5G NR Reference Signals (DMRS, PTRS, SRS and CSI-RS)

Last updated on Dec 28, 2023

5G New Radio (NR) introduces several types of reference signals that play crucial roles in various functionalities, including channel estimation, synchronization, and feedback. Let's delve into each of them:

1. DMRS (Demodulation Reference Signals):

Purpose:
The primary role of DMRS is to assist in demodulating and decoding the transmitted data symbols at the receiver side. DMRS provides a known reference signal that the receiver uses to estimate the channel's state.

Structure and Details:

Time and Frequency Domain: DMRS can be present in both time and frequency domains. In the time domain, it's typically inserted between data symbols in the time domain, aiding in symbol demodulation. In the frequency domain, it occupies specific Resource Element (RE) positions.
Mapping: DMRS symbols are mapped to specific resource elements within a Resource Block (RB). The structure depends on the configuration, such as localized or distributed mapping.
Configurations: Different configurations like Type 1 (localized), Type 2 (distributed), and Type 3 (interleaved) DMRS can be used depending on the deployment scenario and requirements.

2. PTRS (Phase Tracking Reference Signals):

Purpose:
PTRS is introduced to provide phase tracking and assist in maintaining phase coherence, especially in scenarios with high mobility or high Doppler shifts.

Structure and Details:

Phase Tracking: In environments with significant phase shifts due to mobility, PTRS helps in maintaining phase coherence by aiding in the phase tracking process.
Deployment: PTRS can be deployed in certain symbol positions within the slot, offering phase tracking benefits while not occupying as many resources as DMRS.

3. SRS (Sounding Reference Signals):

Purpose:
SRS is used for uplink channel sounding, allowing the network to estimate the uplink channel quality and derive necessary parameters for uplink transmission.

Structure and Details:

Periodic Transmission: Devices send SRS periodically based on certain configurations. This helps the base station (gNodeB) estimate the uplink channel condition and adjust its transmission parameters accordingly.
Frequency Hopping: SRS can use frequency hopping to avoid interference and provide better channel estimation over a wider bandwidth.

4. CSI-RS (Channel State Information Reference Signals):

Purpose:
CSI-RS is utilized to obtain Channel State Information (CSI) at the receiver. CSI provides insights into the channel's characteristics, enabling efficient beamforming, MIMO transmission, and other advanced techniques.

Structure and Details:

Beamforming and MIMO: CSI-RS aids in beamforming by allowing the receiver to determine the best beamforming weights based on the channel's state. It's crucial for maximizing the system's spectral efficiency and capacity in MIMO deployments.
Configurations: CSI-RS can be configured in various ways, such as using different antenna ports, configurations, and patterns, depending on the deployment scenario and requirements.

Conclusion:

Each of these reference signals in 5G NR serves specific purposes and plays critical roles in ensuring reliable, efficient, and high-performance communication. They facilitate functionalities like channel estimation, synchronization, adaptive modulation and coding, beamforming, and more, contributing to the overall performance and capabilities of 5G networks.