P25 Phase 2 decoder for SDR: what it is and how to receive it

P25 Phase 2: two time slots, talkgroups, unit IDs, voice channel grants, site status (receive only; encrypted calls are only reported; AMBE+2 voice needs your own codec plug-in).

Group: Two-way radio
In the mode list: Digital voice, 12.5 kHz, 2 slots

What P25 Phase 2 is

APCO Project 25 Phase 2 (TIA-102.BBAB) TDMA, two time slots, 6000 symbols/s differential QPSK in 12.5 kHz: talkgroups, unit IDs, voice grants and the trunking system. The frame layout is a first reading of public descriptions and has not been tried on a real signal (receive only; encrypted calls are only reported).

Frequencies and settings

APCO Project 25 Phase 1 (TIA-102), C4FM, 12.5 kHz, conventional and trunked.

OnAir takes any frequency you type; the numbers above are the mode's starting point and the range its table describes. In the app the shortcut label is Digital voice, 12.5 kHz, 2 slots.

What OnAir shows

Calls with talkgroups, unit IDs and emergency flags; trunking: WACN, system, NAC, RFSS / site, adjacent sites, channel identifiers, voice grants, talkgroup activity; voice through your mbelib. Receive only: encrypted calls are only reported as encrypted, nothing is decrypted. Follow the rules on receiving radio where you live.

Limits and what it was checked with

Checked with generated signals and impairments; recordings of real signals welcome.

Which radio you need

This is a shortwave mode, so the radio has to tune HF. An SDRplay, Airspy HF+ or an RTL-SDR in direct-sampling mode do that directly; with a HackRF an HF upconverter or a direct-sampling receiver is the usual route. A long wire or loop antenna matters more than the radio.

Receiving and the law

Receiving and the law. In many countries it is illegal to intercept or disclose private radio communications. OnAir is a receiver: encrypted calls show up only as encrypted and are never decrypted, and voice needs a library you install yourself. Follow the law where you live. See also Legal.

Questions about P25 Phase 2

Can I decode P25 Phase 2 with an RTL-SDR?

An RTL-SDR can receive HF only in direct-sampling mode, which OnAir offers in its radio settings; results depend on the antenna. SDRplay and Airspy HF+ radios are better suited.

Is it legal to receive P25 Phase 2?

In many countries it is illegal to intercept or disclose private radio communications. OnAir is a receiver: encrypted calls show up only as encrypted and are never decrypted, and voice needs a library you install yourself. Follow the law where you live.

Can I try P25 Phase 2 in OnAir without a radio?

Yes. Most modes include a built-in synthetic test signal, so you can open the screen and try the controls before you own a radio.

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