What is CSI?
CSI describes the known radio propagation properties of a communication channel at a given instant — including signal strength, phase, multipath characteristics, and interference levels. Accurate CSI is the enabler of Massive MIMO, adaptive modulation, and beamforming in 5G NR.
How Does CSI Work?
The gNB transmits CSI Reference Signals (CSI-RS). The UE measures these pilots, estimates the channel, and reports CSI feedback including: CQI (Channel Quality Indicator — suggests the best modulation/coding scheme), PMI (Precoding Matrix Indicator — suggests the best precoding matrix for MIMO), and RI (Rank Indicator — suggests the number of useful spatial layers). The gNB uses this to optimise scheduling and beamforming.
Use Cases
MIMO precoding and beam selection decisions, adaptive modulation and coding (link adaptation), MU-MIMO scheduling, interference coordination between cells.
3GPP / Standards Reference
3GPP TS 38.214 (Physical Layer Procedures for Data), TS 38.331 (RRC Protocol — CSI reporting configuration)
Related Terms
Massive MIMO | Beamforming | MU-MIMO | QAM | gNB
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
