.12.4.2 Precoding Matrix Indicator (PMI) Precoded transmission is a well-known technique where the transmitted data is precoded based on the actual channel on which it is going to be transmitted, such that he energy is radiated toward the location of the receiver. While the previous statement is more of an ideal case, in practical systems, the precoder is obtained based on the feedback from the UE, while one of the forms of precoding is based on searching among a set of PMI forming a so-called a codebook, and sending this PMI back to the transmitter (a gNB in a 5G system). Similar to LTE, 5G selected the use of PMI-based operation; however, it diversified the codebook options such that various MIMO use cases are met. Indeed, NR has specified two types of codebooks: the Type I codebook, which better fits single-user transmission, has a standard resolution in terms of CSI feedback, and has a lower payload which needs to be fed back to the transmitter. The Type II codebook is envisioned for multi-user application and has in this respect a higher-resolution CSI feedback, as it is known that in MU MIMO, the CSI resolution has a critical impact, as orthogonality between the users needs to be achieved. The Type II codebook is based on a linear combination of multiple beams and has DFT beams and port selection options. Both Type I and II codebooks are designed for cross-polarized antennas. The basic principle of precoded transmission is the following: the UE measures the channel based on the CSI-RS (as shown in Figure 6.38), it computes the CSI 6.12 5G MIMO and Beamforming Operation CSI-RS transmission of gNB transmitter up to 32 ports CSI components feedback Figure 6.38 Obtaining the CSI feedback: measurements and reporting based on CSI-RS port estimation. (consisting of the rank, PMI, CQI, etc.), and then sends such CSI information back to the transmitter. The number of CSI-RS ports the UE can measure is limited. In practice, however, it evolved step by step in LTE, and the first release of NR supports estimation of up