RACH: 2-step (MsgA/MsgB)

Cutting One Round Trip from Initial Access

Release 16 introduced the two-step RACH procedure to reduce initial access latency by eliminating one complete round-trip exchange. Instead of four separate messages, the procedure condenses everything into two composite messages. MsgA combines the preamble and UE payload that were previously split across Msg1 and Msg3, transmitting both on PRACH and PUSCH without waiting for an uplink grant. MsgB combines the RAR and contention resolution from Msg2 and Msg4 into a single gNB response. This approach saves approximately four to ten milliseconds of access latency compared to four-step RACH.