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P25 Question

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Premium Subscriber
Dec 8, 2005
Dupage County, Illinois
To my knowledge there are 4 types of P25 modulation C4FM, CQPSK, WIDE, and the new Phase 2 TDMA. My question is when are these P25 settings used, for example I know CQPSK is used on simulcast 9600 Baud 100% P25 trunked systems especially when narrow banded, but is it also used on conventional P25 Simulcast repeaters or is all conventional P25 C4FM? Basically what I would like to know is what circumstances determine what modulation is used on a P25 signal whether it is trunked or conventional? Also is there any conventional use of P25 Phase 2 or is that limited to trunking applications only?

Thank you for any answers,
Robert Klamp
Last edited:


Feb 24, 2001
For phase 1, the mobiles and handhelds transmit C4FM. The repeater (trunked or not) may transmit C4FM or CQPSK for simulcast.

The phase 2 waveform is the same for single site and simulcast.

I don't think phase 2 is limited to trunking but can't say for sure.

Note that the control channel is always either of the two phase 1 waveforms.

I have no idea what "wide" is as a P25 modulation.


Jul 26, 2002
Sydney, Australia
WIDE comes from the era of Motorola ASTRO gear that was IMBE CAI compatible (post 1996) but still pre P25 compliant (prior to 2002-2003 or so)

WIDE is C4FM with modified deviation, symbol parameters and Nyquist ISI/RRC filtering that made use of the FM capture effect to make simulcast work better. It also requires a 25khz wide channel - hence the name.

Wide was replaced in P25 Phase 1 with CQPSK LSM style modulation, since its far more spectrally efficient and fits in a 12.5khz channel.

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