CSI-RS explanation

According to the protocol, the number of ports in a CSI-RS resource can be a single port or multi-port. A maximum of 32 ports can be configured. A concept of CDM is used during multi-port mapping, that is, multiple CSI-RS ports may be distinguished and mapped on a same time-frequency resource in a CDM manner. How did that happen?

According to 3GPP specifications, there are four CDM types of CSI-RSs. The CDM types can be determined based on the parameter cdm-Type in the RRC. {noCDM, fd-CDM2, cdm4-FD2-TD2, cdm8-FD2-TD4} indicates that the CSI-RSs are mapped to only one RE without code division. The other three CDM types are as follows:

FD-CDM2: multiplexing of two ports on two REs of two carriers in the frequency domain and one symbol in the time domain

CDM4-FD2-TD2: multiplexing four ports on four REs of two carriers in the frequency domain and two symbols in the time domain

CDM8-FD2-TD4: multiplexing eight ports on eight REs with two carriers in the frequency domain and four symbols in the time domain

Using CDM2 as an example, how to multiplex two ports on two REs? The key lies in CDM code division. Actually, the two CSI-RS ports use mutually orthogonal sequences on the two REs, so that the two ports can be distinguished.

You also can see the post:

https://forum.huawei.com/enterprise/en/5g-nr-system-csi-rs-signal/thread/733957-100305

Hello,

CSI-RS port is a specific combination of time-frequency resources, such as a symbol in the time domain, a RE in the frequency domain as a port, consisting of 1, 2, 4 basic ports, and the number of other ports as a combination of basic ports.

For example, a set of 8 ports of CSI-RS refers to the number of downlink beams, and 8 ports will be mapped in 64 RF channels of AAU, that is, one port will map 8 AAU channels. Since existing 5G terminals support 2 transmits and 4 receivers, only 4 ports can be distinguished. During testing, the port of the access terminal can be selected by setting phase shifter attenuation.You can also set the phase shifter Angle so that the terminal automatically selects the port.

Generally one time (single symbol) frequency (single RE) position can represent two ports, which are separated by orthogonal code.

CSI-RS signal is a downlink reference signal. After receiving CSI-RS, the terminal will feedback the uplink CSI, and the base station will select the appropriate codebook according to the CSI for downlink service transmission.

CSI-RS is an important reference signal for monitoring channel quality. There are various ports in 5G NR. A deep understanding of port can help you better grasp the data transmission.

I hope my understanding of this will help you.

:D