n design. As we emphasized, coexistence and forward compatibility with LTE are crucial for NR. To achieve this, it is necessary to have as few blocked resources as possible. So, even though SS block transmission is periodic, having the option to turn off certain SS block locations or opportunities will help the network avoid interference with LTE or other featured services. SS block The SS block is composed of four consecutive OFDM symbols that contain one symbol for PSS, another symbol for SSS, and two symbols for the PBCH. The SS blocks are grouped in SS bursts and those are grouped again in SS burst sets. The SS burst set is periodically transmitted at each system default value of 20 milliseconds, and all the SS blocks are contained in 5-millisecond windows at the beginning of the 20-millisecond period, as shown in the following figure: 5G Air Interface and Physical Layer Procedures - Part 2 One SS burst set (one half-frame of 5ms, or shorter) SSB #1 SSB #2 SSB #L SSB #1 SSB #2 SSB #L SS burst set period (dafault 20 ms) Figure 5.5 - SS blocks in SS bursts Each of these SS blocks can be transmitted over different spatial directions to implement the beam- sweeping technique we described earlier. This is represented on the right hand of the preceding diagram. Now, we will continue going into more detail about the SS block. First, we will give a definition of the SS burst set, and then we will discuss its composition, mapping locations, and SS block utilization in LTE-NR coexistence. SS burst set definition So, as we saw in the previous figure, all SS blocks are confined in a 5-millisecond period that repeats every 20 milliseconds. This is the default periodicity that each device will assume. This indicates that a network can change periodicity to another value, but the devices that have not accessed the network yet will have to assume this periodicity as 20 milliseconds. Once the devices are connected, they will be notified of the actual value set by the gNB. The same happens in the case of the network turning of