Project 25, P25 Radio

Notes and summary or tutorial of the basics of P25 radio also known as Project 25, or APCO-25 - a digital land mobile radio system used mainly in North America.

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Project 25, also known by its short name P25 or APCO-25 is a digital radio system used for land mobile applications, especially by the federal, state or province and local public safety agencies. P25 radio is most widely used in North America and can often be thought of as being equivalent to TETRA radio, although it does not interoperate with it.

Project 25, APCO-25 or P25 radio is a set of standards that defines the radio system.

P25 radio, APCO & TIA

Project 25, P25 radio has been defined under the auspices of APCO - Association of Public-Safety Communications Officials. APCO International is an organization of public safety communications professionals. It aims to serve the needs of public safety communications users and hence provide for the welfare of the general public as a whole. APCO provides expertise, professional development, technical assistance, advocacy and outreach.

APCO promotes many projects. These are given project numbers, and hence the project to develop a standard for digital radio communications for the emergency services was given a project number - Project 25. The name Project 25 or P25 radio has remained with the technology.

The standards themselves are held under the auspices of the TIA, Telecommunications Industry Association.

The Project 25, P25 standard was established in October 1989 when APCO2, NASTD3, NCS4, NTIA5 , and NSA6 collaborated in the creation of the APCO-NASTD-Fed Project 25 which is now known as Project 25. The system is now widely used in North America.

What is Project 25, P25 radio

Project 25, or P25 radio is a standard that defines a form of interoperable digital two-way wireless communications products. The P25 radio system was developed in North America with state, local and federal representatives and Telecommunications Industry Association (TIA) governance.

The P25 radio standards are administered by the TIA (Mobile and Personal Private Radio Standards Committee TR-8).

P25 radio technology uses digital techniques for the transmission of voice and data.

The P25 radio technology is being developed in two phases:

  • P25 Phase 1:   The phase 1 Project 25 radios can operate in 12.5 kHz analogue, digital, or a mixed mode format. The Phase 1 radios use a form of modulation known as Continuous 4 level FM, C4FM.

    Phase 1 P25 compliant systems are backward compatible and interoperable with legacy FM systems. In addition to this, the P25 standards provide an open interface to the RF Sub-System to facilitate interlinking of different vendors' systems.
  • P25 Phase 2 :   Phase 2 P25 radio systems are based around a 6.25 kHz channel bandwidth> within this they achieve one voice channel or a minimum 4800 bps data channel. P25 Phase 2 uses FDMA - frequency division multiple access and the modulation format is CQPSK. An alternative 2 slot TDMA solution is currently under development. Phase 2 implementation achieves the goal of improved spectrum utilization. Also being stressed are such features as interoperability with legacy equipment, interfacing between repeaters and other sub-systems, roaming capacity and spectral efficiency/channel reuse.
  • P25 Phase 3:   With the growing need for high speed data in many areas, Phase 3 of the P25 radio standards will address this requirement. Additional features are aimed at providing functionality for a new aeronautical and terrestrial wireless digital wideband/broadband public safety radio standard that can be used to transmit and receive voice, video and high-speed data in wide-area, multiple-agency networks.

P25 modulation

Different forms of modulation of the RF carrier are used between Phase 1 and Phase 2 of P25. These different forms of modulation help enable the system to meet the different requirements between these two phases.C4FM is used for Phase 1 and CQPSK is used for Phase 2.

  • C4FM:   C4FM is form of frequency modulation where the carrier is shifted in frequency at a particular rate to a particular location around a centre frequency - there are four positions, hence the name. This allows for each of the 4 "states" to represent a binary number. Each state is a known as a "Symbol" which contains two bits of information.

    C4FM modulation may be viewed as a type of Differential Quadrature Phase Shift Keying, DQPSK. Each symbol is shifted in phase by 45° from the previous symbol. Although the phase and hence the frequency is modulated for C4FM, the amplitude of the carrier is constant, generating a constant envelope frequency modulated waveform. This is of particular interest in preserving battery power. For constant envelope modulation schemes are able to use non-linear power amplifiers which are more efficient, converting more power to usable RF energy for a given DC input. This means that for a given RF power output less input power - hence battery supply is needed, enabling a battery to be smaller or last for longer.
  • CQPSK:   CQPSK modulates the phase and simultaneously modulates the carrier amplitude to minimize the width of the emitted spectrum which generates an amplitude modulated waveform.

The data rate for the modulated carrier is 4800 symbols per sec, and each symbol conveys two bits of information as detailed in the table below.

(Binary data)
Symbol C4FM Deviation CQPSK Phase Change
01 +3 +1800 Hz +135°
00 +1 +600Hz > +45°
10 -1 -600Hz -45°
11 -3 -1800Hz -135°

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