Reference Signals are for Phy layer only.
In 5G NR Reference signals are used for both downlink and uplink.
There are 4 different types of reference signals in 5G:
1. DM-RS: Demodulation Reference Signal
2. PT-RS: Phase Tracking Reference Signal
3. STS: Sounding Reference Signal
4. CSI-RS: Channel State Information Reference Signal.
1. DM-RS: Demodulation Reference Signal
> It is used for acquisition and channel estimation.
> It is used for both uplink and downlink.
> It is UE specific and is transmitted on demand.
> It is applicable for PBCH, PDCCH, PDSCH, PUCCH and PUSCH.
> DMRS is used to acquire PBCH, PDSCH etc.
2. PT-RS: Phase Tracking Reference Signal
> It is used to track phase of the LO at transmitter and receiver.
> This will help for phase tracking and phase noise compensation for PDSCH and PUSCH.
> It is used for both uplink(PUSCH) and downlink (PDSCH.
> PTRS is associated with one DMRS port during transmission.
3. SRS: Sounding Reference Signal
> It is used for UL channel estimation when PUCCH and PUSCH are not scheduled.
> It is specific to UE.
> In time domain, it spans 1/2/4 consecutive symbols which are mapped within last six symbols of the slot.
> It will always be transmitted in the last six symbols of the slot.
4. CSI-RS: Channel State Information Reference Signal.
> It is used for downlink channel measurement , acquisition of downlink channel state information, beam management, RRM / RLM measurement and refined time-frequency tracking, mobility management
> It is triggered by UE.
3GPP TS 38.211