Sign in Subscribe

NR-PDSCH DMRS (NR Physical Downlink Shared Channel Demodulation Reference Signal)

Last updated on 


The NR-PDSCH DMRS, or New Radio Physical Downlink Shared Channel Demodulation Reference Signal, is a fundamental element in 5G New Radio (NR) that assists user equipment (UE) or devices in demodulating and decoding the data transmitted on the Physical Downlink Shared Channel (PDSCH). This signal is essential for reliable communication in the downlink direction, allowing the UE to accurately extract the data sent by the base station (gNB or gNodeB). Let's delve into the technical details of NR-PDSCH DMRS:

Purpose of NR-PDSCH DMRS:

  • Demodulation and Channel Estimation: The primary purpose of NR-PDSCH DMRS is to aid the UE in demodulating the data carried on the PDSCH. It provides a known reference signal that the UE can use to estimate the downlink channel conditions.
  • MIMO Spatial Multiplexing: In MIMO (Multiple-Input, Multiple-Output) communication, the NR-PDSCH DMRS plays a crucial role in spatial multiplexing. It helps the UE disentangle the signals transmitted from different antennas at the base station.

Positioning in Time and Frequency:

  • NR-PDSCH DMRS is positioned within the resource elements of the PDSCH. The exact location of the DMRS symbols within the PDSCH varies depending on the NR numerology and the configuration of the transmission.
  • It is important that the DMRS symbols are placed in such a way that they can be easily identified by the UE.

Sequence Generation:

  • The DMRS sequence is generated based on a predefined pattern, known to both the transmitter (base station) and the receiver (UE). This pattern is determined by higher-layer signaling and follows standardized procedures.
  • The NR-PDSCH DMRS may vary in terms of the sequence length and the number of symbols, depending on the specific NR configuration and channel conditions.

Modulation and Mapping:

  • The DMRS symbols are modulated using specific modulation schemes, typically QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), depending on the NR configuration and modulation scheme used for data.
  • The modulated DMRS symbols are then mapped onto the resource elements within the PDSCH.

Transmission Scheme:

The NR-PDSCH DMRS can be transmitted in various schemes, such as:

  • Time-Domain DMRS: The DMRS symbols are spread across different time slots within a subframe.
  • Frequency-Domain DMRS: The DMRS symbols are distributed across different frequency resources within a subcarrier spacing.
  • Beamformed DMRS: In scenarios involving beamforming or MIMO, multiple DMRS sequences may be transmitted to represent different beams or antenna ports.

Channel Estimation:

  • Upon receiving the DMRS symbols, the UE uses them to estimate the channel conditions for each antenna port or beam.
  • The channel estimates are crucial for coherent demodulation and decoding of the PDSCH data symbols, especially in MIMO scenarios.

DMRS Configuration and Reporting:

  • The configuration of NR-PDSCH DMRS, including the type, length, and positioning, is communicated by the network to the UE through higher-layer signaling and system information.
  • The UE reports its channel estimation results back to the network, which can be used for adaptive transmission schemes and beamforming adjustments.

In summary, the NR-PDSCH DMRS is a vital reference signal in 5G NR downlink communication. It assists UEs in demodulating and decoding the data carried on the Physical Downlink Shared Channel (PDSCH) by providing known reference symbols. These symbols are positioned within the PDSCH resource elements, modulated according to standardized procedures, and used for channel estimation, especially in MIMO and beamforming scenarios. The configuration and positioning of the NR-PDSCH DMRS are determined by system parameters and are essential for achieving reliable and high-throughput downlink communication in 5G networks.