Unlike LTE, 5G NR does not have cell-specific reference signals. It must configure reference signals that a device can monitor and report on. These are called CSI-RS.
The Following Figure and the below summarizes the key aspects of the CSI-RS. Various configuration options enable the CSI-RS to be sent on multiple ports with various scheduling options such as Periodic, Semi Persistent, and Aperiodic.
1- NR CS-RS can be configured with up to 32 Ports: Port 3000 to 3031
– Increasing the number of configured ports(Ex 32 Port) provides better granularity in the beamforming and offers better throughput, rank, and 256Qam but will introduce extra overhead
2- Scheduling Options: 3 Options available Periodic, Semi-Persistent or Aperioidic
– Periodic: Transmitted at fixed Nth Slots,
– Semi-Persistent: Similar to Periodic, but CSI-RS Transmission can be switched on and off using MAC CE.
– Aperiodic: Event-based; Transmission is triggered by the DCI when required
3- Time/Freq Resource Locations: This is identified through multiple parameters(Frequencydomainallocationrownumber, Density, and firstOFDMSymbokInTimeDomain) And is mapped to the Resource elements 3GPP table, which has 18 Combinations
4- CDM Types: Ranges: No CDM, FD-CDM2, CDM-4 or CDM-8
– Applicable when multiple ports share the same Resource Element allocation. In this case, code division multiplexing is used to differentiate the transmissions from each port.
– Fd-CDM2 means that CDM is used to differentiate 2 Resource Elements occupying different subcarriers.
– Cdm4-FD2-TD2 means that CDM is used to differentiate 2 REs, which occupy a grid of 2 subcarriers X 2 Symbols
– Where: CDM: Code Division Multiplexing FDM: Freq Division Multiplexing
Some of the mentioned Characteristics can be checked through the following messages:
-In SA networking, check the RRCReconfiguration message” csi-MeasConfig” and “PDSCH-CONFIG”
-In NSA networking, check the SgNB Addition Request Acknowledge