ts of reporting the CSI for each beam formed from the CSI-RS. The multiple CSI-RS method limits the number of antenna ports per beam to 8. The returned CSI relates to the best CSI-RS received and contains a CRI (CSI Resource Index) corresponding to this CSI-RS. In the case of the multiple CSI-RS method, the following mechanisms have been introduced to improve efficiency of CSI-RS resources, i.e. to allow more mobiles to share a pool of CSI-RS resources: - aperiodic CSI-RS, in which a mobile is configured to measure the CSI-RS physical signal in a given sub-frame; - multi-tap CSI-RS, in which a mobile is configured to measure the periodic CSI-RS physical signal for a specified period of time. Two-step resource configurations are introduced to allow flexible selection of resources while maintaining control over CSI reporting overload. For these configurations, RRC messages and Medium Access Control (MAC) elements are combined. The enhancements also include the optimization of CSI reports, obtained from measurements of the CSI-RS physical signal. The first mechanism aims to configure the size of the return information by defining two types of reports: - CSI TYPE 1: normal (PMI) feedback in the case of SU-MIMO giving the direction of the most important beam. - CSI TYPE 2: enhanced (explicit or codebook-based) in the case of MU-MIMO by providing more feedback information from the terminal to the base station. 5G-NR Radio Interface - MIMO and Beamforming The second mechanism aims to improve the flexibility of CSI-RS resources, either by reducing the density of the resource elements (see Chapter 4) or by transmitting a measurement of all the beams in order to transmit only the highest power beam (only for the multiple-CSI method). 6.4.2. Codebook configuration The 5G codebook is an evolution of the 4G codebook specified in Release R.13 to implement FD-MIMO. The 3GPP specification defines a W precoding which consists of two sub-matrices W1 and W2 with W = W1W2. The matrix X is therefore integrated into W. The matrix W1 r