PRACH Config Index Lookup
Decode any PRACH Config Index (0–255) to preamble format, periodicity, and RACH occasion details per 3GPP TS 38.211.
The PRACH Configuration Index is a single small integer in 5G NR that packs an enormous amount of information: which preamble format the cell uses, how often PRACH occasions occur in time (periodicity in radio frames), which subframes or slots carry them, the starting symbol inside the slot, and how many PRACH occasions fit per slot. 3GPP TS 38.211 splits the mapping across three tables — 6.3.3.2-2 for FR1 paired / SUL (FDD), 6.3.3.2-3 for FR1 unpaired (TDD), and 6.3.3.2-4 for FR2 unpaired. This lookup tool lets you enter a config index with your duplex mode and frequency range and instantly see the decoded parameters, together with a reminder of what each preamble format is useful for (format 0/1/2/3 for long preambles and larger cells; A/B short formats for micro and FR2; C formats for restricted conditions).
How It Works
Each row of the 3GPP PRACH configuration tables specifies:
- Preamble format — 0, 1, 2, 3 (long, 1.25/5 kHz SCS) or A1–A3, B1–B4, C0, C2 (short, SCS up to 120 kHz).
- x, y — PRACH occurs in radio frames where n_f mod x = y. So x = 16 means every 16 frames = 160 ms.
- Subframe number(s) / slot number(s) — exactly when in the frame the PRACH occasions start.
- Starting symbol — OFDM symbol offset inside the slot.
- Number of PRACH slots per subframe and RO per PRACH slot.
We ship a curated subset of ~60 of the most commonly deployed entries. If the index you need is not in the lookup table, you can still check the full row in TS 38.211 — we link the exact table for each duplex/FR combo.
3GPP References
- • TS 38.211 §6.3.3.2 — Random access preamble generation
- • TS 38.211 Table 6.3.3.2-2 — PRACH configurations for FR1 paired / SUL (FDD)
- • TS 38.211 Table 6.3.3.2-3 — PRACH configurations for FR1 unpaired (TDD)
- • TS 38.211 Table 6.3.3.2-4 — PRACH configurations for FR2 unpaired
- • TS 38.213 §8 — UE random access procedure
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