 to 32 antenna ports. This means that it is possible to transmit a maximum of 32 CSI-RSs in the digital domain. This operation is similar to LTE Class A operation. Another form of obtaining CSI feedback is based on beam selection, where multiple CSI-RSs are beamed and CRI (CSI-RS Resource Indicator) is fed back. This operation is similar to LTE Class B operation (Figure 6.39). The gNB arrays may be formed of single or multiple panels. A single-panel array per- forms the combination of RF beamforming and digital precoding at baseband. Different form factors of such panels may exist, from square, horizontal, or vertical placement of the cross-polarized antennas (Figure 6.40). Multiple panels are a combination of RF beamforming and digital precoding at base- band (Figures 6.41 and 6.42). 6.12.4.2.1 Type I Codebooks As mentioned in the previous section, the NR Type I code- book consists of single- and multiple-panel designs. Type I Single-Panel Codebook The design of the Type I codebook supports the configu- ration of 4, 8, 12, 16, 24, and 32 ports, while for 2 ports it is a simpler structure that we describe next. gNB transmitter 8 port CSI-RS 8 port CSI-RS 8 port CSI-RS 8 port CSI-RS CSI components feedback Figure 6.39 Obtaining the CSI feedback: measurements and reporting based on CRI and CSI-RS. 6 5G Physical Layer Figure 6.40 Single-panel antenna arrangements: square, horizontal, or vertical placement. gNB transmitter Figure 6.41 Multiple-panel SU MIMO transmission. gNB transmitter Figure 6.42 Multiple-panel MU MIMO transmission. For 2 ports, the Type I codebook for rank 1 is given by W E { 1 , n = 0,1,2,3 , while for rank 2 it is 1 For 4 and more transmission ports, the Type I codebook supports up to rank 8 trans- mission. The Type I codebook for single panel uses a dual-stage structure similar to LTE codebooks, where W = W1W2 and W 1 contains the beam selection and is reported as a wideband component, while W2 2 contains the QPSK co-phasing between the two polar- izations and possibly also beam selection and c