Coherent Differential Phase-Shift Keying or
CDPSK is a sibling form of
PSK.
CDPSK is not used much (if at all) in consumer equipment, or business equipment, it's Military grade and is used under the 188 standards of the military's telecommunications equipment (MIL-STD-188).
In CDPSK, the carrier signal is discretely modulated in relation to the phase of a reference signal and in accordance with data to be transmitted. The modulated carrier signal is always of constant amplitude and frequency.
A phase comparison is made of successive pulses, and information is recovered by examining the phase transitions between the carrier and successive pulses rather than by the absolute phases of the pulses.*
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* Quoted from http://www.tiab2b.com/glossary/index.cfm?term=%27%234%5FQSB%3FKITE%24%0A