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APCO Project 25 (P25) Trunked Radio Systems (TRSs) are systems that follow the open APCO Project 25 Standard for Public Safety TRSs. Multiple vendors make and sell Project 25 systems and compliant radios. Audio on these systems is exclusively digital using the APCO-25 Common Air Interface (P25 CAI) standard.

There are several "subsystem" features defined as part of the Project 25 Standard to enhance interconnectivity & interoperability and allow equipment from various manufacturers to work together.

These standards include
P25 Common Air Interface (P25 CAI)
Over-the-air modulation (digital audio)
P25 Inter-RF Subsystem Interface (ISSI)
The Inter-RF Subsystem Interface (ISSI) standard allows P25 systems from different manufacturers to be directly interconnected at the controller level, allowing seamless cross-system intercommunication, and system-to-system roaming for same-band systems.
P25 Console Subsystem Interface (CSSI)
The Console Subsystem Interface (CSSI) allows dispatch consoles from different manufacturers to be connected to the controller/core of other manufacturers' systems. For example, a Zetron console could be connected to a Motorola system, or a Harris console could be connected to a Tait system.

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Different Types of Project 25 Trunking/Modulation
Project 25 uses either MBE vocoder. IMBE stands for Improved Multi-Band Excitation or AMBE Advanced Multi-Band Excitation, and both were developed & licenced by DVSI Inc. Numerous vendors have produced Project 25 capable subscriber equipment, including EF Johnson, Motorola, M/A COM, Racal, Uniden, and others. There are both Conventional and Digital Trunk solutions which are able to use P25 Digital Voice using a compatible transmitter, transceiver and/or receivers.

Currently, Motorola's implementation of P25 digital data & voice is marketed as "ASTRO-25". both older Motorola Type II and Phase I systems for two of their types of trunking solution, they utilize the P25 IMBE vocoder, with their newer AMBE or AMBE2 radios are backwards compatible with systems that use the IMBE packaging.
Motorola ASTRO IMBE
Motorola ASTRO IMBE is a Motorola digital solution which is not compliant with the Project 25 standard. It is also called the "ASTRO Digital CAI (Common Air Interface) Option". This is a proprietary trunking solution that uses the Project-25 vocoder as its digital voice solution on top of a standard Motorola Type II Smartnet/Smartzone system.

The Motorola ASTRO IMBE solution uses the Motorola Type II 3600 Baud Smartzone control channel.
The Motorola ASTRO IMBE solution can allow both analog voice and P25 CAI digital voice radios to operate on the same network.
Project 25 Digital Trunking
This is the Project 25 (P25) Digital voice & data trunking solution, it is one that is vendor independent and designed around the Project 25 Digital Trunking standards. Phase I(PI) is 4800 symbols per second - where each symbol encodes two bits of data for a raw bit rate of 9600 bps. Phase II(PII) is 6000 symbols per second where each symbol encodes two bits of data for a raw bit rate of 12000 bps and utilizes the AMBE vocoder.

P25 Phase I uses a 4800 baud, 9600bps control channel.
P25 Phase II uses a 6000 baud, 12000bps control channel with AMBE2 vocoder at half-rate.
All radios on a Project-25 Digital trunking system must use digital voice - NO analog voice capability is provided, except via patches.
Project 25 Phases
Project 25 Phase I "FDMA"
Phase I FDMA consists of C4FM modulated signal or a CQPSK modulated signal. Both fit in a 12.5 kHz channel. Subscriber equipment transmits using C4FM. Site equipment may transmit in C4FM or CQPSK. Simulcast uses CQPSK modulation, however older Motorola ASTRO equipment used C4FM simulcast in a special mode called "WIDE pulse" which is not P25 compliant. P25 CQPSK Linear Simulcast Modulation is P25 compliant and is referred to as LSM. LSM is defined in the P25 standards.

Motorola "X2-TDMA"
Prior to the final Phase II standard being approved, Motorola developed and implemented their own TDMA protocol known as "X2-TDMA" uses the same modulation as Phase 1.

Project 25 Phase II "TDMA"
The Phase II standard is a 2-slot TDMA signal that fits inside a 12.5 kHz wide channel, providing two 6.25 kHz-equivalent channels. Fixed site output modulation is H-DQPSK with subscriber units using H-CPM on the input. This allows existing 12.5 kHz wide license holders to double call capacity by upgrading their infrastructure to Phase II. The Phase II standard was finalized and approved in November 2010 [1], and Motorola has begun shipping Phase II systems as of August 2011 [2] [3].

Motorola ASTRO-25 Phase II systems can also have an optional feature known as Dynamic-Dual-Mode (DDM), which will seamlessly revert a whole talkgroup (TGRP) to FDMA operating mode if a Phase I-only radio affiliates with a Phase II TDMA TGRP, and only go back to TDMA once all Phase I-only are unaffiliated with said TGRP.

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Phase 1
Phase 1 radio systems operate in 12.5 kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level FM (C4FM) modulation—a special type of 4FSK modulation[12]—for digital transmissions at 4,800 baud and 2 bits per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the IMBE codec, 2,800 is forward error correction, and 2,400 is signalling and other control functions. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviation at symbol time as C4FM. Phase 1 uses the IMBE voice codec.

These systems involve standardized service and facility specifications, ensuring that any manufacturers' compliant subscriber radio has access to the services described in such specifications. Abilities include backward compatibility and interoperability with other systems, across system boundaries, and regardless of system infrastructure. In addition, the P25 suite of standards provides an open interface to the radio frequency (RF) subsystem to facilitate interlinking of different vendors' systems.

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Phase 2
To improve spectrum use, P25 Phase 2 was developed for trunking systems using a 2-slot TDMA scheme and is now required for all new trunking systems in the 700 MHz band.[13] Phase 2 uses the AMBE+2 voice codec to reduce the needed bitrate so that one voice channel will only require 6,000 bits per second (including error correction and signalling). Phase 2 is not backwards compatible with Phase 1 (due to the TDMA operation), although multi-mode TDMA radios and systems are capable of operating in Phase 1 mode when required, if enabled. A subscriber radio cannot use TDMA transmission without a synchronization source; therefore direct radio to radio communication resorts to conventional FDMA digital operation. Multi-band subscriber radios can also operate on narrow-band FM as a lowest common denominator between almost any two way radios. This makes analog narrow-band FM the de facto "interoperability" mode for some time.

Originally the implementation of Phase 2 was planned to split the 12.5 kHz channel into two 6.25 kHz slots, or Frequency-Division Multiple Access (FDMA). However it proved more advantageous to use existing 12.5 kHz frequency allocations in Time Division Multiple Access (TDMA) mode for a number of reasons. It allowed subscriber radios to save battery life by only transmitting half the time which also yields the ability for the subscriber radio to listen and respond to system requests between transmissions.

Phase 2 is what is known as 6.25 kHz "bandwidth equivalent" which satisfies an FCC requirement for voice transmissions to occupy less bandwidth. Voice traffic on a Phase 2 system transmits with the full 12.5 kHz per frequency allocation, as a Phase 1 system does, however it does so at a faster data rate of 12 kbit/s allowing two simultaneous voice transmissions. As such subscriber radios also transmit with the full 12.5 kHz, but in an on/off repeating fashion resulting in half the transmission and thus an equivalent of 6.25 kHz per each radio. This is accomplished using the AMBE voice coder that uses half the rate of the Phase 1 IMBE voice coders.

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" SILENCE IS CONSENT "

Jim N7UAP - Bellingham, WA / InterceptRadio.com