5G NR Reference Signals | DMRS,PT-RS,CSI-RS,SRS

5G NR Reference signals viz. DMRS, PT-RS, CSI-RS, SRS. These 5G NR reference signals are used in the downlink and uplink. Functions,contents and mapping of DMRS, PT-RS, CSI-RS, SRS are covered. The page also mentions tabular difference between 4G LTE SRS and 5G NR SRS.

5G NR DMRS

• DMRS refers to Demodulation Reference Signal.
• It is used by 5G NR receiver to produce channel estimates for demodulation of associated physical channel.
• The design and mapping of DMRS is specific to each of the 5G physical channels viz. PBCH, PDCCH, PDSCH, PUSCH and PUCCH.
• DMRS is UE specific which is transmitted on demand.

• DMRS is used to acquire PBCH, PDSCH etc. DMR for PBCH is spread across same bandwidth as used for PBCH (on same symbols).
• It does not extend outside of the scheduled physical resource of the channel it supports.
• DMRS supports massive multi-user MIMO.
• It can be beamformed and supports upto about 12 orthogonal layers.
• DMRS sequence for CP-OFDM version is QPSK based on Gold Sequences.

PDSCH DMRS

Front loaded DMRS symbols (either 1 or 2) are located as follows.
➤Slot based (DMRS mapping type-A): Fixed OFDM symbol regrdless of PDSCH assignment. It configurable between lo = {2,3}
➤Non-slot based (DMRS mapping type-B): First OFDM symbol assigned for PDSCH i.e. Mini-slots

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

PUSCH DMRS

In uplink two waveform types are supported viz. CP-OFDM and DFT-S-OFDM. Gold sequence is used in CP-OFDM where as Zadoff-chu is used in DFT-S-OFDM.

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

5G NR PT-RS

• PTRS stands for Phase Tracking Reference signal.
• It's main function is to track phase of the LO at transmitter and receiver.
• This will enable suppression of phase noise and common phase error specially at higher mmwave frequencies.
• It is present both in uplink (in PUSCH) and downlink (in PDSCH) channels.
• Due to phase noise properties, PTRS has low density in frequency domain and high density in time domain.
• PTRS is associated with one DMRS port during transmission. Moreover it is confined to scheduled BW and duration used for PDSCH/PUSCH.

5G NR CSI-RS

• Like LTE, it is used for DL CSI acquisition.
• It is also used for RSRP measurements used during mobility and beam management. It is also used for frequency/time tracking, demodulation and UL reciprocity based precoding.
• CSI-RS is configured specific to UE. But multiple users can also share the same resource.
• 5G NR standard allows high level of flexibility in CSI-RS configurations.
• A resource can be configured with up to 32 ports.
• CSI-RS resource may start at any OFDM symbol of the slot and it usually occupies 1/2/4 OFDM symbols depending upon configured number of ports.
• CSI-RS can be periodic, semi-persistent or aperiodic (due to DCI triggering)
• For time/frequency tracking, CSI-RS can either be periodic or aperiodic. It is transmitted in bursts of two or four symbols which are spread across one or two slots.

5G NR SRS

• SRS stands for Sounding Reference signal.
• It is used for UL channel sounding.
• In contrast to LTE, it is configured specific to UE.
• In time domain, it spans 1/2/4 consecutive symbols which are mapped within 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 is same as used in LTE.

Difference Between 4G LTE SRS And 5G NR SRS

Following pdf describes Sounding Reference Signals used in 4G LTE and 5G NR. It also mentions difference between 4G LTE SRS and 5G NR SRS.