 help the UEs in RRC_CONNECTED and potentially RRC_IDLE mode to receive downlink data/control in unused SSBs. The structure and timing of SSB transmis- sion with various numerologies is illustrated in Fig. 4.32, where a number of symbols are reserved for downlink control at the beginning of the slot, and some symbols are reserved for guard period and uplink control to allow UL/DL switching and fast uplink feedback. The SSB pattern corresponding to 15 and 30 kHz subcarrier spacing can provide more UL/ DL switching opportunities in TDD mode. The SSB pattern for 30 kHz subcarrier spacing can be used to facilitate LTE-NR coexistence in the downlink in an FDD system, consider- ing the locations of LTE PDCCH and cell-specific reference signals in the symbols with LTE default 15 kHz subcarrier spacing. 484 Chapter 4 Within a PBCH transmission time interval update period of 80 ms, there are 16 possible positions for an SS burst set, if we consider the minimum period for an SS burst set to be 5 ms. The 16 possible positions of an SS burst set can be identified by the three least sig- nificant bits of the SFN and one-bit half radio frame index. The SSBs can be repeated within an SS burst set. When the UE detects an SSB, it will acquire the timing informa- tion from its PBCH, from which the UE is able to identify the radio frame number, the slot index in a radio frame, and OFDM symbol index in a slot. The timing information includes 10 bits for SFN, 1 bit for half radio frame index, and 2, 3, or 6 bits for SSB time index for frequency ranges up to 3, 3-6, and 6-52.6 GHz, respectively. Within the SSB indices, two or three LSBs are carried by changing the DM-RS sequence of PBCH. Thus, for the sub-6 GHz frequency range, the UE can acquire the SSB index without decoding the PBCH. It also facilitates PBCH soft combining over multiple SSBs as these SSBs with different indices carry the same PBCH payload [72]. As shown in Fig. 4.32, for a half frame with SS/PBCH blocks, the first symbol indices of the candidate SS/PBCH blocks are 