Project 25 (P25)
Phase I FDMAFrequency Division Multiple Access
Phase 1 radios use Continuous 4 level FMFrequency Modulation (C4FMContinuous 4-Level Frequency Modulation) modulation—a special type of 4FSK modulation—for digital transmissions at 4800 baudBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. and 2 bits per symbol, yielding 9600 bits per second total channel throughput.
Phase I FDMAFrequency Division Multiple Access consists of C4FMContinuous 4-Level Frequency Modulation modulated signal or a CQPSKContinuous Quadrature Phase-Shift Keying modulated signal. Both fit in a 12.5 kHzKiloHertz (kHz) 10^3 Hz channel. Subscriber equipment transmit in C4FMContinuous 4-Level Frequency Modulation. Site equipment may transmit in C4FMContinuous 4-Level Frequency Modulation or CQPSKContinuous Quadrature Phase-Shift Keying. Simulcast uses CQPSKContinuous Quadrature Phase-Shift Keying modulation, however older Motorola ASTRO equipment used C4FMContinuous 4-Level Frequency Modulation simulcast in a special mode called "WIDE pulse" which is not P25 compliant. P25 CQPSKContinuous Quadrature Phase-Shift Keying Linear Simulcast Modulation is P25 compliant and is referred to as LSM. LSM is defined in the P25 standards
Phase II TDMATime Division Multiple Access
The Phase II standard is a 2-slot TDMATime Division Multiple Access signal that fits inside a 12.5 kHzKiloHertz (kHz) 10^3 Hz wide channel. Fixed site output modulation is H-DQPSKDifferential Quadrature Phase-Shift Keying with subscriber units using H-CPM on the input. This allows existing 12.5 kHzKiloHertz (kHz) 10^3 Hz wide license holders to double call capacity by upgrading their infrastructure to Phase II. The Phase II standard was approved in November 2010, and as of August 2011 Motorola has begun shipping Phase II systems.
Motorola ASTRO 25 systems can also have an optional feature known as Dynamic Dual Mode (DDM), which will seamlessly revert a talkgroup to FDMAFrequency Division Multiple Access operating mode if a Phase I only capable radio affiliates with a Phase II capable talkgroup.
|Trunking Control Channel||Encrypted Voice Channel|
|Trunking Control Channel|
Additional SamplesP25 Phase 1 Encrypted Voice Bursts:
Usually around ~860 MHzMegaHertz (MHz) 10^6 Hz or ~500 MHzMegaHertz (MHz) 10^6 Hz.
- Hobby Level Software
- RTL-SDR: Decoding P25 Phase I QPSK with DSD and SDR#
- Decoding P25 with SDR#, DSD, and RTLSDR #2
- P25 Decoding with SDR# and DSD