Field Guide · technology

Also known as: CQPSK

CQPSK (compatible QPSK, also linear simulcast modulation, LSM) is the linear phase-modulation counterpart to C4FM used on P25. It produces the same symbol stream as C4FM so a single demodulator can receive either.1

linear phase transitions (compatible with C4FM detection)
CQPSK conveys the same symbols as C4FM on a linear (phase) path, so one receiver design handles both.

How it works

CQPSK uses a linear amplifier and shapes the signal so its phase trajectory matches C4FM’s four levels. Simulcast systems favour it because linear modulation behaves better when overlapping transmitters are received together.

Relevance to SDR

A P25 receiver can demodulate both C4FM and CQPSK; on the constellation the recovered symbols look alike.

Sources

  1. Project 25 — Wikipedia, for the CQPSK/LSM linear path and its compatibility with C4FM on P25. ↩

See also

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