RACH: 4-step (Msg1 → Msg4)

Contention-Based Random Access

After reading SIB1, the UE initiates the four-step contention-based RACH procedure to establish its connection with the gNB. The procedure is called contention-based because the UE randomly selects one preamble from a pool of 64 sequences, and multiple UEs may independently choose the same preamble on the same RACH occasion. This potential collision is resolved through a contention resolution mechanism embedded in the fourth message. The four messages form two round-trip exchanges between UE and gNB, each carrying progressively more information until the UE is uniquely identified and granted…