An Introduction to CSI-RS

What is CSI-RS?

Channel State Information Reference Signal (CSI-RS) is a reference signal (RS) that is used in the Downlink (DL) direction in 5G NR, for the purpose of Channel Sounding and used to measure the characteristics of a radio channel so that it can use correct modulation, code rate, beam forming etc. UEs will use these reference signals to measure the quality of the DL channel and report this in the UL through the CQI Reports. gNB sends CSI Reference signals to report channel status information such as CSI-RSRP, CSI-RSRQ and CSI-SINR for mobility procedures. Specific instances of CSI reference signals can be configured for time/frequency tracking and mobility measurements.

CSI-RS in 5G NR Channel mapping
CSI-RS in 5G NR Channel mapping

What is CSI?

Channel state information (CSI) is the way of indicating certain reports by the UE to the network. These are well defined reporting parameters and comprises of:

  • Channel Quality Indicator (CQI)
  • Precoding Type Indicator (PTI)
  • Precoding Matrix Indicator (PMI)
  • Rank Indicator (RI)
  • Layer Indicator (LI)

What is the difference between CSI-RS and CRS?

CSI-RS is configured per device rather than per cell, as it was in LTE. In contrast to Cell Specific Reference signal (CRS) from LTE Release 8, LTE release 10 introduced this concept of CSI-RS with the addition of up to 8-layer spatial multiplexing where the RS are not transmitted continuously, which led to requirement of 8-layer channel estimation while only 4-layer spatial multiplexing was used in LTE Release 8. Extending CRS to 8 layers would have added more signaling overhead than desired, resulting in introduction of CSI-RS. Also, CSI-RS is transmitted on different antenna ports (15-22) than CRS and CSI-RS uses code-domain orthogonality along with time/frequency orthogonality unlike CRS which used only time/frequency orthogonality.

As 5G NR minimizes always on strategy (Except for SS Block), there are no CRS-like signals in NR, but the same concept of CSI-RS is reused and extended in NR to provide support for beam management and mobility in the connected mode.

Why SS Block is not used for channel sounding in 5G NR?

SS Block is transmitted over a limited bandwidth but with a much larger periodicity compared to LTE CRS. It can be used for power measurements to estimate path loss and channel quality but due to limited bandwidth and low duty cycle, SS Block is not very suitable for more detailed channel sounding aimed at tracking channel properties that vary rapidly in time/frequency.

What are the advantages of using CSI-RS?

  • CSI-RSs consume fewer air interface resources to perform channel measurement/feedback.
  • Different sets of CSI-RSs can be allocated to different UEs to form directional beams.

5G Base Stations can configure the UEs to use CSI-RS for:

  • Beam Management (CQI, RI, PMI Measurements): Measurements should be sent by UE to BS in order to understand and estimate the correct direction of beams.
  • Connected Mode mobility: For calculating RSRP, RSRQ, SINR
  • Radio link failure detection: To check if channel is out of sync or in sync
  • Beam failure detection and Recovery: Based on estimation of such signals, UE can be forced to perform contention free random-access attempt (when BS assigns dedicated preamble)
  • Time and Frequency Synchronization (Tracking Reference Signals)
  • Coordination and Multi Point transmission

CSI-RS Structure

A configured CSI-RS may correspond to up to 32 antenna ports, each corresponding to a channel to be sounded. Depending on the number of antenna ports, there can be 2 possible arrangements:

  1. Single-Port: A single port CSI-RS occupies a single Resource element (RE) within a block corresponding to one slot in time domain and one resource block in frequency domain
  2. Multi-Port: multiple orthogonally transmitted per-antenna-port CSI-RS, sharing the overall set of REs assigned for the configured multi-port CSI-RS. Sharing can be based on combinations of: Code domain sharing CDM (different orthogonal patterns), Frequency Domain Sharing FDM (Different subcarriers within an OFDM symbol) or Time Domain Sharing TDM (Different OFDM symbols within a slot)

Below is an example showing single-port and multi-port CSIRS:

Single Port and Two port CSI-RS structure
Single Port and Two port CSI-RS structure

Here, in case of Two-port CSI-RS Structure, we can see that how 2 adjacent RE’s in the frequency domain can be shared by means of CDM, which allows for code domain sharing between 2 per-antenna port CSIRS.

Below are the orthogonal patterns of each port:

 W0W1
1st Port+1+1
2nd Port+1-1

Note: CSI-RS can be configured to occur within an RB/slot block but to avoid collision with other downlink physical channels and signals with the same block, there are some restrictions. So, a UE/ Device can assume that the transmission of a configured CSI-RS will not collide with:

  • Any CORESET configured for the device,
  • Transmitted SS Blocks,
  • DMRS associated with the PDSCH transmissions scheduled for the device.

