than one nonzero-power CSI-RSs, it can report a set of N UE-selected CSI-RS resource-related indices. This is a very useful indicator as it can quickly point to the N best CSI-RS resources the network can use further. As we have seen for the indication of the CSI-RS, a similar indication is possible for SS Blocks; the indicator is called SSBRI. The LI indicates which column of the precoder matrix of the reported PMI corre- sponds to the strongest layer of the codeword corresponding to the largest reported wideband CQI. If two wideband CQIs are reported and have equal value, the LI corre- sponds to the strongest layer of the first codeword. The RI is a well-known indicator in MIMO, and it is a measure of the total number of layers that can be transmitted. 6.12.5 Uplink MIMO Transmission Schemes NR supports two transmission schemes for the PUSCH: codebook-based transmission and non-codebook-based transmission. These are described in the next subsections. 6.12.5.1 Codebook-Based Uplink Transmission When codebook-based transmission is used, the UE precodes the PUSCH based on a precoder which is indicated by the gNB. The following steps are performed: The UE transmits SRS resources to the gNB. The gNB estimates the UL channel based on these resources and hence determines the UL transmission rank and also iden- tifies the best SRS, each of the SRS resources being tagged with an SRI if multiple SRS resources are configured. In addition to the SRI and rank, the gNB estimates the precoder to be applied over the layers that corresponds to that SRS resource. This pre- coder is selected from a predefined UL codebook over the SRS ports in the selected SRS resource by the SRI. At the end of all these processing stages, the gNB has the SRI, Transmit Precoder Matrix Indicator (TPMI) (in the uplink direction, the PMI is denoted by Transmit PMI to differentiate it from the downlink PMI), and rank, which can be used by the UE for the UL transmission. If a single SRS resource is con- figured, no SRI needs to be indicated to the UE; 