulation Reference Signal.
• The 5G NR receiver uses it to estimate the channel for demodulating associated physical channels.
• The design and mapping of DMRS are specific to each 5G physical channel: PBCH, PDCCH, PDSCH, PUSCH, and PUCCH.
• DMRS is UE-specific and transmitted on demand.
• It’s used to acquire PBCH, PDSCH, and other channels.
• DMRS for PBCH is spread across the same bandwidth used for PBCH (on the same symbols).
• DMRS does not extend beyond the scheduled physical resource of the channel it supports.
• Supports massive multi-user MIMO.
• It can be beamformed and supports up to approximately 12 orthogonal layers.
• The DMRS sequence for the CP-OFDM version is QPSK-based, using Gold Sequences.

PDSCH DMRS

Front-loaded DMRS symbols (either 1 or 2) are positioned as follows:

• Slot-based (DMRS mapping type-A): Fixed OFDM symbol regardless of PDSCH assignment. Configurable between $l_0 = \{2, 3\}$.
• Non-slot-based (DMRS mapping type-B): The first OFDM symbol assigned for PDSCH (i.e., Mini-slots).

Additional DMRS symbols can be configured for high-speed scenarios and are always present for broadcast/multicast PDSCH.

PUSCH DMRS

Uplink supports two waveform types: CP-OFDM and DFT-S-OFDM.

• Gold sequences are used in CP-OFDM.
• Zadoff-Chu sequences are used in DFT-S-OFDM.

Front-loaded DMRS symbols (either 1 or 2) are located at the first OFDM symbol assigned for PUSCH.

5G NR PT-RS

• PTRS stands for Phase Tracking Reference Signal.
• Its primary function is to track the phase of the local oscillator (LO) at the transmitter and receiver.
• This enables suppression of phase noise and common phase errors, especially at higher mmWave frequencies.
• It is present in both uplink (PUSCH) and downlink (PDSCH) channels.
• Due to phase noise characteristics, PTRS has low density in the frequency domain and high density in the time domain.
• PTRS is associated with one DMRS port during transmission and is confined to the scheduled bandwidth and duration used for PDSCH/PUSCH.

5G NR CSI-RS

• Like LTE, it’s used for downlink CSI acquisition.
• It’s also used for Reference Signal Received Power (RSRP) measurements during mobility and beam management.
• Additionally, it’s used for frequency/time tracking, demodulation, and uplink reciprocity-based precoding.
• CSI-RS is configured specifically for each UE, but multiple users can share the same resource.
• The 5G NR standard allows for high flexibility in CSI-RS configurations.
• A resource can be configured with up to 32 ports.
• CSI-RS resources can start at any OFDM symbol of the slot and typically occupy 1, 2, or 4 OFDM symbols, depending on the configured number of ports.
• CSI-RS can be periodic, semi-persistent, or aperiodic (triggered by Downlink Control Information (DCI)).
• For time/frequency tracking, CSI-RS can be either periodic or aperiodic, transmitted in bursts of two or four symbols spread across one or two slots.

5G NR SRS

• SRS stands for Sounding Reference Signal.
• It’s used for UL channel sounding.
• In contrast to LTE, it’s configured specifically for each UE.
• In the time domain, it spans 1, 2, or 4 consecutive symbols mapped within the last six symbols of the slot.
• Multiple SRS symbols allow coverage extension and increased sounding capacity.
• The design of SRS and its frequency hopping mechanism are the same as used in LTE.

Difference between 4G LTE SRS and 5G NR SRS

The following content describes Sounding Reference Signals used in 4G LTE and 5G NR, highlighting the differences between them.