Reference signals Part-2

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In this section, we will learn about the different reference signals. Now in order to keep the transmissions contained within a slot without having to depend on other slots and beams as well as to increase the protocol efficiency NR introduces the following reference signals:-

(QUS: what are the types of reference signals in NR (5G)?)

Demodulation reference signal (DMRS)
Sound reference signal (SRS)
Tracking reference signal (TRS)        
Phase tracking reference signal (PTRS)
Channel state information reference signal (CSI-RS)

Why are reference signal required?

RS are required for the demodulation of user data and channel estimation. The reference signal transmission frequency is adaptive in NR.

DMRS:

(QUS: What is DMRS?) & (QUS: Can we transmit DMRS in DL and UL both direction in NR?)

DMRS is used to evaluate the radio channel for demodulation. It is UE specific, confined in a scheduled resource, can be beam formed and transmitted in both UL and DL but only when necessary (when DUT is connected the mode that time only DMRS transmitted or we can say it will transmitted  on demand).

Multiple orthogonal DMRS ports can be scheduled to support multiple layer MIMO transmission, one for each layer. The orthogonality is achieved by TDM and CDM and FDM. In time domain DMRS uses low density for low speed scenarios. To support low latency uses the pattern of DMRS is front loaded as its design takes into account the early decoding requirement to support low latency uses.

However, the time density is increased for high speed cases to track fast changes in the radio channel. DMRS for PDSCH are intended for channel estimation at the device as part of coherent demodulation. They are available only in the resource blocks used for PDSCH transmission. Similarly, the DMRS for PUSCH allowes the gNB to coherently demodulate the PUSCH.

What are the downlinks Channel will use DMRS in 5G and what is functionality?

Required for PDSCH, PDCCH and PBCH demodulation.

What are the uplinks Channel will use DMRS?

PUSCH, PUCCH.

Note: in the case of LTE, for Uplink DMRS was used initially, but later 3gpp release introduced DL DMRS but it was not used. In the case of 5G, DMRS is introduced for DL and UL both.

SRS:

(QUS: What is SRS?)

SRS are the UL reference signals transmitted by the device and are used for UL channel state estimation at the base stations.
In contrast to LTE, SRS is configured specific to UE This signal is transmitted in uplink to perform CSI measurements mainly for scheduling and link adaption.

Sounding Reference Signal will also be utilized for reciprocity based pre-coder design for massive-MIMO and UL beam-management (For NR case). To support different procedures and UE capabilities, it is likely that SRS will have a modular and flexible design.

In time domain, it spans 1/2/4 consecutive symbols that are mapped within last 6 symbols of the slot. Multiple SRS symbols allow increased sounding capacity and coverage extension. The frequency hopping mechanism and the design of SRS is same as LTE.

TRS:

TRS are the sparse signals intended tp assist the device in frequency tracking and time tracking. The purpose of TRS is served by a specific CSI-RS configuration. TRS is a DL transmission which allows the UE to track frequency and time variations with a high resolution.

The Synchronization Signals allow a relatively coarse synchronization while the TRS allows fine tuning. Improved synchronization can benefit the performance of data transfer in both the UL and DL directions. The TRS can be viewed as being similar to the CRS used by LTE. 
However, the TRS generates a lower overhead by occupying a reduced percentage of Resource Elements and using only a single antenna port. 3GPP has adopted the CSI Reference Signal as a solution for the TRS. This means that the TRS is a CSI Reference Signal Resource Set with a specific configuration.

What is TRS functionality?

It is used for time tracking in NR.

TRS can be configured as periodic and aperiodic.



In the next section, we will learn about different NR reference signals. So keep reading……. And Stay tune……………
Pinal Dobariya………..






Reference:
3GPP TS 38.211,
3GPP TS 38.212,
3GPP TS 38.213,
3GPP TS 38.214



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