 using beam sweeping, as shown in Figure 5.2. In NR, both the synchronization signals and the broadcast channels, or the Physical Broadcast Channels (PBCHs), transmitted in units of four OFDM symbols called Synchronization Signal (SS) blocks. The gNB will transmit SS block multiplexes in time, and each SS block will use a different beam or a spatial direction to cover the desired coverage area. Eventually, one of these SS blocks' spatial direction will be better suited for communicating with the device, which it will detect. The device will be able to decode the basic system information that is carried in the SS block, thus initiating the random access procedure. SS Block 1 SS Block 2 SS Block 3 SS Block 4 SS Block 5 Figure 5.2 Beam sweeping So, the first step in the initial access procedure from a device point of view is identifying the best SS block within those transmitted by a network. The device must obtain the time index of the received SS block that uniquely identifies the selected block. 5G Air Interface and Physical Layer Procedures Part 2 The time index discovery is very crucial because, without any other time reference, the device will not know where the frame starts and where to transmit random access preambles. That is why all SS blocks carry a time index indication dividing between the PBCH Demodulation Reference Signal (DMRS) and explicit indication in the PBCH payload. With this time index information, the device can completely identify the best SS block. The device transmits the Physical Random Access Channel (PRACH) on a set of resources that depend on the time index of the received SS block, by implicitly linking a set of PRACH resources to a particular SS block. Then, the gNB will know which SS block is received by the device in the downlink. With that information, the gNB can continue the rest of the initial access procedure in the same beam spatial direction, as notified implicitly by the device in the PRACH. On the left pane of the following figure, the gNB is transmitting the SS blocks in 