5G Random Access Simulator

How to use the 5G Random Access Simulator

1. Set the preamble index. Type a ra-PreambleIndex from 0 to 63 — the value flows into the RAPID echoed in MSG2 and into the derived TC-RNTI and contention identity.
2. Pick a preamble format. Choose a format (0, A1–A3, B1/B4, C0/C2); the panel updates the sequence length, PRACH subcarrier spacing and the rough maximum cell radius that format supports.
3. Step through the messages. Use Next step to advance MSG1 → MSG2 → MSG3 → MSG4, or hit Auto-play to let it cycle on a timer.
4. Read the per-message fields. For each step, inspect the table of channels, RNTIs, the TA command and the identities so you can map a log capture to the right message.
5. Reset and try a collision case. Hit Reset, then re-run with a different preamble index to see how the RAPID and contention identity change.

Frequently asked questions

What are the 4 steps of 5G RACH?

Four-step contention-based random access is MSG1 to MSG4. MSG1 is a PRACH preamble the UE picks at random; MSG2 is the Random Access Response (RAR) the gNB sends on PDSCH, scheduled by a DCI scrambled with the RA-RNTI; MSG3 is the UE’s first scheduled PUSCH transmission, usually carrying an RRC Setup Request; MSG4 is contention resolution plus the RRC Setup that promotes the TC-RNTI to a C-RNTI.

What is the RA-RNTI and how is it computed?

The RA-RNTI is the identity the UE uses to find its RAR on PDCCH. It is derived purely from where the preamble was sent in time and frequency: RA-RNTI = 1 + s_id + 14·t_id + 14·80·f_id + 14·80·8·ul_carrier_id, where s_id is the first OFDM symbol of the PRACH occasion, t_id the first slot, f_id the frequency-domain occasion, and ul_carrier_id flags the SUL carrier. Two UEs that pick different preambles in the same occasion share the same RA-RNTI but are told apart by the RAPID inside the RAR.

What is contention resolution in MSG4?

When several UEs choose the same preamble in the same occasion they all read the same RAR and transmit MSG3 on the same grant. MSG4 settles the tie: the gNB echoes the 48-bit UE Contention Resolution Identity MAC CE, which is the leading bits of the CCCH SDU the winning UE sent in MSG3. A UE that sees its own identity considers itself the winner and moves to RRC_CONNECTED; the others back off and restart RACH.

When does a 5G UE trigger random access?

At initial access from RRC_IDLE, on RRC re-establishment after radio-link failure, at handover (where the target cell can hand the UE a dedicated preamble for contention-free access), on transition out of RRC_INACTIVE, when uplink data arrives but the UE has no valid PUCCH resource for a scheduling request, and to re-establish uplink synchronisation when the timing-advance timer expires.