Technical Analysis – 5G NR Reference Signals

We are surrounded by signals of various kinds and our world is driven by them to a large extent. Let us take you through a detailed analysis of what 5G NR Reference Signals are, their relevance and their uses.

How are NR reference signals different from LTE Reference signals ?

• In NR, there is no Cell-Specific Reference Signal (C-RS)
• In NR, a new Reference signal i.e., PTRS (Phase-Tracking Reference Signal) has been introduced for Time/Frequency spacing
• In NR, DMRS (Demodulation Reference Signal) has been used for both downlink and uplink channel
• In NR, Reference Signals are transmitted only, when necessary, while in LTE reference signals are constantly exchanged to manage links.

What are 5G NR Reference signals and what are their functionalities :

Reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The NR specification includes several types of reference signals transmitted in different ways and is intended to be used for different purposes by a receiving device such as Cell search and initial acquisition, Downlink channel quality measurements, and Downlink channel estimation for coherent demodulation/detection at the UE. One of these is transmitted from each downlink antenna port mapped onto different physical channels. Before we see how they are mapped, let’s see how many types of them are there.

There are four main reference signals in 5G NR –

• Demodulation Reference Signal (DMRS)
• Phase-Tracking Reference Signal (PTRS)
• Sounding Reference Signal (SRS)
• Channel-State Information Reference Signal (CSI-RS)

A detailed explanation of each signal’s function and purpose:

1. Demodulation Reference Signal (DMRS)

• DMRS is used to estimate the radio channel for demodulation.
• DMRS is UE-specific, it can be beamformed or confined in a scheduled resource, and transmitted only, when necessary, both in DL and UL.
• To support multiple-layer MIMO transmission, multiple orthogonal DMRS ports can be scheduled, one for each layer.
• DMRS design and mapping are specific to each downlink and uplink NR channel viz NR-PBCH, NR-PDCCH, NR-PDSCH, NR-PUSCH, NR-PUCCH.
• For low-speed scenarios, DMRS uses low density in the time domain. However, for high-speed scenarios, the time density of DMRS is increased to track fast changes in the radio channel.

2. Phase-Tracking Reference Signal (PTRS)

The ‘phase noise’ of a transmitter increases as the frequency of operation increases. The PTRS can therefore be utilized at high carrier frequencies (such as mmWave) to mitigate phase noise.

PTRS is UE-specific, confined in a scheduled resource, can be beamformed and presents in both uplink (i.e., PUSCH) and downlink (i.e., PDSCH) channels.

PTRS is configurable depending on the quality of the oscillators, carrier frequency, OFDM subcarrier spacing, and modulation and coding schemes used for transmission.

One of the main degradations caused by phase noise in an OFDM signal is an identical phase rotation of all the subcarriers, known as common phase error (CPE). PTRS is designed so that it has a low density in the frequency domain and high density in the time domain since the phase rotation produced by CPE is identical for all subcarriers within an OFDM symbol, but there is a low correlation of phase noise across OFDM symbols.

3. Sounding Reference Signal (SRS)

SRS is used as a UL-only signal. It is transmitted by the UE to help the gNodeB obtain the channel state information (CSI) for each other.

Channel State Information describes how the NR signal propagates from the UE to the gNodeB and represents the combined effect of scattering, multipath fading, doppler and power delay with distance.

The system uses the SRS for resource scheduling, link adaptation, Massive MIMO and beam management.

SRS has a modular and flexible design to support different procedures and UE capabilities.

Multiple SRS symbols allow coverage extension and increased sounding capacity.

4. Channel-State Information Reference Signal (CSI-RS)

CSI-RS is used as a DL-only signal. The UE receives the CSI-RS signal from gNodeB to estimate the channel and reports the channel quality information back to the gNodeB.

CSI-RS is used for RSRP (Reference Signal Received Power) measurement as a DL-only signal.

It is used during mobility and beam management. Also used for frequency/time tracking and demodulation. It is configured specifically to UE. It can be periodic, semi-persistent or aperiodic (due to DCI triggering).

Note: At Expertlancing, our team of experts has expertise in 5G data channels and other 5G– related technologies. Please reach out to us for any kind of technical analysis related to 5G and telecom-related technologies.

–  Written by: Anusha & Ankit Puri