Also known as: P25 Phase 2, P25 Phase II, Phase 2 P25
P25 Phase 2 is the second-generation air interface of Project 25, using two-slot TDMA to carry two simultaneous voice conversations in a single 12.5 kHz channel — effectively 6.25 kHz per call.1 It reuses Phase 1’s Common Air Interface addressing and signalling but replaces the FDMA voice channel with a time-shared one.
Overview
Phase 2 was introduced to meet FCC spectrum-efficiency goals. Where Phase 1 gives each call its own frequency, Phase 2 divides a traffic channel into two repeating timeslots, doubling voice capacity in the same bandwidth. It uses the more efficient half-rate AMBE+2 vocoder and a phase-shift modulation family: H-DQPSK on the outbound (base-to-mobile) link and H-CPM on the inbound (mobile-to-base) link. The two are chosen so that a mobile’s transmitter can stay simple and constant-envelope while the base station uses the more linear form. Crucially, Phase 2 is a traffic-channel technology: a system almost always keeps a Phase 1 control channel to run the trunking.
Technical characteristics
| Property | Value |
|---|---|
| Access | TDMA, 2 slots |
| Channel | 12.5 kHz (6.25 kHz equivalent capacity) |
| Modulation | H-DQPSK (outbound) / H-CPM (inbound) |
| Symbol rate | 6000 symbols/s per channel |
| Vocoder | AMBE+2 (half-rate, ~3.6 kbps incl. FEC) |
| Control channel | Usually a C4FM Phase 1 TSBK control channel |
| Addressing | shared with Phase 1 (NAC, talkgroup, radio ID) |
A common deployment keeps a C4FM Phase 1 control channel — carrying TSBK grants and the WACN / System ID / RFSS identity — while voice traffic rides Phase 2 TDMA slots. The grant tells a listening radio (or scanner) not just which frequency but which of the two timeslots to decode.
History
Phase 2 was standardised by the TIA within the TIA-102 suite to follow narrowbanding and 6.25 kHz-equivalent spectrum-efficiency mandates, and has been deployed on large metropolitan and statewide systems since the early 2010s.12 The switch to AMBE+2 (from Phase 1’s IMBE) was necessary because two calls now share the bit budget of one, so each needs a lower-rate vocoder.
Deployment
Phase 2 is widely used by busy urban and countywide public-safety systems where channel capacity is at a premium, frequently mixed with Phase 1 on the same network: Phase 1 control channel and mutual-aid channels, Phase 2 for the bulk of routine voice traffic. As with Phase 1, much of this traffic is AES-encrypted on many modern systems.
Decoding it with GopherTrunk
GopherTrunk follows the (Phase 1) control channel, reads the TSBK channel grant, tunes to the assigned Phase 2 traffic channel and slot, and decodes the AMBE+2 voice. Because two calls interleave in time on one frequency, the decoder demultiplexes the bursts by slot before running the vocoder. Clear and known-key calls produce audio; keyed-encrypted calls are logged with their metadata only. See Status.
Sources
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Project 25 — Wikipedia, for the P25 Phase 2 two-slot TDMA air interface, H-DQPSK/H-CPM modulation, the AMBE+2 vocoder, and TIA standardisation. ↩ ↩2
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P25 (Project 25) — CISA SAFECOM, for the TIA-102 suite and the 6.25 kHz-equivalent spectrum-efficiency mandate that motivated Phase 2. ↩