e a high-frequency-antenna solution can accommo- date a larger number of antenna elements. The antenna architectures are in any case different: in low carriers, one is using digital transmission where power amplifiers are present for each antenna element, whereas in higher carriers, this is not possible, and hence one power amplifier serves a set of antenna elements, which may be placed on a so-called antenna panel. Both the gNB and UE can have multiple panels: while multiple panels at the gNB enable the multiplexing of multiple UEs, as each panel can transmit toward one UE, multiple panels at the UE would enable the reception of signals in a challenging environment where UE movement, rotation, or beam blockage would ben- efit from receive signal diversity. Hence, there is a need and motivation for the placing of multiple panels on the device. One of the fundamental elements of beam management is the concept of a beam pair between the gNB and UE. Both the transmitter and receiver can form beams in different directions; it is said that when the beams "face each other," we can have a beam pair. One should not restrict this possible only to line of sight, as such beam 6.12 5G MIMO and Beamforming Operation Figure 6.31 Beam pair between gNB and UE. Figure 6.32 Downlink beam training use cases for linear movement of UE (diagonal hash beam = old beam, square dots beam = new beam). pairs can also occur due to reflections from scatterers. In Figure 6.31,we depict a transmitter having multiple beams and a UE which also operate with multiple beams. Note that these beams can be used for both transmission and reception. If the UE utilizes the same beam for transmitting and receiving, it is said to be capable of beam correspondence. There are a few challenges in the process of keeping alive beam pairs. Figure 6.32 shows three different intra-cell downlink beam training use cases for UE movement. In the all cases, it is assumed that the UE moves its position linearly with respect to the location of a base station. In the left 