Taking example of 3 different structures for 8-port CSI-RS:

  • Frequency Domain CDM over 2 RE’s (2 x CDM) in combination with 4 times frequency multiplexing.
  • Frequency Domain CDM over 2 RE’s (2 x CDM) in combination with frequency and time multiplexing, consisting of 4 subcarriers with 2 OFDM symbols
  • Time and Frequency Domain CDM with 4 RE’s (4 x CDM) in combination with 2 times frequency multiplexing
8-Port CSI-RS
8-Port CSI-RS

In the last example, we will consider one of possible structures of 32 port CSI-RS based on a combination of 8 x CDM and 4 times frequency multiplexing, which also suggests that CSI-RS antennas ports in the frequency domain need not occupy consecutive subcarriers. Similarly, CSI-RS ports separated in time domain need not occupy consecutive OFDM symbols.

32-Port CSI-RS
32-Port CSI-RS

CSI-RS Characteristics

CSI-RS are used for beamforming support, but they can be configured by layer 3 to be either beam-specific or UE specific. They are always mapped onto certain resources in frequency and time domain. These signals play an important role in performing tasks such as beam acquisition and evaluation, adaptation of the beam (beam refinement), decision making for beam switching and UE tracking with steerable beams.

The network could schedule the CSI-RS as a specific RS per beam to allow them to be distinguished from one another and on the other hand, RE’s carrying the CSI-RS can be configured to be either zero power (ZP-CSI-RS) or non-zero power (NZP-CSI-RS). One objective could be to provide a configuration that contains transmission gaps so that the UE can perform interference measurements and provide feedback. In addition, it may be used for optional beamforming implementations where the zero and non-zero power concept can be used to distinguish between beams.

NZP-CSI-RS is used for most of the procedures like Channel Measurement, Beam Management, Beam measurement, connected mode mobility etc. There is a dedicated signaling from BS to UE to configure the reception of such signals.

ZP-CSI-RS are special empty resource elements, used mostly for interference measurement. It defines a set of REs which do not contain any transmission for the UE. These REs may however contain transmissions for other UE so the name ‘Zero Power’ can be misleading. The important point is that these REs puncture the PDSCH so the UE does not expect to receive any DL data within them, i.e. ZP-CSI-RS are used to configure a RE puncturing pattern for the PDSCH when some REs are allocated for other purposes.

Below figure shows a possible usage example for such a mapping of ZP-CSI-RS and NZP-CSI-RS. Assuming a beam mobility condition, the gNB here used 2 beams with identical physical layer settings such as the bandwidth part and CORESET definition as well as the CSI-RS resources. gNB has configured the CSI-RS resources in an alternating mapping such that for each CSI-RS Instance in the time domain, only one of the 2 beams would have a non-zero CSI-RS.

Example of ZP-CSI-RS and NZP-CSI-RS
Example of ZP-CSI-RS and NZP-CSI-RS

Here, UE must decide which beam is the best, means having the highest CSI-RSRP per beam. So, based on the CQI reports in the uplink direction, gNB can decide which beam to use and whether to apply a beam switching procedure.

CSI-RS Resource Sets

In addition to being configured with CSI-RS, a device can be configured with one or several CSI-RS resource sets, officially referred to as NZP-CSI-RS-ResourceSets. Each such resource set includes references to one or several configured CSI-RS. The resource set can then be used as part of report configurations describing measurements and corresponding reporting to be done by a device. Also, NZP-CSI-RS-ResourceSet may include pointers to a set of SS blocks, which suggests that some device measurements, especially measurements related to beam management and mobility, may be carried out on either CSI-RS or SS block.

CSI-RS Resource Indicator

The UE can be configured with a set of NZP-CSI-RS resources out of which it may be asked to report a subset. The identification of such NZP-CSI-RS is done by a CSI-RS RI. When a UE is configured with more than one nonzero-power CSI-RSs, it can report a set of N UE-selected CSI-RS resource-related indices. CRI can be used during Beam Management procedures when identifying the best downlink beam(s). The

CRI allow the Base Station to switch between CSI Reference Signal beams which are typically more directional than SS/PBCH beams. This is a very useful indicator as it can quickly point to the N best CSI-RS resources the network can use further.

