rs for beamforming is the sweeping sub-frame. Each four-symbol downlink sweeping sub-frame consists of the PSS, SSS, and PBCH, as shown in earlier in Figure 6.4. This structure is often referred to as the SS Block. The SS Block is the basis for UE measurement taking from a cell, as well as gaining access to the network. Further, with beamforming, the beam identification is based on the detection and measurement of the SS Block. With the beam sweeping operation, the synch block is repeated several times, as shown in Figure 6.13. With the lower frequency bands (below 2.5 GHz), typi- cally one can support at most 4 beams (with 15 kHz or 30 kHz subcarrier spacing), while with bands up to 6 GHz, up to 8 beams could be supported. With the mmW bands, the 120 kHz or 240 kHz subcarrier spacing enables up to 64 beams. As the symbol dura- tion is shorter with the higher subcarrier spacing, the necessary synch block sequence duration remains reasonable with higher frequency bands even with a larger number of beams in use. Beam 0 Figure 6.13 SS Block sequence enabling beam-related feedback. Beam 1 4 symbols Beam 2 4 symbols Beam 3 4 symbols 4 symbols Synch block sequence 6.5 5G Channel Structures and Beamforming Basics Figure 6.14 Beam quality feedback. Beam 0 Beam 1 Beam 2 Beam 3 Beam 4 Beam 5 Beam Sweep 5G UE Beam Quality Feedback In the case of the beamforming operation, the SS Block is repeated for each beam during the sweeping sub-frame. The aim is to have the UE identify which beam was received the best and provide corresponding feedback for the gNB (Figure 6.14). Important beamforming concepts have been introduced in 3GPP. One of the funda- mental aspects is beam correspondence, which basically means the UE capability to send in the uplink to the beam direction corresponding to the one it has measured in the downlink direction to be the best one. This becomes even more important when a UE can use more than one set of antennas, as is likely to be the case with frequency bands above 24 GHz (3GPP Frequency Range 2). The 3