Editing Project 25 (section)

====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>