SS block 4 Uplink 1 Uplink 2 Uplink 3 Uplink 4 ((H)I)) PRACH transmission (same PSS, SSS, PBCH reception transmit beam direction as in the downlink transmit beam) UE TX PUSCH RAR window NT1+0.5 ms UE RX PDSCH PDSCH Slot in Slot n+k2+ NT1+NT2+0.5 ms NT1 A time duration of N1 symbols related to PDSCH reception time for UE Last symbol of First symbol of Msg2 on PDSCH Msg3 on PUSCH processing capability 1 when additional PDSCH DM-RS is configured. NT2: A time duration of N2 symbols related to PUSCH preparation time for UE processing capability 1. Figure 4.85 Random-access procedure 8,12]. 630 Chapter 4 gNB UE (1) Random access preamble (1) MsgA: PRACH preamble + data (Msg1 + Msg3) (2) Random access response (2) MsgB: random-access response (Msg2 + Msg4) (3) Msg3 transmission on PUSCH (4) Contention resolution Figure 4.86 Comparison of two-step and four-step contention-based random-access procedures [11]. In NR Rel-15, the RACH procedure is triggered, when uplink data becomes available at the UE buffer and the UE is either in the RRC_IDLE/INACTIVE state, where the RACH pro- cedure is triggered for state transition, or in the RRC_CONNECTED state, if the uplink is not synchronized, where the RACH procedure is used to reestablish uplink synchronization, or in the RRC_CONNECTED state, if the UE has no PUCCH resources available for SR or the SR procedure fails, where the RACH procedure serves as an SR. In addition, the RACH procedure is used for beam failure and recovery, on-demand SI request, or it can be explic- itly triggered by the network with RRC for handover. In NR Rel-15, the uplink data cannot be transmitted until the RACH procedure is success- fully completed. It is observed that for small packet transmission, four-step RACH is not efficient in terms of latency and signaling overhead; thus the two-step RACH has been pro- posed to simplify the RACH procedure to achieve lower signaling overhead and latency. It is possible to allow Msg3 in four-step RACH procedure to carry data in order to reduce the latency and ove