example, a linear movement introduces the need to change the TX beam at the base station while using the same RX beam at the UE. In the middle example, only the RX beam needs to be changed while the same TX beam is used. In the right, both TX and RX beams need to be changed. Figure 6.33 presents intra-cell beam training use cases for UE rotation and beam blockage. In the left example, we have UE rotation, which causes the RX beamformer at the UE to change while the TX beam at the gNB is unchanged. In the case of beam blockage, the signal path is blocked by an obstacle, leading to a significant drop of signal quality, for example, tens of decibels in received power, at the receiver. An alternative path, for example, due to reflections (blue beams), may become the best link between the base station (BS) and UE. Figure 6.33 Necessity for beam change: movement of the UE (left figure), UE rotation (middle figure), and beam blockage (right figure). 6 5G Physical Layer Beam management consist of a set of procedures used to assist the UE to set and maintain its receive spatial filter used to transmit and receive. It can be categorized by a few steps: Initial beam acquisition Beam measurement, refinement, and reporting Beam recovery 6.12.2.1 Initial Beam Acquisition The process of finding the beam pairs is based on beam sweeping and it is done by both the gNB and UE, as shown in Figure 6.34. This means essentially that the transmitter (being the gNB or UE) sweeps a set of beams, while the receiver measures these receive beams. As both the gNB and UE need to perform this procedure, there are ways in which one end sweeps through beam while the other performs measurements, and the other way around. The process ends when a beam pair is established between the transmitter and receiver. Initial beam acquisition occurs already during the initial access. The gNB transmits the synchronization signal in such a sweeping manner; in fact, all the transmission points from the network perform this operation while the UE performs measure