ing with beam correspondence operation. Digital beamforming Hybrid beamforming Number of MIMO layers 3.5 GHz Below 7 GHz Above 24 GHz Above 52.6 GHz Frequency Figure 6.16 Number of MIMO layers feasible as a function of frequency. The use of the mini-slot together with hybrid beamforming allows the transmission resources to be reserved for a shorter time for a particular direction, compared to the use of the full 14-symbol slot. This improves the resource usage efficiency, especially with smaller packet sizes and durations. Random Access Random access with 5G supports different PRACH formats and numerologies to cope with different deployments. The random access sequences themselves are based on the Zadoff-Chu sequences: Sequence length L = 839, with subcarrier spacings of 1.25kHz and 5 kHz. This sequence can be used for very large cells (up to 100 km). Sequence length of L = 139, with subcarrier spacings of 15, 30, 60, and 120 kHz. These offer suitable solutions for different coverage situations. 6.7 Downlink User Data Transmission Downlink sweeping blocks Beams for uplink PRACH reception Beam association Figure 6.17 Beam association for uplink PRACH transmission. Several formats are supported for different coverage situations. Due to the cyclic prefix duration, a short sequence length is suited for cell radio on the order of 4.7 km. After this distance, some degradation appears; thus, for larger cells, it is better to use a longer sequence. With 30 kHz TDD deployment and a single uplink slot in the frame structure, only a shorter sequence is possible. Use of a longer sequence would require allocation of two consecutive slots in the uplink, something not supported with either of the TDD patterns shown earlier in Figure 6.7. The beamforming operation has an impact on the RACH procedure. Once the UE has detected the cell from the strongest SS Block, it needs to use RACH resources that correspond to the best beam direction toward the UE. Thus, the UE needs to estab- lish beam association, as shown in Figure 6.17. Oth