Also known as: TETRA, Terrestrial Trunked Radio
TETRA (Terrestrial Trunked Radio) is an ETSI digital trunked-radio standard built for public-safety and professional users, especially across Europe and much of the world outside North America. It divides each 25 kHz carrier into four TDMA timeslots and modulates them with π/4-DQPSK (phase-shift keying), encoding speech with an ACELP vocoder.1 It is a complete system standard rather than merely an air interface, defining call control, gateways, security, and interworking.
Overview
TETRA is a complete system standard, not just a modulation scheme. It specifies group and individual calls, fast call setup (often quoted under 300 ms), direct mode (DMO) for radio-to-radio operation off the network, packet data, and a rich security suite with authentication, air-interface encryption, and end-to-end encryption options. Splitting a 25 kHz carrier into four timeslots gives four simultaneous talk paths per channel — the same spectral-efficiency goal as narrowband FDMA systems, but reached through time division. One slot on a carrier typically acts as the main control channel (MCCH), continuously broadcasting system information and handling registration, affiliation, and channel grants, while the others carry traffic. The choice of π/4-DQPSK — a differential QPSK whose phase transitions avoid the origin — keeps the signal envelope well-behaved for efficient amplification while carrying 2 bits per symbol at 18 kbit/s gross.
Technical characteristics
| Property | Value |
|---|---|
| Access | TDMA, 4 slots per carrier |
| Channel spacing | 25 kHz |
| Modulation | π/4-DQPSK, 18 kbit/s gross |
| Symbol rate | 18 000 sym/s (36 kbit/s raw before differential mapping) |
| Vocoder | ACELP (~4.567 kbit/s speech) |
| Control | Continuous main control channel (MCCH) on one slot |
| Modes | Trunked (V+D), Direct Mode (DMO), packet data |
| Data evolution | TEDS (TETRA Enhanced Data Service) in “TETRA 2” |
History
TETRA was standardised by ETSI from the mid-1990s (first specifications published around 1995) as a pan-European digital replacement for the fragmented analogue and proprietary trunked systems then serving emergency services.12 It was designed deliberately as an open, interoperable standard so agencies from different countries and vendors could share infrastructure and roam across networks. A later evolution, sometimes called TETRA 2, added the higher-throughput TEDS data service and additional modulation options while preserving backward compatibility with the original voice-and-data air interface. TETRA competes in its home market with the French Tetrapol system, a different, FDMA-based public-safety standard.
Deployment
TETRA underpins national public-safety networks across Europe, the Middle East, Asia, Africa, and Latin America, along with transport (airports, metros, railways), utilities, and military/government users. It is essentially absent from North America, where P25 dominates public safety for regulatory and historical reasons. Because it is a mature standard with a large installed base, many countries are now weighing long-term migration of mission-critical voice toward broadband LTE/5G push-to-talk, though TETRA remains the workhorse for guaranteed, infrastructure-independent group communications.
Decoding it with GopherTrunk
TETRA uses a distinct physical layer — π/4-DQPSK at 18 kbit/s on a 25 kHz carrier with four TDMA slots — that differs from the C4FM/4FSK family GopherTrunk handles for P25/DMR/NXDN, and its speech uses the ACELP vocoder rather than an AMBE variant. Following a TETRA system means demodulating DQPSK, recovering the slot/frame timing, and parsing the MCCH signalling to track calls onto traffic slots. Note that GopherTrunk’s downconversion path normalises TETRA to a 144 kHz channel rate (versus 48 kHz for the 4800-baud C4FM family), reflecting its wider symbol rate. Air-interface and end-to-end encryption (TETRA TEA) are out of scope for a receiver-only decoder. Consult the Status page for GopherTrunk’s current TETRA coverage.
Sources
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Terrestrial Trunked Radio — Wikipedia, for the ETSI TETRA standard, its four-slot TDMA air interface, π/4-DQPSK modulation, and the ACELP vocoder. ↩ ↩2
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ETSI — TETRA — ETSI, the standards body, for the TETRA specifications, the main control channel, direct mode, and the TEDS data evolution. ↩