and the numerology to be used is contained in the PBCH payload. The numerology used for RMSI is the following: 15 or 30 kHz for frequencies below 6 GHz 60 or 120 kHz for frequencies above 6 GHz A dedicated Control Resource Set (CORESET) is used for delivery of the downlink grant for the RMSI. More system information is transmitted to a device via the Physical Downlink Shared Channel (PDSCH), per the device's demand. Note that the system information delivery is different from the LTE in the sense that, apart from the SS block and RMSI, there is no periodic transmission of System Information Blocks (SIB); instead, the information is provided on a device-specific manner on a demand basis. This reduces the number of blocked resources due to broadcast information transmission to a minimum. This is also due to NR having an ultra-lean design. Message 1, 2, 3, and 4 transmissions Now, let us discuss how the gNB and a device can transmit synchronization signals and the PRACH respectively. The random access procedure is based on a four-step approach, as shown in the following figure: RAR window ( TRAR) PDCCH/ PUCCH/ PDCCH/ PRACH PDSCH PUSCH PDSCH (Msg 1) (Msg 2) (Msg 3) (Msg 4) Successful Msg 1& Msg 2 transmission and reception PUCCH/ PUSCH (Msg 3) PDCCH/ PDSCH (Msg 4) Figure 5.4 - A four-step random access approach 5G Air Interface and Physical Layer Procedures Part 2 Note that this was the case for the LTE as well. So, up to four different messages are exchanged in this procedure. Message 1 (Msg 1) or PRACH defines a random access response window (RAR window), where the device expects a Message 2 (Msg 2) response from the gNB. After that, Message 3 (Msg 3) and Message 4 (Msg 4) follow with a set of timers that are configured to trigger retransmissions of the messages, in case they are not received correctly. Let us now analyze the message flows for the random access procedure in detail. Msg 1 or PRACH is sent from the device to the gNB to start the random access procedure. Keep in mind that the different Subcarrier Spaci