NR - Channel State Information Reference Signal (CSI-RS)

Introduction

In NR system, channel state information reference signal (CSI-RS) plays a variety of roles, including

CSI measurement
Beam management
Tracking
Mobility management
Rate match

Generally, there are two types of CSI-RS, termed non-zero-power (NZP) CSI-RS and zero-power (ZP) CSI-RS, respectively. The former is configured by .

Sequence Generation

CSI-RS symbols are a sequence of QPSK-modulated Gold sequence, which is initialized with \[\begin{aligned} c_{\text{init}} = \left[ 2^{10}(N_{\text{symb}}^{\text{slot}}n_{s,f}^{\mu} + \ell + 1)(2n_{\text{ID}}+1) + n_{\text{ID}} \right] \mod 2^{31} \end{aligned}\] where

$N_{\text{symb}}^{\text{slot}}$ is the number of symbols in a slot.
$n_{s,f}^{\mu}$ is the index of the slot within a frame.
$\ell$ is the index of the symbol within a slot.
$n_{\text{ID}}$ is configured by .

Resource Mapping

Parameters

Regarding the resource mapping of CSI-RS, following parameters are involved.

Density $\rho$ is configured by .
Port number $X$ is configured by .
Power offset $\beta_{\text{CSIRS}}$ is configured by .
CDM type is configured by .
Time-domain location
- Slot-level: period and offset are indicated by .
- Symbol-level: $\ell_0 \in \left\{0, 1, \ldots, 13\right\}$ and $\ell_1 \in \left\{2, 3, \ldots, 12\right\}$ are respectively configured by .
Frequency-domain location
- RB-level: the starting position and number are configured by
  - , or
  - .
- Subcarrier-level: indicated by a bitmap configured by .

Patterns

In NR systems, 1/2/4/8/12/16/24/32-port CSI-RS are defined. The multi-port CSI-RS are constructed based on 2/4-port CSI-RS using frequency domain (FD) and time domain (TD) orthogonal cover code (OCC). The coordinate of a CSI-RS resource in the resource grid is indicated by a bitmap.

A single-port CSI-RS resource occupies one RE.
A 2-port CSI-RS resource occupies 2 x 1 RE's.
A 4-port CSI-RS resource occupies 4 x 1 or 2 x 2 RE's.

1-port CSI-RS

In case of $\rho = 3$, there are 4 candidates available, as shown in Figure fig:pat-1port; Otherwise, i.e., $\rho = 1$ or $0.5$, there are $C_{12}^1$ candidates for the subcarrier index.

./fig/nr_csirs_1port.png — 1-port CSI-RS ($\rho = 3$)

2-port CSI-RS

For 2-port CSI-RS, as shown in Figure fig:pat-2port, there are $C_6^1$ candidates.

./fig/nr_csirs_2port.png — 2-port CSI-RS

4-port CSI-RS

As shown in Figure fig:pat-4port, there are $C_3^1$ and $C_6^1$ candidates for single-symbol and double-symbol 4-port CSI-RS, respectively.

./fig/nr_csirs_4port.png — 4-port CSI-RS

8-port CSI-RS

As shown in Figure fig:pat-8port, there are $C_6^4$ and $C_6^2$ candidates for single-symbol and double-symbol 8-port CSI-RS, respectively.

./fig/nr_csirs_8port.png — 8-port CSI-RS

12-port CSI-RS

As shown in Figure fig:pat-12port, for single-symbol case, there is only one candidate; while for double-symbol case, there are $C_6^3$ candidates.

./fig/nr_csirs_12port.png — 12-port CSI-RS

16-port CSI-RS

The pattern of 16-port CSI-RS can be shown in Figure fig:pat-16port. There are $C_6^4$ candidates.

./fig/nr_csirs_16port.png — 16-port CSI-RS

24-port CSI-RS

For 24-port CSI-RS, there are $C_6^3$ candidates, as shown in Figure fig:pat-24port.

./fig/nr_csirs_24port.png — 24-port CSI-RS

32-port CSI-RS

For 32-port CSI-RS, there are $C_6^4$ candidates, as shown in Figure fig:pat-32port.

./fig/nr_csirs_32port.png — 32-port CSI-RS

Functionality

CSI-RS for CSI Measurement

Framework

Just as its name indicates, CSI-RS can be used to measure CSI, e.g., CQI, PMI, CRI, SS/PBCH block resource indicator (SSBRI), layer indicator (LI), RI, L1-RSRP.

A UE can be configured with

$N \ge 1$ , in which
- The time domain behavior is indicated by .
- CSI-related or L1-RSRP-related quantities are indicated by ;
- Reporting granularity in the frequency domain is indicated by .
- Time domain restriction for channel and interference measurements (IM) are indicated by , respectively.
- Codebook-related configurations are indicated by .
$M \ge 1$ , in which
- $S \ge 1$ CSI resource sets are indicated by .
- The time domain behavior is indicated by have the same time domain behavior.
- The NZP CSI-RS resource(s) for channel/interference measurement and CSI-IM resource(s) for IM corresponding to one .
One or two lists of trigger states (.

NZP CSI-RS

For each , with each consisting of $K \ge 1$ NZP CSI-RS resource(s).

All the CSI-RS resource(s) except the NZP CSI-RS resources used for IM within one set are configured with the same .
All the CSI-RS resource(s) within one set are expected to configured with the same .

CSI-IM

A UE can be configured with one or more CSI-IM resource set(s) by .

CSI-RS for Tracking

In NR system, a single-port CSI-RS is used as tracking reference signal (TRS), targetting for time/frequency tracking.

Periodic TRS is essentially a periodic CSI-RS resource set, which is configured by high layer signalling.
Aperiodic TRS has the same structure as periodic TRS. The difference lies that it is triggered by downlink control information (DCI).

CSI-RS for Rate Match

In NR system, zero-power (ZP) CSI-RS can be used for rate match, which is exclusive for PDSCH transmission. There are 3 types of ZP-CSI-RS, periodic, semi-persistent, and aperiodic.