5G Random Access Simulator
Walk through the 4-step contention-based Random Access procedure on 5G NR, message by message, with RA-RNTI computation, TA command, TC-RNTI and contention resolution identity — per 3GPP TS 38.321 §5.1 and TS 38.213 §8.
L=139, SCS=15 kHz — Short format, small cells (FR1/FR2)
| Channel | PRACH (physical random access channel) |
| Preamble index (ra-PreambleIndex) | 23 / 63 |
| Preamble format | Format A1 |
| Sequence length (L_RA) | 139 |
| PRACH SCS | 15 kHz |
| Max cell radius | ~0.94 km |
| RA-RNTI formula | 1 + s_id + 14·t_id + 14·80·f_id + 14·80·8·ul_carrier_id |
| RA-RNTI (with s_id=t_id=f_id=0) | 1 (0x1) |
About 5G Random Access
Random Access (RACH) is how a UE establishes uplink synchronization with a gNB. It happens at initial access, after long DRX, on handover, after radio-link failure, or when the UE has uplink data but no PUCCH resources. 5G NR defines a 4-step contention-based procedure (MSG1–MSG4) and also an optional 2-step procedure (MSGA / MSGB) introduced in Rel-16.
MSG1 is a Zadoff–Chu sequence preamble on PRACH. The gNB detects the preamble and replies with MSG2 (the RAR) scheduled with a DCI 1_0 scrambled by RA-RNTI. The RAR carries the Timing Advance command, an UL grant, and a temporary C-RNTI (TC-RNTI). The UE then sends MSG3 on the granted PUSCH — typically carrying an RRC Setup Request with a 39-bit 5G-S-TMSI or a random identity. Finally MSG4 echoes back the 48-bit UE Contention Resolution Identity MAC CE and carries RRCSetup, promoting TC-RNTI to C-RNTI and moving the UE into RRC_CONNECTED.
Who uses this simulator?
Protocol engineers use it to trace failure points (preamble collision, RAR miss, MSG3 loss, contention resolution timeout). Trainers use it in 5G certification courses. Test engineers use the RA-RNTI formula and RAPID matching to correlate log captures with over-the-air preambles.
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