le closed loop transmission schemes are the main design point, these being based on some forms of CSI feedback from the UE, transparent transmit diversity is possible, for example, precoder cycling based on a set of rules, like using the same precoder on the frequency allocation where the same DM-RS is used. Having set the type of reference signals used for data transmission, the design of 5G targets the number of layers which are transmitted to a single UE and the rules needed for performing multi-user MIMO transmission in an orthogonal way between the users. Indeed, the two main types of transmissions are single- and multi-user MIMO. While single-user MIMO can support a total of 8 transmission layers to a single UE, in the multi-user domain, the network can multiplex up to 12 orthogonal DM-RS ports, meaning that a total of 12 layers can be orthogonally multiplexed between a number of UEs, for example, 12 UEs if each is receiving a single layer or 3 UEs if each is receiv- ing 4 layers. Nevertheless, in the multi-user domain, the network can multiplex even more than 12 layers, if, for example, there is better spatial separation between the users or some forms of orthogonalizations are performed at the transmitter, such as zero forcing. It is worth noting that codeword-to-layer mapping has the following rules: - One to four layers are mapped to a single codeword. - If there are more than five layers, these are mapped on two codewords: the first floor (L/2) layers are mapped on codeword 0, while the remaining layers are mapped on codeword 1. 6.12.2 Beam Management Framework Beam management is one of the most important components of 5G NR. These mech- anisms become necessary when 5G is operated in higher carrier bands, such as above 6 GHz, where radio propagation is more challenging, and beamforming is mandatory for both the gNB and UE in order to compensate for the path loss. The good news is that as the carrier frequency is increasing, the antenna sizes at both the transmitter and receiver become smaller, and henc