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
Overview
Phase 2 was introduced to meet spectrum-efficiency goals. Where Phase 1 gives each call its own frequency, Phase 2 divides a traffic channel into two repeating timeslots, doubling capacity. It uses the more efficient AMBE+2 vocoder and a phase-shift modulation (H-DQPSK on the outbound link, H-CPM on the inbound).
Technical characteristics
| Property | Value |
|---|---|
| Access | TDMA, 2 slots |
| Channel | 12.5 kHz (6.25 kHz equivalent capacity) |
| Modulation | H-DQPSK (outbound) / H-CPM (inbound) |
| Vocoder | AMBE+2 (half-rate) |
| Control channel | Usually a C4FM Phase 1 control channel |
A common deployment keeps a C4FM Phase 1 control channel while voice traffic uses Phase 2 TDMA slots.
History
Phase 2 was standardised by the TIA to follow narrowbanding and spectrum-efficiency mandates, and has been deployed on large metropolitan and statewide systems since the early 2010s.1
Deployment
Phase 2 is widely used by busy urban public-safety systems where channel capacity is at a premium, frequently mixed with Phase 1 on the same network.
Decoding it with GopherTrunk
GopherTrunk follows the control channel, tunes to the assigned Phase 2 traffic channel and slot, and decodes the AMBE+2 voice. 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