rom the previous chapter that for LTE, u=0 has a subcarrier spacing of 15 kHz. The yellow SS block will not interfere with the LTE cell reference signals, while, in contrast, the green one does interfere. Therefore, the gNB must turn it off to avoid interference. This is the underlying reason for the LTE-NR coexistence requirement that we mentioned. In a nutshell, LTE and NR coexistence forces the SS block to use 30 kHz subcarrier spacing, and the gNB must turn off any colliding SS blocks with LTE cell reference signals. A consequence of this is 5G Air Interface and Physical Layer Procedures - Part 2 that LTE-NR coexistence requires a minimum bandwidth of 10 MHz, since this is the bandwidth used by the SS block at this 30 kHz subcarrier spacing. SS burst set composition In the following figure, the SS block locations are displayed relative to a 5-millisecond window. Blue and orange colors indicate the slots containing two SS blocks, whereas the green and yellow colors show a set of two slots containing four SS blocks in this time window. 5 ms window 15 kHz (L 4) 15 kHz (L 8) 30 kHz (L 4) 30 kHz (L 8) 120 kHz (L 64) 240 kHz (L 64) 0.5 ms Slot containing 2 Set of two slots SS-blocks containing 4 SS-blocks Slot containing 2 Set of two slots SS-blocks containing 4 SS-blocks Figure 5.10 - SS burst set composition As we detailed in previous sections, below 6 GHz, we have a total of either four or eight SS blocks, depending on exactly which frequency band is used, and above 6 GHz, we have a total amount of 64 SS blocks. SS block time index indication We saw that the SS blocks are transmitted using beam sweeping. If we recall the beginning of this section, it is important to identify these SS blocks with the time indexes SO that devices can derive the resources to transmit the PRACH. In NR, three bits of the SS block time index will be carried out by selecting the PBCH DMRS sequence. This is enough for the following 6 GHz case, where the number of SS blocks in an SS burst set is eight or less. For higher frequencies, up 