Английская Википедия:Incremental frequency keying
Incremental frequency keying, also known as IFK or IFK+, is a modified type of MFSK modulation where the data to be transmitted is represented by the difference in frequency between the previously received tone and the currently received tone.[1][2]
This modulation produces a signal which is much more tolerant of receiver mis-tunings and frequency drift than MFSK modulation. Additionally, IFK modulation is more resistant to multipath interference and intersymbol interference caused by multipath propagation than traditional MFSK.[3][4] This combination of features makes IFK modulation well suited for high frequency communications.
This modulation is used in the amateur radio data-modes DominioEX and THOR.
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