CSI Report
How the gNB Closes the Downlink Link-Adaptation Loop
The CSI report is the heart of closed-loop MIMO in 5G NR. The base station periodically transmits a CSI Reference Signal (CSI-RS) on a configured set of antenna ports, up to 32 ports for the most capable arrays. The user equipment (UE) measures the channel matrix on those ports, estimates the interference and noise from an associated CSI interference measurement (CSI-IM) resource, and condenses the result into a compact report. Because the report is reverse-link feedback, it is rate-limited; sending the full complex channel matrix every slot would be prohibitively expensive on the uplink, so 3GPP defines codebooks and indicators that quantize the channel down to a few bits to a few hundred bits.
Reporting is configured per report setting, which links a measurement resource (the CSI-RS), a report quantity (such as cri-RI-PMI-CQI), and a time-domain behavior. Periodic and semi-persistent reports ride on the Physical Uplink Control Channel (PUCCH) when small, while aperiodic and larger reports are multiplexed onto the Physical Uplink Shared Channel (PUSCH). A report is split into two parts: CSI Part 1 has a fixed payload (RI, CQI for the first codeword, and the number of non-zero coefficients for Type II) so the receiver can blindly decode it, and CSI Part 2 carries the variable-size PMI whose length depends on the rank and Part 1 contents.
Latency matters. The interval from the CSI-RS measurement to the corresponding report defines the channel-aging budget. At sub-6 GHz with pedestrian mobility the channel is stable enough for periodic reports every 20 to 80 slots, but at FR2 millimeter-wave frequencies with vehicular Doppler the coherence time can drop below a few milliseconds, pushing designs toward frequent aperiodic reports and toward exploiting TDD channel reciprocity instead.
Report Composition and Selection Order
RI → PMI → LI → CQI
Spectral efficiency from CQI:
η ≈ (R / 1024) × log2(M) [bits/symbol]
MIMO capacity ceiling (informs RI):
C ≈ ∑k=1r log2(1 + λk × SNR / Nt) [bits/s/Hz]
Where R = code rate × 1024, M = modulation order (QPSK…256-QAM), r = reported rank, λk = k-th channel eigenvalue, Nt = transmit antennas. CQI targets 10% BLER (or 0.001% for the URLLC CQI table). Example: CQI 9 maps to 16-QAM, R≈616/1024, η ≈ 2.41 bits/symbol.
CSI Report Quantities at a Glance
| Field | Full name | Typical size | Range / values | What the gNB does with it |
|---|---|---|---|---|
| CQI | Channel Quality Indicator | 4 bits (per codeword) | Index 0 to 15 | Picks modulation & code rate (MCS) |
| PMI | Precoding Matrix Indicator | 4 to 200+ bits | Codebook index (i1, i2) | Selects the downlink precoder |
| RI | Rank Indicator | 1 to 3 bits | 1 to 8 layers | Sets number of MIMO streams |
| LI | Layer Indicator | 0 to 2 bits | Strongest of reported layers | Maps phase-tracking RS to layer |
| CRI | CSI-RS Resource Indicator | 0 to 4 bits | Selects 1 of N CSI-RS | Beam / resource selection |
| L1-RSRP | Reference Signal Received Power | 7 bits | -140 to -44 dBm | Beam management decisions |
Frequently Asked Questions
What is the difference between periodic, semi-persistent, and aperiodic CSI reports?
Periodic CSI (P-CSI) is RRC-configured with a fixed period (5 to 320 slots) and always rides on PUCCH for low-overhead background estimates. Semi-persistent CSI (SP-CSI) is also periodic but is activated and deactivated by a MAC CE or DCI and may use PUCCH or PUSCH. Aperiodic CSI (A-CSI) is a one-shot report triggered by a CSI request field in an uplink-grant DCI, always carried on PUSCH, and supports the largest payloads including Type II high-resolution PMI for MU-MIMO.
How are CQI, PMI, RI, and LI related inside a single CSI report?
They are computed in a dependency chain. The UE first chooses the RI (1 to 8 layers), then the PMI (best codebook precoder for that rank), then the LI (strongest reported layer), and finally the CQI, a 4-bit MCS index assuming the gNB uses the reported rank and precoder at 10% target BLER (or 0.001% for the URLLC table). Because CQI is conditioned on the reported PMI and RI, the four quantities must be interpreted together, not in isolation.
Why does a CSI report use CSI-RS rather than the synchronization signal block?
CSI-RS spans up to 32 antenna ports with configurable density, so the UE can estimate the full per-subband MIMO channel matrix needed for PMI and RI. The SSB carries only a few ports and is built for coarse beam sweeping and initial access, so it cannot resolve the per-port phase needed for Type II precoding. A paired CSI-IM (or zero-power CSI-RS) resource provides the interference estimate that makes CQI accurate. SSB-based L1-RSRP is used for beam-management reports, which are separate from the CQI/PMI/RI link-adaptation report.