Editing Project 25 (section)

=== P25 phases ===
[[File:Motorola hand-held.jpg|right|thumb|200px|A hand-held Project 25 radio used in US systems]]
P25-compliant technology has been deployed over two main phases with future phases yet to be finalized.

====Phase 1====
Phase 1 radio systems operate in 12.5&nbsp;kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level [[FM broadcasting|FM]] (C4FM) modulation—a special type of 4[[Frequency-shift keying|FSK]] modulation<ref>{{Cite web |url=http://www.aeroflex.com/ats/products/prodfiles/appnotes/Understanding%20P25%20Modulation%20Fidelityiss1.pdf |title=Aeroflex: Application Note - Understanding P25 Modulation Fidelity |access-date=2012-03-26 |archive-url=https://web.archive.org/web/20120320083432/http://www.aeroflex.com/ats/products/prodfiles/appnotes/Understanding%20P25%20Modulation%20Fidelityiss1.pdf |archive-date=2012-03-20 |url-status=dead }}</ref>—for digital transmissions at 4,800 [[baud]] and 2 [[bit]]s per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the [[Improved multi-band excitation|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 [[Absolute deviation|deviation]] at symbol time as C4FM. Phase 1 uses the [[Improved multi-band excitation|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.

====Phase 2====
To improve spectrum use, P25 Phase 2 was developed for trunking systems using a 2-slot [[Time-division multiple access|TDMA]] scheme and is now required for all new trunking systems in the 700&nbsp;MHz band.<ref>{{cite web|url=http://www.p25phase2.com/p25-phase-2|title=P25 Phase 2|access-date=9 December 2016}}</ref> Phase 2 uses the [[Advanced Multi-Band Excitation|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&nbsp;kHz channel into two 6.25&nbsp;kHz slots, or Frequency-Division Multiple Access (FDMA). However it proved more advantageous to use existing 12.5&nbsp;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&nbsp;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&nbsp;kHz per frequency allocation, as a Phase 1 system does, however it does so at a faster data rate of 12&nbsp;kbit/s allowing two simultaneous voice transmissions. As such subscriber radios also transmit with the full&nbsp;12.5&nbsp;kHz, but in an on/off repeating fashion resulting in half the transmission and thus an equivalent of 6.25&nbsp;kHz per each radio. This is accomplished using the AMBE voice coder that uses half the rate of the Phase 1 IMBE voice coders.<ref>{{cite web|url=https://www.teleco.com.br/tutoriais/tutorialprojeto25/default.asp|title=P25 in Brazil|access-date=4 March 2020}}</ref>

====Beyond Phase 2====<!--[[Project MESA]] redirects here-->
From 2000 to 2009, the [[European Telecommunications Standards Institute]] (ETSI) and TIA were working collaboratively on the Public Safety Partnership Project or '''Project MESA''' (Mobility for Emergency and Safety Applications),<ref>{{cite web|url=http://www.projectmesa.org/ |title=Mobile Broadband for Public Safety - Home Page |publisher=Project MESA |access-date=2014-06-06|url-status=usurped|archiveurl=https://web.archive.org/web/20081020103132/http://www.projectmesa.org/|archive-date=2008-10-20}}</ref> which sought to define a unified set of requirements for a next-generation aeronautical and terrestrial digital wideband/broadband radio standard that could be used to transmit and receive voice, video, and high-speed data in wide-area, multiple-agency networks deployed by public safety agencies.<ref name=projectmesa_tia>[https://www.itu.int/dms_pub/itu-r/oth/0A/0E/R0A0E0000540001PDFE.pdf Advanced Mobile Broadband For Public Protection & Disaster Relief Professionals]. David Thompson. Telecommunications Industry Association</ref><ref name=projectmesa_itu-t>[https://www.itu.int/itudoc/itu-t/workshop/ets/s5p1.pdf Project MESA: Broadband Telecommunications for PPDR]. David Thompson. Telecommunications Industry Association</ref>

The final functional and technical requirements have been released by ETSI<ref>{{Cite web|url=http://www.projectmesa.org/ftp/Specifications/|archiveurl=https://web.archive.org/web/20100613000949/http://www.projectmesa.org/ftp/Specifications/|url-status=dead|title=www.projectmesa.org - /ftp/Specifications/|archivedate=June 13, 2010}}</ref> and were expected to shape the next phases of American Project 25 and European DMR, [[dPMR]], and TETRA, but no interest from the industry followed, since the requirements could not be met by available commercial off-the-shelf technology, and the project was closed in 2010.{{citation needed|date=May 2013}}

During the [[United States 2008 wireless spectrum auction]], the FCC allocated 20&nbsp;MHz of the 700&nbsp;MHz [[UHF]] radio band spectrum [[Digital television transition in the United States|freed in the digital TV transition]] to public safety networks.  The FCC expects providers to employ [[Long Term Evolution|LTE]] for high-speed data and video applications.<ref>{{Cite web|url=https://www.fcc.gov/700-mhz-public-safety-narrowband-spectrum|title=700 MHz Public Safety Spectrum|date=March 17, 2011|website=Federal Communications Commission}}</ref>