What does the PRACH configuration index control?

The index (0–255) is a lookup key into the TS 38.211 PRACH configuration tables. One value fixes the preamble format, the system-frame and subframe numbers where RACH occasions appear, the periodicity, the starting symbol, and the number of time-domain occasions per slot. The gNB signals it in SIB1 (prach-ConfigurationIndex) so every UE knows when to send MSG1.

Which table applies — FR1 or FR2?

38.211 has separate tables for FR1 paired (FDD), FR1 unpaired (TDD) and FR2 unpaired spectrum, so the same index number maps to different occasions in each. Long preamble formats (0–3) live in the FR1 paired/unpaired table; the short formats (A1–C2) appear across both ranges. Always read the index against the table that matches your duplex mode and frequency range.

What is the difference between long and short preamble formats?

Long formats (0–3) use 1.25 or 5 kHz PRACH subcarrier spacing and a long sequence sized for large cells and wide coverage. Short formats (A1–C3) use 15/30/60/120 kHz spacing, are much shorter in time, and pack several occasions into a slot — better for small cells, TDD and FR2 beam sweeping where you need many RACH chances quickly.

PRACH Config Index Lookup

Decode any PRACH Config Index (0–255) to preamble format, periodicity, and RACH occasion details per 3GPP TS 38.211.

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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).

Config Index (0–255)

Duplex Mode

Frequency Range

Preamble Format

PRACH SCS: 1.25 kHz · max cell radius ≈ 14.5 km · 1 ms + 0.1 ms CP

Periodicity

10 ms

n_f mod 1 = 0

Subframe(s)

Every frame

Starting Symbol

0.000

Config index 27 (FR1, TDD) uses preamble format 0 with a periodicity of 10 ms. Source: TS 38.211 Table 6.3.3.2-3.

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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How to look up a PRACH configuration index

Pick the frequency range and duplex. Select FR1 FDD, FR1 TDD or FR2 so the tool reads the correct 38.211 table.
Enter the index. Type the prach-ConfigurationIndex value (0–255) from SIB1 or your config.
Read the decoded fields. See the preamble format, periodicity, and the subframe/slot positions of the RACH occasions.
Check the occasions. Note the starting symbol and number of time-domain occasions to confirm the cell has enough RACH capacity.

Frequently asked questions

What does the PRACH configuration index control?: The index (0–255) is a lookup key into the TS 38.211 PRACH configuration tables. One value fixes the preamble format, the system-frame and subframe numbers where RACH occasions appear, the periodicity, the starting symbol, and the number of time-domain occasions per slot. The gNB signals it in SIB1 (prach-ConfigurationIndex) so every UE knows when to send MSG1.
Which table applies — FR1 or FR2?: 38.211 has separate tables for FR1 paired (FDD), FR1 unpaired (TDD) and FR2 unpaired spectrum, so the same index number maps to different occasions in each. Long preamble formats (0–3) live in the FR1 paired/unpaired table; the short formats (A1–C2) appear across both ranges. Always read the index against the table that matches your duplex mode and frequency range.
What is the difference between long and short preamble formats?: Long formats (0–3) use 1.25 or 5 kHz PRACH subcarrier spacing and a long sequence sized for large cells and wide coverage. Short formats (A1–C3) use 15/30/60/120 kHz spacing, are much shorter in time, and pack several occasions into a slot — better for small cells, TDD and FR2 beam sweeping where you need many RACH chances quickly.

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