Section 5 - Reference Signals

Question 1

What physical signals where used in LTE?

Answer 1

CRS, CSI-RS, DMRS

Question 2

What are the new reference signals introduced in NR?

Answer 2

PTRS, TRS, SSB, SRS

Question 3

What is the CSI-RS used for?

Answer 3

• DL/UL channel estimation
• Beam Management
• Time/Frequency tracking
• Radio Link Monitoring (RLM) Procedures
• Interference Measurement

Question 4

What are the types of CSI-RS?

Answer 4

NZP used for CSI feedback and measurements, ZP used as masking tool to protect certain REs, and for interference management

Question 5

How many ports for CSI-RS are supported in NR?

Answer 5

Up to 32 ports

Question 6

How are the CSI-RS resources defined?

Answer 6

By their density(how many RE are occupied by the CSI-RS), their CDM type (sending only one or multiple), CSI RS patterin in Y, Z, and by how many symbols there are in the same slot.

Question 7

What characteristic of NR DMRS enables low latency applications?

Answer 7

The fact that it is front loaded

Question 8

What is DMRS used for?

Answer 8

Channel estiamtion and demodulation of physical channels

Question 9

Where is DMRS located with type A mapping?

Answer 9

Symbol 2 or 3 of PDSCH allocation?

Question 10

Where is DMRS located with type B mapping?

Answer 10

First symbol of PDSCH allocation (it might be a partial slot allocation, for low latency)

Question 11

How many symbols can be allocated to DMRS?

Answer 11

1 or 2

Question 12

How many additional positions in a slot can be configured for DMRS?

Answer 12

Up to 3

Question 13

What are the configuration types for DMRS (PDSCH and PUSCH)?

Answer 13

Type 1 with senser pattern (50% of the SCs in a symbol), type 2 occupying 2 out of 6 SCs in a symbol

Question 14

How many orthogonal sequences are available for each DMRS type?

Answer 14

Orthogonality can be achieved via time, frequency and code. The code components gives a set of 2 DMRS as the basic unit (two different orthogonal DMRS per RB). Time component defines single symbol or double symbol given 2 degrees of freedom. Frequency defines he locations int he frequency domain. For type 1 only two locations in frequency domain can be used. Time 1 can have 1x2x2 or 2x2x2 orthogonal signals (timefreqcode). Type 2 can have 1x3x2 or 2x3x2.

Question 15

What are the functionalities of the PTRS signal?

Answer 15

• Compensates local oscillator phase noise at high frequencies (mmWave)
• Embedded into the PDSCH or PUSCH resource allocation
• Presence and density depends on MSC and RB-allocation size
• Up to 2 PTRS ports configurable (both DL and UL) and associated with DMRS port group
• If PTRS config IE is not present in RRC configuration, default PTRS config is used (TD = 1, FD = 2)

Question 16

What are the functionalities of the SRS signal?

Answer 16

SRS is used for uplink CSI measurements, scheduling and link adaptation and/or UL beam management
- Same concept as LTE with different SRS density
- UE-specific configuration with multiple resource sets
- Transmission periodicity: Periodic, aperiodic, or semi-persistent
- 5G NR support up to 4 antenna ports
- Transmitted within last 6 symbols of a slot vs last 2 symbols in LTE
- Frequency hopping is supported

Question 17

What determines the PTRS density?

Answer 17

PTRS density depends on MCS and BW of PUSCH/PDSCH

Question 18

What are the functionalities of the TRS signal

Answer 18

Because in NR there is no CRS, TRS is used to compensate the variations in time and frequency. It is for sub6 and mmWave. It is a NZP CSI-RS resource set. A TRS burst can have 4 symbols for FR1 and 2 or 4 symbols for FR2