Why Codebooks?
The CSI feedback channel is bandwidth-limited. The full channel matrix would take thousands of bits. Codebooks define a finite set of precoder candidates indexed by a few bits — UE picks the closest match and reports its index.
Type I — Single-Beam
Type I codebooks select one DFT beam from a 2D oversampled grid (configurable N1 × N2 with O1, O2 oversampling). Compact (~10 bits feedback), used for typical eMBB. Subtypes: Type I single-panel and multi-panel.
Type II — Multi-Beam
Type II combines L beams (typically L=2-4) with per-beam complex coefficients per layer per sub-band. Much higher resolution than Type I but feedback can be 100+ bits. Best for high-rank (8-16 layer) high-throughput scenarios.
eType II (Rel-16) — Compressed
Enhanced Type II compresses the per-sub-band coefficients in the frequency domain via a DFT (or DCT-like) transform with a small "frequency-domain compression matrix." Typical feedback shrinks to 30-40 bits while preserving 90%+ of Type II throughput gain.
Sub-band Reporting
CSI is reported per sub-band (groups of 4-32 PRBs depending on BWP size). The UE picks PMI per sub-band; CQI per sub-band; RI is wideband. Frequency-selective channels benefit most.
Group-Based Beam Reporting
For mmWave beam-pair-link tracking: UE reports two beams in a group (one for primary, one for backup) so handover-class beam switches can happen quickly.
Rank Determination
UE picks the rank that maximizes its expected throughput. Rank 1 for cell-edge or single-stream reliability; rank 2 for typical sub-6 eMBB; rank 4-8 for high-SNR high-MIMO scenarios.
Frequently Asked Questions
What is the difference between Type I and Type II MIMO codebooks?
Type I selects a single DFT beam — compact ~10-bit feedback, optimal for typical eMBB. Type II combines multiple beams with per-beam coefficients — higher resolution ~100+ bit feedback, optimal for high-MIMO high-throughput.
What is eType II?
Enhanced Type II (Rel-16) compresses Type II's frequency-domain coefficients via a DFT-like transform. Feedback shrinks 3-5× while preserving most of Type II's throughput gain.
How is rank determined in 5G NR?
The UE computes the rank that maximizes its expected throughput given the current channel SNR and reports it as RI in the CSI report. The gNB scheduler may choose to use a lower rank.