TETRA
A digital trunked radio standard used mainly outside North America.
What does it sound like?
A continuous, dense digital hiss.
Who uses it?
Emergency services, transport and utilities, mostly in Europe and elsewhere.
Where and when can I hear it?
Around 380-400 MHz for emergency services in Europe, and other UHF bands.
The science
TETRA packs four conversations into one 25 kHz carrier by dividing time into frames of four slots, each frame lasting about 56.67 milliseconds. The carrier uses pi/4-DQPSK, a phase modulation in which each symbol shifts the signal's phase to carry two bits, at a modulation rate of 36 kilobits per second. Voice is coded with an ACELP speech codec at 4,567 bits per second, which channel coding raises to a gross rate of 7.2 kbit/s.
Why use it?
Four talk paths per 25 kHz carrier gives spectrum efficiency equivalent to 6.25 kHz per path, and a 2016 DHS Office of Emergency Communications fact sheet notes the design suits infrastructure-rich, high-density networks such as those of utilities and transport operators. The standard also defines direct radio-to-radio operation, group calls and air interface encryption.
Lore
ETSI documents from 1995 expand the name as Trans-European Trunked Radio, while by 1997 ETSI was titling the standard Terrestrial Trunked Radio. A 2016 DHS fact sheet said TETRA had gained acceptance in more than 120 countries.
Easily confused with
- DMR digital voice, from the same family
- P25 digital voice, from the same family
- D-STAR digital voice, from the same family