Preamble format
Long formats use L=839 @ 1.25/5 kHz · short formats L=139 follow the data SCS · TS 38.211 Table 6.3.3.1-1/2
cyclic prefixpreamble sequence (repetitions)guard time
Cell radius ≈ c × (TCP − τDS) / 2 with τDS ≈ 5 µs delay spread — the round trip must fit inside the CP.
Sequence planning
Zero correlation zone → root budget
Each Zadoff–Chu root yields ⌊L / NCS⌋ cyclic-shift preambles. Bigger cells need bigger NCS — and therefore more roots to reach 64 preambles.
After Msg1
RA-RNTI calculator
The gNB addresses its random-access response using an RNTI derived purely from where your preamble landed (TS 38.321 §5.1.3).
RA-RNTI = 1 + s_id + 14·t_id + 14·80·f_id + 14·80·8·ul_id
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Common questions
What are the 5G PRACH preamble formats?
TS 38.211 defines long formats 0–3 (sequence length 839 at 1.25 or 5 kHz SCS, durations 1–3.5 ms, for macro cells up to ~100 km) and short formats A1–A3, B1–B4, C0, C2 (length 139, SCS follows the carrier numerology, durations of a few OFDM symbols, for normal and small cells and beam-swept FR2).
How does PRACH format determine cell radius?
The preamble's cyclic prefix must absorb the full round-trip delay plus delay spread: radius ≈ c × (T_CP − τ_DS) / 2. Format 0's ~103 µs CP supports ~14 km; format 1 reaches ~100 km; a short A1 at 30 kHz with ~2.3 µs CP covers only a few hundred metres.
What is the zero correlation zone (N_CS)?
Preambles are cyclic shifts of a Zadoff–Chu root sequence. N_CS is the minimum shift between them — it must exceed the maximum round-trip delay in samples, or a far UE's preamble would alias into a neighbour's shift. zeroCorrelationZoneConfig indexes the N_CS table; larger cells ⇒ larger N_CS ⇒ fewer preambles per root ⇒ more roots consumed.
How is RA-RNTI calculated?
RA-RNTI = 1 + s_id + 14×t_id + 14×80×f_id + 14×80×8×ul_carrier_id, where s_id is the first symbol of the PRACH occasion, t_id its slot number, f_id its frequency-domain index and ul_carrier_id 0/1 for NUL/SUL. The UE monitors PDCCH for this RNTI to receive its random-access response.
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