to implement the MIMO mechanism using a baseband precoder. The precoder is defined by its matrix W, and the baseband beamforming mechanism is defined by a beam precoding X matrix. All the codes are registered in a codebook. The base station chooses the precoding code from the CSI measurement report sent by the mobile. The mobile calculated CSI from the CSI-RSs. 5G-NR Radio Interface - MIMO and Beamforming DM-RS CSI-RS1 DM-RS Codeword Mapping RE Precoding Spatial layer Precoding Beamforming mapping MIMO W Codeword Mapping RE DM-RS CSI-RSn Figure 6.12. Massive-MIMO transmission chain The CSI-RS are not precoded by the precoding matrix W but can be precoded by beamforming X (see Figure 6.12). The received CSI-RS signal corresponds to the convolution product [H].[X]. The DM-RS are precoded (see Figure 6.12). The received DM-RS allows data to be decoded without knowing the code word used since the receiver receives the effect of MIMO and BF precoding: [H] [X][W]. The mobile UE transmits a measurement report to the base station from the CSI-RSs. The gNB base station can transmit CSI-RSs without beamforming. In this case, the matrix X is the identity matrix, also called the single-CSI method. The gNB base station can transmit CSI-RSs with beamforming, also called the multiple-CSI method. Single CSI-RS method Multiple CSI-RS method Figure 6.13. Single CSI-RS and multiple CSI-RS methods The massive-MIMO mechanism introduces two methods for the feedback of CSI to the gNB entity: the single CSI-RS method and the multiple CSI-RS method. NG-RAN and 5G-NR The single CSI-RS method is applied to the antenna partitioning model. CSI is collected per antenna element and per polarization of a group from a CSI-RS physical signal, not precoded by the matrix X. The single CSI-RS method defines a number of antenna ports and a number of non-precoded CSI-RS which can vary from 1 to 32. Single CSI-RS allows horizontal or vertical beamforming and 8x8 MIMO. The multiple CSI-RS method is applied to the complete connection model, and it consis