TETRA
From SpenchWiki
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 two nice pictures of the waveform and spectrum!
| Property | Value |
|---|---|
| Modulation | π/4 DQPSK |
| Symbol rate | 18000 |
| Bit rate | 36000 |
| Slots/frame | 4 |
| Frame length | 255 symbols (56.7ms) |
| Frame 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.
The Fast Auto-correlation GUI block is used above.
De-modulation
Passing the baseband input into a constellation sink with the appropriate parameters results in:
This is also reflected in the eye diagram:
It should be noted that tuning plays an important role (at least with the follow setup) in ensuring a lock is maintained within the MPSK demodulator. Best results are found when tuning slightly off-centre of the (frequency domain) signal.
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