Differential Phase Shift Keying (DPSK):

Also called dibit phase shift keying, it is a modulation technique used to improve the efficiency with which the naturally analog electromagnetic waveform is employed to carry digital bits in a digital bitstream. DPSK is a form of “coherent demodulation,” in which the phase of the incoming signal is compared to a replica of the carrier waveform. The carrier waveform (the carrier frequency “carries” the data, and the waveform is characteristic of all electromagnetic energy), is used as a reference point. With DPSK, the carrier waveform reference point serves to record changes in the binary data code. In other words, a “1” in the PSK (Phase Shift Keying) signal is denoted by no change in the DPSK signal, and a “0” is denoted by a change in the DPSK signal.

DPSK works much better than PSK because so many things can foul up the “absolute” value of a signal sent over an Unshielded Twist Pair (UTP) cable pair or over a microwave radio channel. ElectroMagnetic Interference (EMI) of all sorts can cause the “absolute value” of an originating signal to be “questionable” on the receiving end. Assuming some reasonable level of consistency in the impact of such factors from transmitter to receiver, it helps a lot to have a reference point. DPSK does that.

Takeaway: Differential Phase Shift Keying has emerged as the type of phase modulation most likely to reach commercial deployment since Phase Shift Keying (PSK) is not considered practical due to its extremely difficult method of reception required. Today, air-traffic-control radio communications employ the use of DPSK modulation due to its clear performance advantage over alternative modulation schemes and the availability of rather inexpensive DPSK demodulators (to remove the carrier signal so that the data can be interpreted).