CSI Related Measurements

CSI-RSRP

CSI Reference Signal Received Power measurements are used for connected mode mobility, power control calculations, and beam management. Measurements can be generated and reported at both layer I and layer 3. For example, a UE can provide CSI-RSRP measurements at Layer I when sending CSI to the BS. Alternatively, a UE can provide CSI-RSRP measurements at Layer 3 when sending an RRC Measurement Report. CSI-RSRP represents the average power received from a single RE allocated to the CSI-RS. Measurements are filtered at Layer 1 lo help remove the impact of noise and to improve measurement accuracy.

CSI-RSRQ

CSI Reference Signal Received Quality measurements can be used for mobility procedures. In contrast to RSRP measurements, RSRQ measurements are not used when reporting CSI. CSI-RSRQ is defined as:

CSI-RSRQ = CSI-RSRP / (RSSI / N)

where N is the number of Resource Blocks across which the Received Signal Strength Indicator (RSSI) is measured, i.e. RSSI / N defines the RSSI per Resource Block. The RSSI represents the total received power from all sources including interference and noise. The RSRP and RSSl are both measured across the same set of Resource Blocks. The RSSI is measured during symbols which contain CSI RS REs.

CSI-SINR

CSl-RS Signal to interference and Noise Ratio measurements can be used for connected mode mobility

procedures. The CSI-SINR represents the ratio of the wanted signal power to the interference plus noise power. Both the wanted signal power and the interference plus noise power are measured from REs used by the CSI-RS.

Properties of Different CSI Configurations

Frequency Domain Property

A CSI-RS is configured for a given DL Bandwidth Part (BWP) and is then assumed to be confined within that BWP and use the numerology associated with that BWP. It can be configured to cover the full BW of the BWP or just a fraction of the BW. In the latter case, the CSI-RS bandwidth and frequency-domain starting position are provided as part of the CSI-RS configuration. Within the configured CSI-RS bandwidth, a CSI-RS may be configured for transmission in every resource block, referred to as CSI-RS density equal to one.

However, a CSI-RS may also be configured for transmission only in every alternate resource block, referred to as CSI-RS density equal to 1/2. In the latter case, the CSI-RS configuration includes information about the set of resource blocks (odd resource blocks or even resource blocks) within which the CSI-RS will be transmitted. CSI-RS density equal to 1/2 is not supported for CSI-RS with 4, 8, and 12 antenna ports.

There is also a possibility to configure a single-port CSI-RS with a density of 3 in which case the CSI-RS occupies three subcarriers within each resource block. This CSI-RS structure is used as part of a so-called Tracking Reference signal (TRS)

Time Domain Property

The per-resource-block CSI-RS structure mentioned above describes the structure of a CSI-RS transmission, assuming the CSI-RS is transmitted in a particular slot. In general, a CSI-RS can be configured for aperiodic, periodic, or semi-persistent transmission.

In the case of aperiodic CSI-RS transmission, no periodicity is configured. Rather, a device is explicitly informed (“triggered”) about each CSI-RS transmission instant by means of signaling in the DCI.

Example of CSI-RS Transmission
Example of CSI-RS Transmission

In the case of periodic CSI-RS transmission, a device can assume that a configured CSI-RS transmission occurs every Nth slot, where N ranges from as low as 4, that is, CSI-RS transmissions every 4th slot, to as high as 640, that is, CSI-RS transmission only every 640th  slot. In addition to the periodicity, the

device is also configured with a specific slot offset for the CSI-RS transmission. It is based on RRC measurement.

In the case of semi-persistent CSI-RS transmission, a certain CSI-RS periodicity and corresponding slot offset are configured in the same way as for periodic CSI-RS transmission. However, actual CSI-RS transmission can be activated/ deactivated based on MAC control elements (MAC CE). Once the CSI-RS transmission has been activated, the device can assume that the CSI-RS transmission will continue according to the configured periodicity until it is explicitly deactivated. Similarly, once the CSI-RS transmission has been deactivated, the device can assume that there will be no CSI-RS transmissions according to the configuration until it is explicitly re-activated. It is also based on DCI signaling.

This periodic, semi-persistent or aperiodic transmission is not a property of the CSI-RS itself but rather the property of a CSI-RS resource set. So, activation/deactivation and triggering of semi-persistent and aperiodic CSI-RS, respectively, is not done for a specific CSI-RS but for the set of CSI-RS within a resource set.

Note: All CSI-RS within a semi-persistent resource set are jointly activated/ deactivated by means of a MAC CE command. Likewise, transmission of all CSI-RS within an aperiodic resource set is jointly triggered by means of DCI.

This article was just an introduction. I will come up later with an article related to CSI Measurement and reporting in detail.

References:

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