GPP specifications define the concept of beam correspondence and define two different UE types for that: UEs than meet the correspondence without uplink beam sweeping. UEs that need uplink beam sweeping to meet the beam correspondence require- ments. These UEs need to support the uplink beam management feature. An example of uplink beam sweeping is shown in Figure 6.15. Beamforming is addressed in more detail later in this chapter. The expected order of MIMO transmissions will also vary as a function of the fre- quency bands. While going to higher frequency bands allows a UE to handle more antennas, the number of supported MIMO layers does not increase when going to the mmW range; on the contrary, as illustrated in Figure 6.16, with the mmW spectrum, UEs are only expected support at most two MIMO layers even if the overall number of antennas likely to be more. This is due to the reduced diversity available in typical 28-GHz-range deployment cases with a relatively small cell radius. Thus, 3GPP specifi- cations also define the UE capabilities as follows: Bands below 2.3 GHz: UEs support two receiver antennas. Bands above 2.3 GHz: UEs support four receiver antennas. Above 24 GHz: Only two MIMO streams are required. These definitions will impact the peak data rates achievable, as discussed later in this chapter. The UE with support for the mmW spectrum (24 GHz or higher) is, however, expected to have directivity in the transmission, allowing the UE to select different trans- mission directions from the set of antennas it has. Since antenna sizes with the mmW spectrum are very small, some UEs will have two or more antenna groups, and thus will have capability to change between the antenna groups. 6 5G Physical Layer Beam 0 Beam 1 Beam Sweep Beam 2 Beam 3 3. Select best UL beam from UE Beam 4 transmission Beam 5 UL Beam Sweep 1. Measure the best downlink beam 2. Transmit N candidate 4. Feedback for uplink beams UE on which UL candidate 5. Continue with beam beam to select indicated by gNB Figure 6.15 Uplink beam sweep