RACH Failure — Debugging Preamble / Msg1–Msg4

A UE cannot access a cell in an airport macro cell (gNB-AIR). You've correlated the UE's RACH attempts with the gNB's baseband traces. The UE sends preambles but never receives RRC Setup. Trace the RACH 4-step contention-based procedure (Msg1–Msg2–Msg3–Msg4) and identify at which message it fails. Note that the cell has very high load (peak airport traffic) and covers a wide area — airports frequently reach the edge of standard macro cell radius. Remember the physical side of RACH: PRACH preamble format determines the maximum cell radius the gNB can resolve. A mismatch between the configured format and the UE's actual distance is a classic, silent field issue — preambles are detected, RAR is sent, but Msg3 never decodes.