TETRA

Specs

TETRA (Terrestrial Trunked Radio) is certainly out there, if you look for it. Even with simple AM demodulation, you can hear the characteristic 'buzz'.

If you want to read some TETRA-related bunkum, check out TETRA Watch. At least that particular page has those two pictures of the waveform and spectrum!

TETRA's Magic Numbers

Property	Value
Modulation	π/4 DQPSK
Symbol rate	18000
Bit rate	36000
Slots/frame	4
Frame length	255 symbols (56.7ms)
Frames rate	17.65Hz

• TETRA base station will always broadcast in each slot - if there is no information to transmit, slots are filled with the idle sequence.

Auto-correlation

• The first big peak at ~14ms is indicative of the repeating single-slot idle pattern that fills up all four slots (~56ms).

• Not sure what the peak at ~50.5ms is. If you do, let me know!

• A frequency-correction burst can be seen in the Scope Plot below.

De-modulation

• Passing the baseband input a constellation sink with the appropriate parameters results in:

• It should be noted that tuning plays an important role (at least with the follow setup) in ensuring a lock is maintained in the MPSK demodulator. Best results are found when tuning slightly off-centre of the (frequency domain) signal.