Field Guide · term

Also known as: NCDB, NDB, TETRA SB, TETRA downlink burst

TETRA burst formats are the fixed timeslot layouts that a TETRA transmitter uses to pack data, signalling, and a synchronisation pattern into each TDMA slot.1 A slot is 510 bits — 255 π/4-DQPSK symbols — and four slots make one 56.67 ms TDMA frame. The burst format defines exactly where a receiver will find the training sequence to lock onto and where the two payload blocks (BKN1 and BKN2) sit relative to it, so the framer can slice each slot without ambiguity.2 The main downlink formats are the Normal Continuous Downlink Burst (NCDB), the Normal Downlink Burst (NDB), and the Synchronisation Burst (SB).

BKN1 · 216 bits AACH train AACH BKN2 · 216 bits one slot = 510 bits = 255 symbols (14.167 ms) the training sequence anchors every offset; BKN1/BKN2 are sliced around it
A downlink slot carries two 216-bit blocks (BKN1, BKN2) split by a central training sequence, with the access-assignment channel (AACH) in the two half-blocks flanking it; the framer locates the training sequence, then slices the blocks by fixed offsets.

The Normal Continuous Downlink Burst carries traffic and signalling on a continuously-transmitting downlink carrier. Around its normal training sequence sit two 108-symbol (216-bit) blocks — BKN1 ahead of the training sequence and BKN2 after it — plus the two halves of the access-assignment channel (AACH) that flank the training sequence. GopherTrunk’s traffic extractor measures the geometry in dibits relative to the training-sequence lead dibit L: BKN1 spans [L−115, L−7), the first AACH half [L−7, L), the 11-dibit training sequence [L, L+11), the second AACH half [L+11, L+19), and BKN2 [L+19, L+127). Concatenating BKN1 and BKN2 yields one 432-bit full-slot traffic frame. The Normal Downlink Burst shares that block geometry; the difference between the continuous and discontinuous downlink lies in how the carrier is keyed, not in where the blocks fall.

The synchronisation burst

The Synchronisation Burst is the one a cold receiver hunts first. Instead of two equal blocks it carries a frequency-correction field and a broadcast synchronisation channel (BSCH) block ahead of a longer 38-bit synchronisation training sequence, with a normal-length block after. The SB is transmitted in slot 1 (TN1) of frame 18 of every multiframe, so once detected it anchors the whole slot grid: any burst leading at dibit L then falls in slot (round((L − sbAnchor)/255) mod 4) + 1. A subtlety GopherTrunk pins is that the SB’s synchronisation training sequence sits late in the burst, one NDB-slot after the frame’s TN1 traffic position, so the decoded anchor must be shifted by one slot to line up with the control channel’s granted timeslots. TETRA’s infrastructure-free direct mode reuses the same physical layer but with its own block boundaries — see TETRA DMO burst framing.

Relevance to SDR

internal/radio/tetra/traffic.go encodes the NCDB geometry as the ndbBKN1Start/ndbBKN2Start offsets and slices each detected burst into a raw 54-byte type-5 frame; dmo.go carries the parallel direct-mode geometry. Getting these offsets exactly right — and anchoring the slot grid on the SB — is what lets the framer demultiplex four concurrent slots on one carrier and hand each block to the descrambler and channel decoder. Every TETRA logical channel rides inside one of these bursts, so the burst format is the boundary between raw symbols and decodable content.

Sources

  1. Terrestrial Trunked Radio — Wikipedia, on the TETRA air interface and its TDMA frame structure. ↩

  2. Burst transmission — Wikipedia, on packing payload and synchronisation into fixed time-limited bursts. ↩

See also