Reference signals Part-3

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In the previous section, we learn about the DMRS, TRS, SRS reference signals. in this section, we will learn the remaining reference signals:-

PTRS:


What is PTRS?

Now next is PTRS, this reference signal is introduced in NR to enable compensation of oscillator phase noise. Usually, phase noise increases as a function of the oscillator carrier frequency. Thus PTRS can be utilized at high carrier frequencies such as mm wave to reduce phase noise.

In an OFDM signal, one of the main degradations caused by phase noise is an identical phase rotation of all the subcarriers, called as common phase error or CPE. it is designed so as to have high density in the time domain  and  low density in the frequency domain, as the phase rotation produced by common phase error is identical for all subcarriers within an OFDM symbols, but there is low correlation of phase noise across OFDM symbols.

This RS is UE specific, confide in a scheduled resource and can be beam formed. Also, the number of PTRS [ports can be lower than the total number of ports. The orthogonality between PTRS ports is achieved by means of FDM.

Base on which parameter network (operator) configures PTRS?

PTRS is configurable depending on the carrier frequency, OFDM subcarrier spacing, quality of the oscillators and modulation and coding schemes used for transmission. PTRS can be seen as an extension to DMRS for PDSCH or PUSCH and are intended for phase noise compensation as motioned earlier.

What is PTRS functionality?

It is used for Phase tracking for PDSCH in NR and phase noise compensation.

For which NR channel, PTRS used?

PUSCH and PDSCH


CSI:


What is CSI-RS functionality?

It is used for Beam Management and CSI acquisition in NR.

What is CSI-RS (channel state information reference signal)?

CSI-RS are downlink reference signals intended to be used by UEs to acquire downlink CSI. Specific instances of CSI RS can be configured for mobility measurements and frequency/time tracking.

CSIRS is used for downlink channel state information acquisition, same as LTE. It is also used for RSRP measurements used during beam management and mobility. It is also used for demodulation and UL reciprocity based precoding.

CSI-RS is configured specific to device and multiple users (devices) can also share the same resource. NR standard allows high level of flexibility in CSIRS configurations. CSIRS resource may start at any OFDM symbol of the slot and it usually occupies OFDM symbols (1/2/4) and it depends on the number of antenna ports configured (note: a resource can be configured with up to 32 ports). CSI-RS can be aperiodic, periodic, semipersistent (DCI triggering) for frequency or time tracking. CSIRS is transmitted in bursts of four or two symbols which are spread across one or two slots.

Now, as mentioned above there is no CRS in NR, but why?


Now as we know that there is cell specific reference signal (CRS) in LTE. This CRS is used for downlink channel estimation and this RS (reference signal) is eliminated in 5G. As per the provisions the 36 series protocols, the CRS supports the configuration of 1,2 and 4 antenna ports, and the physical cell IDs of neighboring cells must ensure that the modulo 6 values are different to map the CRS in different positions in frequency domain. 

In more convenient way, for NR ultra-dense networking, CRS will bring great co-frequency interference. For 5G, there are far more than four antenna ports available. Also, at the same time, DMRS will be used for channel estimation, in order to unify the uplink and downlink. In NR DMRS is mapped as per the layers. 

The DMRS on every layer is in a different position and successive operations like precoding are performed with the data. At the reception end when performing channel estimation and demodulation, channel estimation is also performed on the layer. That is from the start of layer mapping to the receiving end as a broad channel.


In the next section, we will learn more 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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